Transcript for:
Basics of Gravitation and Flotation

Hello everybody! Welcome back to your very own Bajjo's 9th and 10th channel. I'm Saurabh, your physics mentor and today we are here to learn gravitation. Chapter is easy. But did you get it? This chapter becomes difficult when we combine it with force, with motion. So that's how this chapter can be made difficult. But the chapter is easy. And today we are here to finish this whole chapter in one go. And that's what we call as one shot. So welcome to gravitation in one shot. So everybody, hello, good evening. It's Saturday evening. And today we are here to finish this chapter, which means I'm really happy you guys are here. Look, we get a day off, but I said, dude, if you want to do your gravitation one shot, then fine. What difference does it make? It's Saturday, it's Sunday, it's Monday, right? So let's do it. What do you say? Yes, good evening. Why gravitation? Dude, Sapnil is a force. Look, Sapnil, if you saw my video on Chandrayaan, What you are trying to say, that's something I've explained over there. Yes, yes Shalini. Everybody, today I came to the office because I had to record something with Vasu sir. And yes, we have done it. Your questions only I have taken with him. It was a marvelous recording. We'll release the video soon. And today we'll do the gravitation. About this chapter, he told something really important which I'll let you know by the end. You'll understand it slowly. Let me tell you what exactly we are going to do. Gravitation in one shot. First of all, we'll talk about Universal Law of Gravitation. After that, value of G, acceleration to gravity. Then, what it depends upon. On what it depends, on what it doesn't. The value of G remains the same, or different, or why it remains the same. All these things we'll talk about. And after that, comes an unexpected guest. In this chapter, we call it, flotation. Yes. In class 9th, Floatation is clubbed with gravitation. You will ask why? And I will tell you that I don't know. Maybe the closest relative to floatation was gravitation because the value of G was something which was used in both the topics. But there is floatation. It's not difficult, it's important. So, we'll cover all of this in today's session. In a good way, completely. It's going to be more than one and a half hour class. Less than two hours, more than one and a half hours. You have to be together, you have to be attentive. Listen carefully. And this whole chapter will end in butter. I promise you that. If you are here for the first time, subscribe. Subscribe quickly. And press the bell icon next to it. If you are coming here regular, press the like button. If you think something good is going to happen, we are standing in front of you. Why are we standing? Like it if you believe that today is going to be awesome. And if you don't feel like liking, then wait. If you like it in the end, then do it. Okay? Everybody, let's start. There will be numericals. I'm good. I'm good. How are you, all of you? Vivaan, Neha, Kanak, Khabib Manu. Oh, you're talking a lot from outside. Doraemon, Dolly, Sanvi, Karma, Kanak, Shalini, Neha. Oh, wow. Come on, everybody. Let's start. Let's start the chapter of gravitation. The story starts with an apple. Apple. Not a working apple, but a edible apple. Story starts from an apple. There is a legend that Sir Isaac Newton was sitting under a tree, it was an apple tree, Yep. Ek sev gira, sar pe gira, saamne gira, I don't know. Kahani toh yehi bolte hai. Sev gira and he started wondering ke yaar sev gira toh niche hi kyun gira? Upar kyun nahi gaya? Idhar kyun nahi gaya? Udhar kyun nahi gaya? Niche hi kyun gira yaar? So that's what made him thinking and slowly slowly he said yaar there must be something right? So it started with things falling to the earth. If you have anything... And if you drop it, if you drop it with your own hands, then what happens? Things fall to the ground. You feel like things are falling to the ground. I would say things are falling towards the center of the earth. Why and how? That's what we are here to study. Yes, very good. Doraemon, I'll explain you. Difference between gravity and gravitation. I'll tell you. If you are asking me this question, means you haven't watched the concert bite session. I have given you concert bite session for gravitation. In that, I have told you what is the difference between gravity and gravitation. You don't watch gravitation, right? Don't lie to me. You are missing it, right? Alright, don't worry. I will tell you. But make sure to watch concert bites. There are some small ones. But you say that there are small ones. We will see it later. Then you leave it. Watch it. Right? Yes. Because of scattering, it's really scattering but I would say scattering of light. Who does it? Air molecules, dust particles, all these things. Right? Come on, let's go. See, there is something called as Universal Law of Gravitation. How does a law is formed? Law is formed when you observe something continuously and then you make conclusions out of it. And you say, you know what, this conclusion, I always see, it becomes a law. Until someone disproves it. Until and unless you find something which contradicts with your hypothesis, till then you say, my law holds. And universal law of gravitation is extremely important. It is called universal. How big is the universe? It's huge. So universal law of gravitation. See, it's very simple. It says, if you have two masses, two mass, something which has matter will have mass. If you have any two masses, Unke beech mein, there will be an attractive force and we'll call it the force of gravitation. And this force will depend upon few things. What are these? Yes? Haan, very good. Being a force of dispro... Nahi yaar, something that's not disproved. Sapnir, scientists from ISRO have said that this is very useful. I'll tell you why. I'll tell you in your age what happens. We think either this is right or this is right. You don't find a balance. Balance is very important. I'll tell you why. Okay? Come, listen carefully. Very good. Nice. Nautic. It depends on few things. First, depends on the masses of the objects. I'm telling you there are two masses, so it will be directly proportional to the product of these two masses. Mass zyada hoga, force zyada hoga. Right? So, this force is directly proportional to the product of masses of two objects. Thik hai? Pehli cheez, pehla observation kya raha? Ke yaar if I increase the mass na, ye force badh jata hai. So, force is proportional to product of these two masses. My first observation. Alright? Next. It is inversely proportional to the square of distance of the centers. Distance, where do we have to measure it? Not from here. This is the biggest mistake that we do. We measure distance many times. Okay, I will start from the boundary of this till the boundary... No, no, no, no, no, not this distance. No. Distance between the centers of these two bodies. Why? Because we consider the centre as the centre of mass. That's why. Okay? If I go more technical, like in class 11th, I'll tell you between two centres of masses. But I'm telling you right now, that this force depends inversely on the square of distance between these two masses. And from where do we measure the distance? From the centre. Right? From the centres. Like this. The inversions of this distance are proportional to how much? Square. proportional to d square, right? Isn't it? y square. Piyush, we observed it. We doubled the distance. Force became 1 by 4. We thought, oh, this is of 1 by square. You observe things. You experiment and then say, this is this much. Okay? Yes. Y square. Apple got me. Alright. Okay. Second observation is, force is inversely proportional to the square of distance. Very good. Now, Let's combine it a little. Whatever you have observed, whatever you have concluded, let's do it together because that's what scientists do. We have made a law. What does the law say? It states that every particle in the universe attracts every other particle. Meaning, Wherever you have two masses, there will be a force of attraction. Now, you can say that there is a particle whose mass is a mass. So, if you have one particle, it will attract every other particle, right? With a force which is directly proportional to the product of their masses. Whose? The particles' mass. Right? And inversely proportional to the square of distance between them. Now, this law is not something different. Whatever you observed, whatever you concluded, you have combined them all and said the statement. Right? This is just a statement which you only observed. Right? Yes? Yes, Piyush! Look, if distance d becomes 2d, what will be the force? 1 upon 2d square. Means, force will be proportional to 1 upon 4d square. So, 1 upon 4 is done, right? Don't worry, we will do a lot of numericals on this. Don't worry, relax. It's just the beginning, listen properly. Right? Right? Absolutely, man. Sanvi, welcome, welcome. Why do feathers float? Drag force, air resistance, right? So, tell me, is this law clear to you? How it came? You observed, then you gave a hypothesis, then you said it is a law. Why? Because you didn't see it being false anywhere. Then this law came. Is this law clear? Tell me quickly. Yes? Tell me quickly, quickly. Perfectly, very good. Okay, easy as pie. Sir, there are two masses. This force is proportional to product of these two masses and inversely proportional to the square of distance between their two centers. That's all. Okay? Okay, very good. So, we said, force is proportional to m1 into m2 upon d2. But whenever you remove the proportionality sign, you have to apply a constant. When you come in class 11, there will be a chapter. The first chapter will be units and dimensions. In that, you will realize what is the value of constant. Why do we know constant? So whenever you remove proportionality sign, you have to put a constant. Here, that constant is named G. Capital G. And do you know what is its name? Universal Gravitation Constant. And this is a very correct constant. This constant has again a word Universal. This constant is such that in the Universe, Same. Yes? Clear? Yes. We will get it. We will get it in our body. We will get it at gravitational speed. Why do we use constant? Shrivali, because we know what we did. We did an experiment. But the value was not the same. We said, there is one thing that we are missing. What is that? But that doesn't change. That becomes a constant. And on this you will do a lot of things in class 11. Don't worry. But today, understand that whenever you remove proportionality sign, you have to put a constant. Okay? Fine. Now listen. G is Universal Gravitational Constant. Now, this Universal Law of Gravitation, if you just say in very simple words, that it acts between two masses, it is attractive in nature. So, gravitation force is such a force which is always attractive. It doesn't discriminate. This doesn't discriminate. It says, come to me. If you have a mass, come to me. It says, if you have a mass, I will attract you. It's an attractive force, always. Unlike charges, electrostatic force, what happens in that? Sir, it repels plus plus in magnetic force. Sir, North pole, North pole repels. But there is nothing like that here, there is no problem. Two masses always attract, simple, okay? Fine. So, that's why we say that this is always attractive in nature. But how many people remember force and laws of motion? First tell me. Do you remember force and laws of motion? And specifically, do you remember Newton's third law of motion? Yes, come on. Absolutely, absolutely, I'll give you time. Centrifugal. Harzimran, this comes in the case of pseudo-force. If you take a non-inertial frame. Now, we'll talk in a relaxed manner, the rest of you will get confused, but it's not difficult. Yes, right? Action-reaction. What is mass? If there is matter in something, molecules, particles, it is called mass. Now, you are right that the space-time fabric is distorted due to which the gravity comes. But why does the gravity come? We didn't go there. We said we observed it. So, I'll tell you why this chapter is important. Do you know that all the missions of ISRO are happening from this chapter? All the calculations, that when should we leave the moon, when you have to launch it, when you have to make sure that it lands on the moon, all these calculations can happen without relativity. All depends on this chapter. I'll tell you exactly what Vasu sir said. But for that, listen carefully. First, learn to listen. Come on. Yes. So, listen to me. Action-reaction. You tell me something. If I call this mass A and I call this mass B, the force of gravitation is between mass A and mass B. If I ask you, let's say this is F1. Mass A attracts mass B with what force? Mass A attracts mass B with what force? Yes. Tanush, we removed proportionality and a constant came in it. We call it G. Yes. F1. Force of gravitation. Okay. Very good. Now you tell me, let's say, forget about F1 and F2. Let's say, this is only F. F is the force of gravitation. A is attracting B with what? Sir, force F. B is attracting A with what force? F. The only difference is, Sir, value of force is same, direction is different. Action reaction. This is the action reaction. What did I explain? What did I tell you? Body A applies force F on body B and body B applies force F on body A in opposite direction. That's what's happening. Force of gravitation is between these two. But if I tell you that body A is applying how much force on body B, you will say, Sir, force of gravitation. In which direction? In that direction. If I ask you that body B is applying how much force on body A, you will say, Sir, force of gravitation. In which direction? In this direction. Clear? So this is what we call as action reaction. A pulled B, B pulled A. Same force applied but the direction was opposite. Easy, right? Yes? We will understand today, it's one shot. Very good. Come. So that's why we are saying that F1 is equal to minus of F2. Sir, minus of F2 Why did you put minus? Minus tells you direction, right? If F1 was in this direction, F2 was in this direction, then how will minus F2 be? This will be minus F2. So that's why, force 1, F1 is equal to minus of F2. What is the value? What is the magnitude? Sir, force of gravitation. g m1 m2 by d2. Easy, right? Clear? Very good. Haan, electric field equation, yes, very good, Saptal. Ye aayega bacha, ham log jab aage aage badenge na, you will understand this. I know you have a lot of questions right now. What is the value of G? I'll come to this. This is the value of G. Dekho bhai, everybody listen to me very carefully. This is something for which Copernicus did very very very precise experiments. Copernicus, in a hut, made a setup and sat outside, that he shouldn't be disturbed by me. And he kept looking from outside, oh, this much is happening. So, that's how he found the value of g. 6.67 into 10 to the power of minus 11. From experiments, we got this value. 6.67 into the power of 10 to the power of minus 11. Yes? Yes. Piyush, the kid will ask questions, I'll tell you. There will be a lot of questions, right? Relax completely. Very good. Newton meter is very good. Spinning king is in the river now. Very good. Right? Faizullah, do you know how this came? It came from an experiment. The value of g was, what we did was, you take the distance of mass of 1 Kg, 1 meter square, how much force is coming, force is also coming, but what is the force? 1 into 1 upon 1 was not coming. Force was coming differently. We said, why is it coming differently? We divided it, that means something is left. That was the value of g. Okay? Fine. So, g equals 6.67 times 10 to the power of minus 11. Now, what will be its unit? If you think about it, unit is easy, right? F equals G m1 m2 by D square. Right? Now, if you just rearrange everything, if you want to find the unit of G, what will you do? What is the unit of force? Sir, Newton. Newton is equal to G Kg into Kg upon, distance how much? Sir, meter. Meter square. So, what will be the unit of G? Newton meter square upon Kg square. This is the unit of G. It's a very common question which I hope you face and you would know the answer. Isn't it easy? Yes, very good. Oh, sorry, I said Copernicus, what? Henry Cavendish, my bad. I said Copernicus, but Copernicus gave us the theory of Kepler and Copernicus, right? Geocentric model of Earth. Henry Cavendish, kids, slight correction, that's Henry Cavendish. Nice, thank you, spinning king. Nice. I got confused with CCC. Why it is a negative ROE, kids? If one force is on this side, the other force is on this side, the direction is opposite, right? And direction means minus. Direction means minus, right? Okay? Yes, yes Yes, correct. Alright, correct. G's value came out, G's unit came out. My bad, it wasn't Copernicus. Copernicus gave the model of heliocentric and geocentric. It came after Kepler, there was a lot of trouble at that time. It's Henry Cavendish. My bad. Yes, got it? Very good. Come on, come on. So, the unit is Newton meter square per kg square. It's an easy thing, isn't it? Now, listen, see. This value of G, What if I say G? What if I say G? Sir, in the universal law of gravitation, the universal gravitation constant, its value is independent of what? Media, mass, size, shape, it doesn't depend on anything. What is in its name? Universal gravitational constant. It is a universal word. It means it will not change. Universal gravitational constant does not depend on anything. Medium, mass, size, shape, doesn't matter. Now, no one will say that sir, G equals F into D square upon M square, so it should depend on D. No, man. This is just to find the units. That is constant, it doesn't depend on anything. Clear? G's value will not depend on mass, not on distance, not on size, not on shape, anything. It doesn't matter, right? Very good. Hello, Jatin. No worries, Jatin. I just explained Universal Law of Gravitation. Law of Theory. Sankalp? Practicality. It came from practicals. Yes. Got it? Very good. Chalo bhai. Ab importance kya hai yaar? What is the importance of Universal Law of Gravitation? And this is where I want to quote an ISRO scientist. I understand all of us are very much interested in, Sir, special relativity kya hota hai? What is general relativity? What is string theory? What is black holes? what are wormholes? But do you know, all this is something which comes later. These are the things which if you want to know, not a bad thing, but basics are necessary. If someone tells you, without even clearing class first, I want to teach you class 10th, how will you feel? You won't enjoy it, right? You'll think, what are they teaching? The basics are not clear. Newtonian mechanics is the basics. And you know, all the space machines nowadays that we are doing, they don't have wormholes and all that. This is simple. Newtonian mechanics. This is simple. Gravitation. It just looks like this. And you can achieve all of this. Relativistic physics comes when you move at the speed of light, when you go near black holes, right? So all these are things which are in theory. This is called theoretical physics. Right, so today I am telling you, this universal law of gravitation is good enough to teach you about falling apple. Sir, what are you saying? That's it. No, no, no, no, no, no. That is only one thing. It can teach you about why things fall towards the earth. That is first. Second, why are you not floating in space and you are stick to the earth? If you jump, you come back. Why? That is from simple, universal law of gravitation. Why moon is orbiting the earth? Why moon doesn't leave us and go away? It says, I am satisfied, I am going. No, it doesn't happen like that because of universal law of gravitation. Why all the planets are orbiting the sun? Universal law of gravitation. Why do we have tides? high tide, low tide because of universal law of gravitation. So, this simple law can explain you all of this without even going into relativistic physics. Right? Yes! Correct? No problem, no problem, I am telling you that this is enough. Whatever you have understood till now, that is good enough. There is no need to go ahead of this. You will realize when you grow up that what is the world ahead of this. Right now it is important that you understand this properly. If you don't understand this and go ahead, then it is a problem. Okay? Let's go. So this is the importance of Universal Law of Gravitation. If you want to write, then you can write. If this comes, tell me three importance of universal law of gravitation. Can you write it? Tell me yes or no? Yes. Dhanush, it's a constant because we experienced, we experimented and we realized that something is missing. And we said it is G. We found the value later. Yes, right? I'm good, thank you. Very good. It's important for the atmosphere. Oh, not a problem. Yes, moon loves Earth. Let's move on. Now, the properties of gravitational force. Another very important question. There are many bigger questions than this. First, the gravitational force is always attractive in nature. It is always attractive. It never says, go away from me. It always says, come to me. So gravitational force is always attractive in nature. It is the weakest force in nature. Sir, which other forces are there? I would tell you there is electrostatic force, there is magnetic force, there is a lot of different forces but gravitation is the weakest force. Sir, it doesn't seem so because you are dealing with large masses. You can see the earth, you can see the sun. Sun is huge. Earth is huge. That's why you can see the value of this force of gravity also huge. Otherwise, it's the weakest force in nature. Alright, it exists as action-reaction pair. I told you this mass is attracting this mass with what force? Force of gravitation. This mass is attracting this mass with what force? Force of gravitation. Equal hai, opposite hai. Action, reaction. Body A ne body B pe gravitation force lagaya. Matlab body B ne body A pe gravitation force lagaya. It is independent of medium. Doesn't matter tum kahan baithe ho. You are sitting in water, you are sitting in space, you are sitting in air. Doesn't matter. It doesn't depend on medium. Next, it obeys the inverse square law. This force is inversely proportional to the square of distance between the two bodies. We have given a special name to this. Inverse square law. That's it. Now, these properties, I hope they are clear. Now, tell me something. Let me ask you something. Yes? Very good. Why gravity is weakest? This is what we observed. Now listen. Yes, nuclear force bhi aati hai, strong nuclear, weak nuclear, suno everybody let me ask you. When you release this object, it falls to the earth. Right? My question to you is, when this body is falling down, when this body is falling down, it is falling down because, it is falling down because, fill in the blanks. It is falling down because, Piyush, same thing, distance, R, notation cover. Same thing, no problem. Gravity, very good. Yes, because of gravity. Alright. Now tell me one thing, who is applying this force? Sir, earth is attracting this object and that is why it is falling down. What would be its reaction pair? Action is fine, earth is attracting this body and this body is falling down. What is the reaction of this? Force? Action reaction happens, right? What is the reaction? It is a non-contact force. Yes, Ruhur ma'am. Yes. Hmm. Good question. I can't take this question right now. Object is attracting earth. Very good. This object is also attracting the earth. With how much force? Sir, equal force. But you tell me something. I can see object is moving but why isn't the earth moving? Earth is not moving. If this object is applying same force on the earth, I don't care if the earth says so, why doesn't the earth care? Tell me that. Yes? Mass. Very good. Force is equal to mass into acceleration. If mass is very high, acceleration is very low. Mana ke 4 same hai, but mass of earth is huge. That is why acceleration of earth is negligible. Negligible matlab 0. Right? So that's why you don't see any effect on the earth. Right? Correct? Hai na? Bot badeeya. Chalo. Is this clear? Haan, very good. Haan, nice yaar, Keshe. Bot badeeya. Moon, aa, sankalp? It's a good question. I have this question in the session. Clear? Jaldi? Yes or no? Fada fad? Come on. Everything is clear here. Universal law of gravitation is clear. Absolutely clear. Let's move forward. Yes. Blue Devils, I'll come to this question. I'll come to this question. After the next topic, I'll answer this question. Very good. Does G depend on Karma? Same. I'll answer this. Come on, move forward. Doraemon, it's done. Study well. That's it. Next is free fall. I have done a concept bite session on free fall, if you remember. How many people have seen it? I don't know. But please watch it. It's a small session. I will explain it to you properly. Now, what is free fall? It has a very simple term. It's a very simple statement. If an object is moving under the influence of the force of gravity alone. Sir, force of gravity? We had said force of gravitation. What is the difference between gravity and gravitation? Look, everybody listen to me. Gravitation can be between any two particles which have mass, right? Force of gravitation can be between any two particles which have mass. Gravity, gravity is a special word we use for planets or heavy, heavenly bodies. The force between planets and objects is called gravity. If we see the asterisk, it is the force of gravitation. So you can say force of gravitation is the bigger umbrella. There is a small part called gravity. We use gravity specially for heavenly bodies. Gravity of sun, gravity of earth. Have you ever heard of gravity of my phone? No. Force of gravitation can happen but gravity only happens or we only use for planets. Okay? Right? Yes? Any case, I'll come to this. Don't worry. Clear? Is it clear? Force of gravitation is the universal thing. It's a bucket. It's a very big umbrella. What is gravity? For planets, we use it. Yes? Right? For heavy objects. You're right, Shruti. Nice, Keshav. Well, it's not downward actually. Ritwa is actually towards center of Earth. I'll tell you what. Duryodhana, I'm telling you that gravitation is the force between any two particles having mass. That is force of gravitation. But force of gravity is given a specific term we use for planets, for heavy heavenly bodies. For them we say that the force of attraction, I am giving you special name, let's call it gravity. That's it. That's the difference. Okay? Fine. Now see, free fall. What is free fall? Anything can be said to be in free fall if there is only and only the force of gravity, meaning heavenly body. Earth pe girna, Jupiter pe girna, Sun pe girna, it could be free fall, right? So free fall, if an object is moving under the influence of the force of gravity alone, then it is said to be in a state of free fall. Abh The acceleration of the freely falling bodies is independent of its mass and is equal to g. Why and how? I'll prove it to you. Don't worry. I'll prove this to you but right now I'm just telling you that When do we use the term free fall? I say free fall we only use when a body is falling under the force of gravity and gravity alone. Clear? If there is only one force on it, that means it is in free fall. Clear? Yes? Very good. Arush, I'll do it, don't worry. Not calculate, I'll show it to you. You can try the calculation, not a problem. I'll give you a... Inverse law of gravitation. Yes, Tanush, there is no derivation of this. It is experimentation and there is a conclusion to it. There is no derivation of this. Right? Let's go. Yes, celestial objects. You are right, Piyush. Let's go. Look, everybody. Listen to me. Acceleration due to gravity G. Now, I want to warn you, I am talking about Earth alone. Now I am talking about Earth only. The force of gravity on Earth. So, the acceleration due to gravity on Earth, I am giving a special word G. Right? Fine. Now listen, what did we say? What did we learn till now? We learned F equals G M1. M2 upon D square. Very good. Now you understand this. Let's say this is earth. Mass of earth is Me. This is a small object. Mass is M. Fine. The force of gravitation between the centers is let's say F. is let's say F. Very good. So F is equal to Sir G mass of earth into mass of object upon this distance this distance or I would say radius of earth. If earth is a ball so this becomes R square. Earth is so big that if I put the object on an empty board, then also it doesn't matter. Right? 6400 kilometers is what we are talking about. This is the Earth's diameter. Fine. So, F equals GME M upon R square. Very good. Now, we have also learned that force equals mass into acceleration. If I want to find the acceleration, I have to divide the force by the mass. What is the mass? I want to find the acceleration of the object which has mass M. How will you find the acceleration? You will say, Sir, divide by M. You will just simply say, that nothing sir, divide by m. Right, just divide by m. So, f upon m, that is equal to acceleration, is equal to gme upon r square. Clear? Now, you tell me something. That this particular g, its value is coming out to be? Universal gravitation constant multiplied by mass of earth divided by the square of radius of the earth. Right? Correct? G, mass and radius of the planet. Now we have talked about earth. You can apply it to any planet also. You found the value of G. This is acceleration. Any doubt here? Yes? Aniket, here small assumption is It's slightly elliptical, but we are considering to be round right now. That is why. Yes? Haan. Me. Acha, me. Correct? Ye clear hua? Tell me. Yes? Haan. Karma? It doesn't depend on mass. I'll tell you why. Haan. What is R square? Shrivali? R square is, if this is your, if this is your earth and this is your object, So, r is the radius of Earth or of any planet you can say. In the case of planet, the r that is the radius of the planet. Right? So, that is r over here. Clear? Now listen. In the case of Earth, in the case of Earth, if we apply the same formula, then, value of the universal gravitational constant, we know it. Right? This is this much. Mass of earth is this much. Sir, how did you get it? Mathematics, calculations. Okay. Then radius of earth is this much. Okay, fine. If you put all these values, the amount of G that you get, it is 9.81 m per second square. It is acceleration only. This is what acceleration is. So, unit will be m per second square. Okay. Clear it now. Right. So, this is how you found the value of G, the acceleration due to gravity. Correct? Why we not take diameter? Shruti, I told you the distance should be from the center of the two objects. So, we will take from the center of the earth. What is the distance from the center to the circumference? It is the radius. Right? Priya, I am sorry, I missed this question. I don't even remember which meme I talked about. Convenience, yes you are right Shrutpa. Can we reduce? Yes, Shankar, it can. If you go to height, if you go to depth. Right? So, value of G near Earth's surface is taken as 9.8 m per second square. Is it clear till here? Now, do you understand why people say take the value of G as 9.8? For convenience, we say sometimes take it as 10, but 9.8 is the average value of acceleration due to gravity on the surface of the earth. Clear? Yes? Right? How D2 equals R2? Faizullah, D2 is the distance between the center of two objects. If one object is earth, it doesn't matter how much it is. Think about it. If I give you two values, 6400 km or 1 m. How much difference is there? It doesn't even mean 1 meter. So I am taking only r. Okay? That's why we say, near the surface of earth, value of g remains the same. Right? Okay? Now listen everybody. Let's, I told you, this chapter can be clubbed very nicely with motion and with force also. In motion, you read these three equations, do you remember? Do you remember? three questions First equation, V equals U plus 80. Second equation, S equals UT plus half 80 squared. Third equation, V squared equals U squared plus 2AS. Do you remember? Tell me quickly. Yes? Yes. Piyush, nothing is left. F upon M, right? So, 1M went below F. That's why you got GM upon R squared. Yes? Yes. Karma, I'll explain it. Yes, right? Correct. Very good. These are the three equations. So, there is nothing in these equations. Acceleration was A. What is G? G is acceleration due to gravity. So, if you are studying objects in the vertical direction, where they are falling on earth, falling on moon, falling on planets, where we just said that acceleration was A, here we used to say G, so let's keep it G. There is no difference. So, acceleration is there, both are acceleration only. So, V equals U plus Gt, S equals Ut plus half Gt square, and V square equals U square plus 2Gs. when downward direction is taken as negative. Now see, this particular thing I want to tell you. When to take plus or minus? How many people are confused about this? How many of you face confusion in this thing? Sir, when to take V equals U plus 80, when to take V equals U minus 80? I explained this to you in motion. I have explained it to you. How many people remember? Tell me quickly, yes or no? Me. It never happens. Very good, Arzimran, you are right. Very good. You will see, I am happy. Very good. No, everybody, look, you take positive value when the motion of the object and the acceleration both are in same direction. If your motion is also up, acceleration is also up, then you say that V equals U plus GT will come. If you say, sir my motion is upwards and acceleration is downwards, then you use V equals U minus GT. There are other ways of doing this, but this is the simplest way I can explain it to you. When downward direction is taken as negative, then you say V equals U minus GT. When downward direction is taken as positive, then you say V equals U plus GT. Nothing is there in this, it is an easy thing. Right? If motion or velocity is same direction as acceleration, you take plus. V equals u plus gt. S equals u t plus half gt square. If motion is opposite to acceleration, uu is minus like Sir we will throw the body upwards if I throw this object upwards means the motion is upwards acceleration is downwards, motion is upwards, acceleration is downwards opposite is done v equals u minus gt if I drop an object, if I drop an object like this means Sir, object is moving down Acceleration due to gravity is also downwards. Same direction. I will use V equals U plus GT. That's it. In this way, it will help you out a lot. There won't be any confusion, right? Tell me, yes or no? Yes. You can, you can. The point is left, you can use Cartesian also. I am telling you the easiest method. I teach this to people in class 11 in a different way. But for you, I am telling so that you don't get confused. This is the easiest way. Yes, okay? Yes, Harishabran, good question. I will come to this. Yes, fine. Let's do the question quickly. Everybody, let's do this question. Rahul and Rohan, they live on two different floors of the same building as shown. 80 meters and 45 meters. If they dropped a ball from their balconies at the same time, what will be the time difference between the two balls to reach the ground? What do you think? You have four options. Solve them. Riya, because? If you take the height of an object, how much will you take? 1 km Earth's radius is 6400 km Now, that is too high, right? So, it doesn't matter if it's 1 km Some say that the radius of Earth is enough How will they do it? I'll tell you How to solve a numeric... Swapnil This is what happens Look, theory is good But it is important to question it The one that is in front of you, understand it properly I am very happy to talk about real district theory, string theory, everything But this is very important Because these are the basics This is important to know Yes? Yes No idea? No problem Ascent and descent, you can write Yes, spinning, you are right I'll tell you Priya. Do it, do it, no problem. Everybody, you cross check, huh? I'm reading the question again. Look, to solve questions on numericals in physics, there is no special trick. It's just you read the question properly. Rahul and Rohan, what does it mean to me? What is its name? I don't care. What is it? They live on two different floors of same building if they dropped a important term. Whenever a question says dropped, what does it mean? Motion you have done. Means you have done it before. Tell me, what is the meaning of dropped? Is it coming little by little? Yes, I don't care. Yes, you are right Sudha. Very good, Dipansh. U equals 0. Very good. Nice. Do you know what drop means? Dropped. Initial velocity was 0. Dropping means the same thing. Every time, whenever a question uses the word dropped, it means u equals 0. Right? At the same time, what will be the time difference between the two balls? Means, if I find the time taken by this ball minus the time taken by that ball. Now, this is at height, so it will take more time. I understand. Now, this question is not of gravitation. This question became motion question. Sir, how? U equals 0. U equals 0. For 45 meter ball. Right? For 45 meter. Correct? You want to find the time. Right? You want to find the time. What do you have? Sir, I have S. I have G. And I have U. Is there any such equation? jo in 3 ko jodti ho, is there any equation which combine these 3 things? Yes. S equals ut plus half gt square. Dono case me the ball is dropped. Motion of the ball is in the direction of acceleration due to gravity. Dono niche ki taraf hai. I will use plus. Iqdam aasaan, koi rikkati nahi hai. Right? So, S kitna hai? Sir, S is 45. is equal to ut plus half gt square. How much is u? u is what u told me. How much? u is 0. If it is dropped, means initial velocity is 0. S that is 45 is equal to ut means 0 into t plus half into g. How much is g? 10 into t square. So, this is 0. 45 equals half into 10 into t square. Solve it. How much will it be? 5 to 5, 9. 9 equals t square. t equals 3 seconds. 45 ball takes 3 seconds. What about 80 meter ball? For s equals 80 meter. Apply the same formula quickly. Sir, 80 is equal to ut. In ut, u is 0 so it is 0. Right, 0 plus half into g means 10 into t square. This is what? How much will it be if you cut it from 5? 5 and then 16 is equal to t square. t equals 4 seconds. The 80 meter ball is taking 4 seconds to reach to the ground. 45 meter ball is taking 3 seconds to reach the ground. What is the time difference? 4 minus 3, 1 second. Easy. That's why option D is correct answer. Yes, it's very easy. Yes, there is 1 second difference. Question was easy. Tell me yes or no. Right? Dipansh, it's okay. You're here. It's the same thing. Correct. Right? Arush, clear? Tell me fast. Everybody. Yes, right? Shruti, Rudpa, Arshamnand. Very good. Let's move on. Now look, everybody. I told you acceleration due to gravity, g value is 9.8. But you know, it is an average value. And what some of you were saying, that Earth is not a perfect sphere. You were right. That is why the value of G varies. What does it mean? Look, listen to me. This is the center. Right? This is the center. The radius towards the equator is how much? 1, 2, 7, 5, 6 by 2. Because the diameter is 1, 2, 7, 5, 6. The radius towards the poles is how much? 1, 2, 7, 1, 4 by 2. Now tell me one thing. Can you conclude that the radius in the equator is more than the radius on poles? And what was the value of g? That is acceleration due to gravity. G equals Universal Gravitation Constant multiplied by mass of Earth upon R square. Now, radius at equator is more than radius at pole. So, can you say G at pole will be more than G at equator? Will you tell? Think about it. If this radius is more, then G will be less. means the value of G at the equator will be less. If radius at equator is more, means the value of acceleration due to gravity at equator will be less. Why? Because the distance from the centre is more. That is why. It is far away. If a body is at pole, the distance from the centre of earth is less compared to the equator. This means the force of attraction will be more. Means, the force of gravity will be more. So we say that acceleration due to gravity is More at the poles compared to the center. So, G at poles is more to G at the equator. Tell me quickly, is it clear? Earth's shape, we call it geoid. It is a little obloid kind of shape. It is not a little surf sphere, it is bulging from the sides. That's why. Yes. Correct? Yes. It means that there are values in front of it, take them out, there is no unity. Yes, an orange is round. Which is that orange? The one with a stick, this one, this one. You can say pumpkin. It is more like pumpkin. How is pumpkin? Or watermelon. Watermelon is like this, a little bit like this. Yes, so this is your earth. Okay, is it clear? So that is why everybody, what is the important thing? Sir, the acceleration due to gravity at the poles is more than acceleration due to gravity at equator. Yeah. If I take a mass on a pole, it will take more force compared to the equator. Why? Because G is more on the pole. Clear? Right? Karma, I'll come to this. Right? So everybody, if we conclude everything, we'll say the value of gravitation acceleration varies with the radius of Earth. Where the radius is more, the force is less. Radius of Earth is lesser on the poles than that of equator. So, G is inversely proportional to radius of the Earth. And that's why the value of G at the polar regions is 9.83. 9.8 is more than 9.8. And at the poles is 9.78. Sorry, at the equator is 9.78. Is the value of G at the equator less? Yes, very good. Elliptical, geoid. Geoid is a more appropriate word. Equator has 9.78 m2 per second, Pole has 9.83 m2 per second. What is its average? Sub 9.81. That's what we said. Average value of G on Earth is 9.81. But on equator, it is less. On poles, it is more. Right? Correct? Right? Yes. Now, some people say that Now let me ask you, if the question is only about the centre, I mean how far you are from the centre, what should be the force of gravity if you stand at the centre of the earth? It's not a trick question but it's a question on which we trip a lot. We like to talk. What is the force of gravity on a particle which is at the centre of the earth? Zero. Why? If you are telling me that on poles, the body is close to the center of the earth, then the object at center is closest to the center of the earth. It should be infinite. Very good, Dipansh. There is derivation. Now listen, everybody. The force of gravity was because earth was attracting the object. Very good, Tarush. Very good. Nice. So when you have an object, when you have an object, at the center of the earth. Where? Sir, at the center of the earth. Do you know what it means? It means earth is attracting this particle in all directions. Like this, like this, like this, like this, like this, like this. And you tell me, if I pull a particle 10 Newton here, 10 Newton here, what is the net force? Sir, zero. It has been cancelled. Same thing is happening in the case of the body which is at the center. Earth is attracting it from all the sides, so the net gravitational acceleration will be zero. Yes, very good. Alitya, we have done a lot of sessions, how to make nodes. Ursh ma'am has done a good session on this. Right? Fine. So that's why, zero at the center. Sir, least at the equator. Maximum at the poles. Right? Right? up. We were talking about Earth every time. Recently we launched Chandrayaan. Who knows what we are doing tomorrow, right? Because we are also planning. Apollo has already done this. NASA has already done this, right? So, on the moon, have you noticed? If you have seen any of the videos, on the moon, if people jump, they can jump higher. Have you seen? If somebody jumps on the moon, they can jump higher. Like this, like this. Oh! Yes? Yes. Absolutely, we'll do numericals. We did one, right Simran? Yes, right? Yes, yes. Earth's center, light and earth, we consider earth's center, we consider it as an earth point object. Yes, right? Very good. So, does it mean that moon attacks less? Does moon attack less? Why? Because mass of moon is less than the mass of Earth. This is the diameter of Moon. So, if you calculate using the simple formula F equals gm1m2 by R square. If you calculate using the formula for acceleration to gravity that is gm of Moon upon radius of Moon square. Both the radius and mass of Moon is lesser as compared to Earth. That's why the value of g that is acceleration due to gravity is also less. Why? g into mass of moon upon r of moon square. If you put all the values, what you get? 1.62 meter per second square. 9.8 1 by 6, you know, you will get the same value. So, the value of g on moon is approximately 1 sixth of the value of g of the earth. Force of attraction on the moon is about 1 sixth of the earth. How did you find it? Simple mathematics. Which law did you use? Your class 9th law. Do you realize this? We did all these things just with the use of simple simple law which you are studying. Right? Oh wow! Zahid, thank you son. Read it. Read it. Okay. Everybody, we have done mathematics. It's here. Value of G on the moon is 1 6th of the earth. Why? Because moon has less mass and moon has less diameter also. Right? Okay? Now, next question. Why doesn't the moon fall on the earth? We had said that everything which earth is attracting falls on the earth. If you release it, it will fall on the earth. Now, you are telling me that moon is also getting attracted by the earth. But why doesn't the moon fall in the earth? Have you ever thought about it? Yes? Come on, tell me quickly. Mass of Earth, measure 5, calculation, mathematics, theory. Very good. Ah, okay Ashika. Very good. Arush, watch the video once. It was very fast. It was breaking down. So he did this. Very good. Nice. Centripetal force. You know, on YouTube, there is a very nice video which Avinash sir has done. Why doesn't moon fall on the earth? How many of you have watched that video? How many of you have watched that video? Tell me, yes or no. Have you seen? We saw it. Very good. Me, me, me. Everybody, huh? Nice. Same thing. Agar aap body ko release karoge, if you drop a body, it will fall on the earth. But if you don't drop a body and you push the body in this direction, means you give velocity in this direction, then scenario will change. Matlab, dekho yahan, if you drop, it will fall on the earth. If you push it, it will fall on the earth but travel some horizontal distance. If you go to a very huge height, this is what will happen. If you give it so much velocity, you will reach a point, that body will start going in this circular motion. This circular motion is what we call as Orbit. Orbit is called, not the one that rises. Orbit means the path in which it is going around the earth. So this has happened because you have given it horizontal velocity which is enough to keep it in this circular motion. So that's why. Now you tell me something. Will this motion be called free fall? Yes. Light and earth, it happened long back that someone pushed the moon and it started moving like this. Right? Centrifugal is different Prisha. Centrifugal is a pseudo force which comes into picture when you are looking at the frame from the moon's perspective. I will not talk today. It's a very nice concept but class 11th. Right? Know it today? Very good. Centrifugal is a fictitious force. Right? Sending objects into orbit, you understood now. I will give you one statement, you tell me true or false. The moon is moving in a circle around the earth, means I can say moon is orbiting the earth. Can I say moon is in free fall? Yes or no? Tell me. Can I say moon is in free fall? Yes. False. Yes. No. No. Yes. Yes. False. False. Yes. No. Oh nice. Everybody, What was the definition of free fall? The definition of free fall was There should be only and only force of gravity on the object. If you take the force on the moon, it is force of gravity of the earth. Think, tell me one thing, if force of gravity of earth was not there, what would have happened? This is the meaning. If force of gravity because of Earth is not there, right? If this force is not there, what will happen? Moon will say, Achcha chalta hoon, duaon mein yaad rakhna. Right? So, Moon will say, main jaa raha hoon. It will go in a tangential path. It will not go in a circular path. This force of gravity is what is making Moon go into this circular path, right? Hai na? Yes, correct. True. So, Moon would have gone straight, tangentially, bye bye, tata, correct? Very good. Yes, it is going far. That 3.8 cm, Faizullah that's true, but the reason is different. But Moon is going in a circle, it is because Earth is attracting the Moon towards the center. Right? This force which makes bodies to move in a circle, we have given a special name to this force, centripetal force. Any force which makes a body to move in a circle, that force is called as centripetal force. Who is providing the centripetal force here? Who is providing the centripetal force? Sir, force of gravity of the earth. Earth was pulling, that's why Moon was moving in a circle. But it is the only force acting. What was the definition of free fall? There should be only and only force of gravity. Is there any other force on Moon? No. Only force of gravity. That is why it is going in a circle. So Moon is in free fall. International Space Station is in free fall. Satellites in the Earth's orbit are in free fall. Is everyone in free fall? Yes. Clear? Very good. Did you enjoy? Oh, Faizullah, I'm sorry, I missed your question. No, Priya, don't do that. It's too late now. centrifugal force...Tanish I would like to answer it but right now I don't know, the rest of the people will get bored. If you want I can give you a small concept by it if you ask me in the comment section I'll give it to you okay but not in this session. So gravity as a centripetal force because of this gravity working as centripetal force what do you see? You see moon's motion around the earth Same thing, if you take a... What is this? Oh wow! I got something. If you take this particular thing... Can you see this? If you take this particular thing and rotate it like this... It is in a circle. What is the force? Because of this string. This string is going in a circle because of this. Same thing as gravity. Right? I did not plan this. I don't know how this was lying here. Right? Correct? So, twirling a ball is also an example of centripetal force. Who is giving it? The force of string. Yes. Thank you, thank you Faiz. Gravity. Divyanshi, I explained this, kid. Gravity is between planets and heavenly bodies. Gravitations between two masses. Yes, it has given velocity, you are right. The moon was given velocity. Someone pushed the moon like this. Who did it? There is a big theory that when the earth was new, an object like Mars came and hit the earth hard. So, the object came out of the earth and started moving outside. There was a lot of dust. That dust came together and became the moon. This is the theory. We will find out the rest. Chandrayaan has gone to tell you. So, the sentimental force acts towards the center. important Centripetal force, it acts perpendicular to the direction of velocity. Any doubt in this? Any doubt? At every instant, this moon or this ball, if you release the force, it will go in this direction. So, the velocity of the particle at each instant, Is tangential. Tangent means? Tangent means a line which is touching at a point. This is called tangent. So the velocity of a particle is tangential to the circular path. Velocity will always be tangential. Acceleration will be towards the center. This is your acceleration. This will be acceleration. Clear? Right? Tell me quickly, is it clear? Yes. Is it clear? Tell me quickly, yes or no? What is velocity? Fez, we have done motion, velocity, rate of change and displacement. Change is only the direction of motion. Velocity is a vector quantity. Velocity needs a magnitude and a direction. Ek baat batao yaar, velocity if it is going in uniform circular motion, I have taught this to you before. Agar koi body uniform circular motion mein hai, iska matlab ho sakta hai uski speed same ho. The body is moving with 2 meter per second in a circle. But, is the direction also same? No. Direction changes at every instant. Right? So, this means velocity changes. If velocity is changing, it means there will be an acceleration. Because acceleration is rate of change of velocity. Speed can be same. But, velocity is not same. Velocity is changing. means, do you know the function of acceleration? Acceleration is used only to change the direction. The magnitude is same. The value of the speed is same. What is changing direction? So, the work of the centripetal force is to change the direction. The magnitude of speed will remain same. Yes, it is getting clear, right? Okay, so, tensor quantity. Tensor quantity is such that direction which is neither scalar nor vector. Like current. We call current as tensor quantity. There is a meaning of direction and also there is no meaning of it. I will tell you more about it later, not now. Right? Right? Are you guys talking? Anshita, same answer. Why moon doesn't fall to earth? Same answer is for why earth doesn't fall into moon. Absolutely. Okay? Yes! Let me ask you something which is very common. How many people think, my weight is increasing. If somebody asks you or let's say you measure your weight on a weighing machine Sir, now I am fit. My weight is showing 70 kg. And your friend says, 70 kg is not your weight, it is your mass. So, what is the difference between weight and mass? Can somebody tell me? Yes? Yes. Shrivali, it is mv2 upon r. You will learn in class 11. Very good, Devanshi. Nice. Oh, it's okay Priya, not a problem. You grow up, it happens to people. We are getting fat, clothes are not fitting. So all these things happen when you grow up. Don't worry now, just have fun. How fast is gravity? Sankalpa! I won't answer this question right now. It will be a long, useless discussion. Yes Priya talking to Shiva. Hey kid, focus here. Mass is the matter. Very good. Nice. Notes I'll tell you. Very good. Priya, focus here. See everybody. See there is a basic difference between mass and weight. Mass is the quantity of matter. How much matter is there in anything? How many molecules are there? So that is what we call as mass of the body. It does not depend on the acceleration due to gravity. Mass has no meaning. Mass is scalar quantity. Unit is kg, kilogram. It is always constant. It can never be zero. If a body is occupying some space, it will have some mass. Right? Now, if we talk about weight, what is weight? It is force. The force of attraction because of Earth. So, W equals M into G because weight is a force. F equals M into A. W equals M into G. It is the same thing. Right? It depends on G. Unit is Newton. Weight is a force. And force is Newton. So weight is Newton. Right? It varies from place to place. It is zero at the center of the earth. Because force is zero at the center of the earth. Now tell me, when someone asks you, how much is your weight? Why do you answer in kg? Why do you answer in kg? Tell me the truth. Yes? Telegram. Oh sorry, not telegram, we are going to send you proper sheet, but for now telegram Do not wait, yes you are right Air have weight, yes master, atmospheric pressure is because of weight of the air Why? I will answer you Shrivani, don't worry No problem, no problem, yes Haha, sure ya, it's a good answer. Newton, it's not fun to speak Newton. This is the truth. That is the truth actually. We don't feel like speaking Newton. We like KG. Why? We use KG. That is why everyone says, you are right. Anshita sorry, I can't read your question fully. It's getting blocked in front of me. Yes, see, what happens is that Weighing machine actually measures your weight in Newton's only. But what it tells you, because you are more comfortable with kg kg kg kg, 5 kg brinjal, sorry not 5 kg brinjal, 5 kg rice, 1 kg brinjal, half kg okra. So you all are comfortable with kg. So nobody tells you your weight in Newton. They tell you in kg. Why? Because you keep hearing kg kg kg. That's why the weighing machine actually finds your weight in Newton's. But what it does, It internally divides it by G. Think about it. If... It finds mg and divides it with the value of g, let's say 9.8. What is left? Mass. So that's how the weighing machine tells you the mass. But they measure weight. Mathematically, it tells you mass. Yes? You'll say, yes, good, good. Yes, very good Prisha. It only tells magnitude. Science exhibition, yes? Oh, do that. My gravitation video. It's easy too. Take your mom's clothes and sari to home. Very good. Right? Okay. So, weight varies because it depends on value of G. Tell me one thing. If I measure my weight on equator and poles, my weight will be more on equator and more on pole. Tell me quickly. My weight will be more on equator and more on poles. Dipansh, we balance for mass. Do you remember weighing scale? The one we use for vegetables. It tells you the mass. How? By comparing. Polls. Very good. Right? So, if you want to lose weight, then go to the equator. If you are in a lot of trouble. Okay? Right? So, weight is a force. Mass is mass. Mass does not change. Okay? Fine. Now, listen. If weight... on the value of G, if G changes the weight will change. Yaini ki earth pe let's say I have some weight, if I go to the moon my weight will change. Why? Because moon pe value of G is different. Kitna hai? Sir value of G on the moon is G by 6. Hai na? 1 sixth hai earth ke. So your weight will be 1 sixth. Your weight will be 1 sixth on the moon. Right? So that's why your weight will become one sixth on the moon. Clear? Yes? How can you measure mass? Everyone knew? You can compare mass. Think, here too, it's G and here too, it's G. Both are G, which means G has been cancelled. Now, what are we comparing? Mass. And that's how this weighing balance tells you mass. Yes, you're right, Shaurya. Strength will be 6 times. Clear? Good evening. Hello, Shaurya. Right? Yes, Arsalan, you're right. Anshita, if the gravity of Earth suddenly becomes zero, Same thing, if a body is going in a circle because of this thread, if you cut the thread, the body will go straight in tangential direction. Same thing, if the moon is going in a circle but suddenly the value of force of gravity of earth becomes zero, what will happen? Chal raha hai, chal raha hai, chal raha hai, chal raha hai, zero. Whoosh! It will go straight, tangential. Moon will do like this. Okay? Fine? Come on, come forward. Question bro, a question for all of you. Solve it quickly. If you weigh 50 kg on Earth's surface, what will you weigh on a planet which has mass 1 fourth times and radius half in comparison to Earth? Good question. I mean this is one question, I will make you 50 questions more on this. It's a pattern. If you solve this, you can solve 50-100 questions more like this. Solve it easily. Come on. No, Aaruj, there is no difference in this much. We are talking about pole. If you go to Antarctica or North Pole, Alaska, then I will talk. Science exhibition idea? Make a projector? No. We have done a session on the channel, how to make a slide projector. Make it, it's good. Is there anything like this? No, Shivalli. You just cut your life. You will also pass away. And 100 more generations will pass away. Then maybe something will happen. Yes. 12.5. D, D, D, D, D, D. D, right? Lot of Ds. Dipay is saying 50. Shruti D. Mega D. 50 Shaurya. Everyone knew mass depends on how much matter inside. So, mass you can say depends only on how much matter is inside. Doesn't depend on anything else. 32 Newton. No, this is wrong. 32 means your weight is 3 kg only. Your mass is 3 kg. No, not sure. C. Tarnas is saying C. 50 Karma. Dipansh B. Priya D. Dipansh B. Aarti D. Tarnas C. Dipansh. Oh, I have read yours. Kavita D. Ansh B. B, 1, 6, 9.8. Okay, listen to me. It's okay if you don't get it. It's okay. Don't worry. Samaran, question is saying, let's say a weighing machine reads your weight as 50 kg on earth. If you take the same weighing machine on moon, sorry not moon, my bad. If you take the same weighing machine on a different planet, which has mass 1 by 4 times of earth and radius half of the earth, what will it read? Your mass. How much will your mass tell you? Why weight in kg? Same thing, when we study, how much is your weight? You say 50 kg, that is why. This is exact, it's a real life question. Faisalullah, bacha aap Adhan kar ke aao, koi baat nahi hai naa Lekin, rukh toh main nahi paaunga, but don't worry, aap miss jo hoga, I'll tell you Nahi nahi, physics tough nahi hai, Shaurya Okay, theek hai, Dimansh Haan suno, everybody listen to me Galti ho raha hai, bohot log ki baise, I'm telling you Dekho, everybody Bhai, 50 kg on Earth's surface Hmm, theek hai Now, you tell me something What will you weigh on a planet which has mass 1 by 4 times? How different will be the G on that planet? That is the first question. The weight depends on the value of G. If your mass is showing 50 kg, it means how much is your weight on earth? 50 multiplied by the value of g on earth. This is your weight on earth. Very good. Now, what is the value of g on this planet? g equals gm upon r square. Now, it is saying that m is 1 by 4 times of the mass of earth. The radius of r is half of the earth. r equals 1 by 2 of r e. Now, the value of acceleration due to gravity on earth is It is Gme upon r e square. Now, put these values in this. What will come? G on the planet is G multiplied by 1 by 4me upon 1 by 2. Re whole square. Solve it. g 1 by 4 into Me upon 1 by 4 Re square. 1 by 4 to 1 by 4 is cancelled. So what you are getting, sir, g Me upon Re square. The value of acceleration due to gravity on this planet is equal to the value of g on Earth. Same is here. Correct? Yes, Aditya? Haan haan, explaining again bacha. We have used the simple formula of G. GM upon R square. Dhyaan se dekho hai naa. Haan very good. I know 50 tumne bola tha. Bhai, on the earth the value of weight or mass you can see is coming out to be 50 hai naa. To humne bola earth pe weight kitna hoga? 50 into value of GE. Ab hum dekh lete hain. Let's find ki bhai how much is the value of G varying on this planet. And for that I have used the same formula. Kuch nahi hai. Mass of this planet is 1 by 4 of earth. Radius of this planet is 1 by 2 of the earth. If you put these values, you will realize that the value of g on this planet is actually equal to the earth. Means the value of weight will not change, you will exactly weigh the same. So what will be the answer? 50. Your weight will read 50 kg only. Right? Square, right? No problem. Yes, it was easy, right? Very good. Correct answer, very good. Was that a good shortcut? You were giving me a shoutout, no worries Alright, very good, come on next question Which of the following is the same on both the surface of the moon and the surface of the earth? Weight of the bar, weight of the astronaut, mass of the astronaut, gravitational force This is the astronaut, right? This astronaut went to the moon and said we will put on the Virgish We will do exercise, I want to do weight lift You tell me something D? Hey, who is speaking? Yes, C. Very good. Yes, mass of astronaut. What does not change? Sir, mass. Because mass is a scalar quantity. It does not depend on anything. What is mass? It depends on the how much matter you have. So, mass is a scalar quantity. So, that's why weight of bar will change. Why? Because it depends on the value of G. And here it will change. Weight of astronaut will change because it is weight. It will depend on G. Gravitational force, it will change, it depends on G. What will not change? Mass. Right? So if you think about it, let's assume that you become astronaut. Here you would have to lift 10 kg. Right? Do you know what will happen on the moon? You will be able to lift 60 kg. Yeah! Right? So what is there on the moon? You can do better. Yes? Nice! Correct? Easy? Very good! It should be like mass. Nice! So, mass of astronaut will not change. Very good! Now, this is the end of gravitation. Gravitation is over. Now, do you know what is remaining? Flotation. Tell me till here, anything, any problem, any sorrow, any complaint, any problem, tell me, any sorrow in your life? Let's go to the moon, but Priya, this is going on, let's go. Gym will be easier on the moon, you're right Prisha. How to join Telegram channel? Tell me Telegram channel, link is given to you. Not coming? What is not coming? Buoyancy, aah. Yes, 5 minutes break, hey, Harjit Pran, there is a session till 7, it's a small topic, it's not difficult. It will end by 7, relax, it will end before 7. Nice, that's true. I respect that. Kids are playing outside and you are studying here. I know. But today is just, tomorrow is Sunday, you have a day off, you will come back from school. What is impulse? It is force into time. How much time does it take to get impulse? F into t is equal to change in momentum. I have taught you in force and laws of motion. See that momentum concept bites. Okay, half chapter is coming. Oh Ansh, read it. You like reading extra. I know. Don't lie. You like reading extra. Now, read it quietly. Come on. Thrust and pressure. Everybody, listen to me. Look, thrust and pressure, if I tell you, thrust is nothing. It is perpendicular force to the surface. In class 8th, there was a chapter called as force and pressure. You have studied this. Thrust is force. What type of force? Yes, special one. Force which is perpendicular to the surface, we have named it as thrust. Right? Very good. So, thrust is the force which is perpendicular to the surface. If thrust is force, unit will be Newton. Easy. What is pressure? It is thrust acting per unit area. Force upon area. Right? That is the pressure. So, what will be the unit? Sir, force upon area or thrust upon area means Newton upon meter square. It has a special name. What? Pascal. Sir Blaise Pascal was a scientist. On his name, we have given the unit Pascal. So, Newton per meter square's special name is Pascal. Force was good. What is thrust? Balraj, nothing. Thrust is force which is acting perpendicular to the surface. That is thrust. Is that clear? Nitya, I have explained this. In this session, I have explained multiple times. Please watch it. Otherwise, watch the concept by Chota Sahib. Liquid pressure. See, this also you have studied in class 8th. That's why I'm telling you. It's easy. Liquid pressure, if you think about it, inside a liquid, there is pressure. If you go to the swimming pool and you deep dive, you will realize there is pressure on you. Not to perform, but water pressure. Okay, so the pressure in liquid it acts in all directions. Sir, how do you know? If you take a bottle and you choose one level and you punch hole, cut cut cut cut cut cut cut cut cut cut, you will realize from that level the water will come out equally in all directions. Means pressure is same, right? So, pressure at a point in a liquid is equal in, is acting in all directions. If you go deeper into the liquid, the deeper you go, the pressure will increase. It will vary with density. If the liquid is denser, then this pressure will be more. Right? Correct? Working is divided by... No, pressure works the same. Basketball also falls down. That's why. Yes, sir Simran. I have studied this well. So everybody, and I had taught you this extra. I gave you this formula in class 8 only. This is coming in front of you now. Pressure at a depth h is rho gh. Rho means density. g is acceleration due to gravity. h is the depth. Depth means? Depth means? Sir? How much is below from above? Right? This is the H. Right? So the only reason I can think of that why this chapter, this flotation is in your gravitation, do you know? Because of this G. This G because of which this chapter came to you. Otherwise, it might not have come. But now, because of this, don't hate G. Don't hate G because of this. Right? It's a small topic. It's easy. Come on. Is this clear? Pressure inside the liquid depends on what? Sir, density. depth and acceleration of gravity. That's it. Nothing else. Neither the shape of the vessel, nor the size of the vessel, nor anything else. Yes? Yes, Tanush, you have done it. Don't worry. Yes. Is it a different chapter? No. Oh, it's a different chapter. I see. Now tell me one thing. How many of you do swimming? How many people like swimming, or have learnt it, or have tried it? Because I don't know swimming, but I have tried. Then I gave up. I gave up. It's true. I have no hesitation. I used to have a headache, so I gave up. Do you swim? Do you like swimming? You have realized, you might have realized, you might have realized that when you are inside the pool, if you jump, you can jump more, you feel lighter while swimming, right? Very good. So why do you feel lighter? Why do you feel lighter? Because in water, There is a force which is acting upwards. Yes, buoyancy, Aditya is here. What is upthasht? Upthast, you asked and here is upthast. Upthast is nothing, see. If not swimming, let me tell you something. When you go for a bath, how many people have tried this, don't lie to me, nobody tells me a lie over here. You might have tried this. You have a bucket full of water and you have your mug. You will take the mug, you will keep it upside down and you will push it inside the... How many people have tried this? You must have done this for sure. Because I used to do it. When we were sent to take a bath, it used to take us half an hour instead of 15 minutes. Because we used to have fun. We used to do this, do this, sometimes we used to put our face in the water. So all these things I used to do. Yes, right? You have done it, right? Yes, obviously, very good. So, upthust, what happens? It is an upward force which is experienced by an object immersed in liquid. It means when you push something in a liquid, you feel resistance, the liquid opposes you. The liquid applies a force in the upward direction. Right? Correct? Bubble, yes you are right. Right? So, when you immerse a liquid inside, when you immerse a solid, a body inside the liquid, there is an upward force acting on the solid because of the liquid. This upward force is called upthrust. Or buoyant force. So, upthrust and buoyant force are the same thing. Boancy! is also the same thing? Kya same hota hai? Is upthrust, buoyant force and buoyancy same? Yes or no? Yes, you're right. Tell me, is upthrust, buoyant force and buoyancy same? Yes, no. Okay, Karma, there is no pressure left. It is thrust upon area. The pressure in liquid remains the same. Pressure depends on depth, value of acceleration of gravity and edge. Oh, sorry, g, g. We still do it? Very good. So, buoyancy is the property. Buoyancy is a property of the liquid which applies a force on the body immersed in it. So, buoyancy is a property. But the name of force is upthrust or buoyant force. Okay? Don't get confused. Now, see. Uphurst or buoyant force. So, if we say at the same level, and this, if you remember concert bites, I have done this derivation in the concert bites. At the same level, what is there? The pressure remains the same. So, this means you can say that the pressure is same. If we do the same area, then force, force, force, it will get cancelled. But the difference is, pressure on the topmost point and pressure on bottom most point. Pressure on the bottom surface and pressure on the upper surface, will it be same? Yes or no? Tell me. Pressure on the upward surface of that block and pressure on the downward surface, will it be same? Yes or no? Yes? No. Why? Sir, depth is different. This surface is on H1. The one below is on H2. Means pressure will be different. Right? So can you say that difference in pressure would be P bottom minus P top? If you multiply it by area. Now do you know what is thrust? What is upthrust? Sir, force in the upward direction. Why? Why is it upthrust? Because force on the bottom surface was more than the force on top surface. Sir, why? Because pressure on top surface was less than the pressure on bottom surface. So, I can say F is equal to, sorry, F upon A is equal to P. So, F is equal to P into area. Right? Now area of top surface and bottom surface is same. Both are square only. Right? So I say P bottom minus P top into A is the force of upthrust. So what is the pressure? rho gh. Right? rho gh2-h1 into A. rho gh2-h1 into A. Look carefully. P bottom minus P top into area. So what is P bottom? Sir, rho gh. So rho gh2 minus Rho g h1 multiplied by area. Okay? Right? If you take rho g out. Rho g h2 minus h1 into a. If you do h2 minus h1 into a, what will you get? This. Right? You will get this. Tell me one thing. What is the volume of a cube? What is the volume of a cube? Yes. Rho, lighten up kiddo, Rho is a Greek alphabet, it denotes density. A cube, very good. S cube, very good. Do you know how S cube came? This is your cube, right? This is A. This is A. and this is A. Area, how much is it? Sir, A square. How much is the height? A. What will be the volume? Sir, A square into A. That is why the volume is Aq. Multiply all the sides, that is the volume. So in this case, area is this. Right? Area is this. If you multiply this area with this height, Will you get the volume? So if you think about it, h2-h1 into a is what? Volume of this body. So, rho g into volume of this body is the force of upthrust. It is upthrust. Clear? Yes? Yes? Study vlogger? Yes, brother, absolutely. We are there. You speak further. Do you understand? Yes? I don't understand. Karma, listen to me. Look, if a body is in the liquid, force on the top surface will be more, sorry, it will be less than force on bottom. Why? Because pressure on top is less than pressure on bottom. Why? Sir, bottom surface is at depth, upper surface is not in that much depth. Right? So that's why. Force of bottom minus force of top is the upthrust. You just calculate using pressure. Simple, right? Yes? Yes, for every object. So, rho g into v is the force of upthirst. Right? So, if you think about it, do you know what is upthirst? Finally, what did you get? rho, that is density of liquid, multiplied by g, multiplied by volume of body. This is the force of upthirst or force of buoyancy. Is this clear? Yes or no? Tell me quickly. Shwarya, there is no atmospheric pressure. It is the pressure because of atmosphere. Because of the weight of the atmosphere on you. Right? Isn't it? Tanush, you will see me. Why do you have a life jacket? Very good. Understood? Now see, what was buoyancy? Property. So, an upward force is experienced by an object immersed in liquid. Right? If there is a body in liquid, It will be affected by upward force. That upward force is called as upthirst. So now look carefully, if a body is floating at the surface, how many forces are acting on this body? How much force is acting on this body? One is the force of gravity which is pulling it down. Second is the force of buoyancy which is pushing upwards. Clear? Yes? Correct? Yes. Gulal, I saw your message. Don't worry. We are figuring out or we are giving you all the notes consolidated. Very good. Why do we say buoyancy? Oh, Shruti, it's a name. Why is my name Saurabh? Tell me. Did you want to? Did you keep it? Right? Yes. Yes, you're right, Shaurya. The atmospheric pressure remains the same, so we forget it. We don't act on it. Yes? Yes. Alright. Now listen, why don't they cancel? If, tell me, if this force and this force, they are competing with each other. If the body floats on the surface, that means Means, this force is less and this force is more. If this force is more, then the body will float. Yes? Correct? If any body floats, means gravity pulls down, buoyant force pushes up, buoyant force is pushing up, gravity is pulling down. If the body is floating on the in the liquid. Means, gravity is weak, buoyant force is heavy, buoyant force is strong. Upward force is strong, downward force, that is gravity is less. And that is when the body will float. Okay? We can say downward force is less than buoyant force. Fine. If both the forces are equal, means the weight of the body is equal to the buoyant force, then what will happen? You take the body. If you keep it in water or in liquid, it will stay where you leave it. If you take this body, you keep it over here, it will stay over here. If you keep this body over here and stay it, it will stay here. This will happen that it will stay where you leave it. If the weight of the body is more than the buoyant force, then what will happen? The gravity force is stronger and buoyant force is weak. Then what will happen? Body will sink. It will sink. Correct? Very good. Because density is more. It depends on density. Simple thing. When will a body sink if weight is more than the point force? Right? So, based on this particular thing, a principle is known as Archimedes' principle. Now, tell me one thing, truthfully, do you remember I told you that the force of upthrust is equal to density multiplied by gravity, acceleration of gravity multiplied by the volume of body. Right? Here it is. We derived it here. We have derived this. Does everyone remember this? Tell me quickly. If this is clear, then the Archimedes Principle will become Halwa. Tell me quickly. Yes. Yes, this is the last topic. Come on, tell me. Is that particular thing clear? It's very easy, right? Yes. Now listen. Archimedes Principle says that, when A body is immersed fully or partially in a fluid. It experiences an upward force that is equal to the weight of the fluid displaced by it. Tell me one thing. And tell me the truth. If you have this body which is irregular in shape, you know how to find the volume of the body, for which you know the answer of cuboid, cube, sphere. If I give you a random stone, how can you find the volume of the stone? Anybody. This is not a trick question, it's a real question. If I give you a random stone and I ask you what is the volume of this stone, how will you tell that? Universal clear, very good. Tell me the story, you'll enjoy it. Yes, tell me. Yes. Put it in water and its length. Length, okay. Yes. Almost, Prisha, almost is correct. Just a little bit less. Argyvandita Prince's... Very good, Tanish. Good. Look, if you take a vessel... Yes, very good, Arishwarya. Nice. If you take a vessel which has water filled to the top, Right? Have you heard that story? Have you heard that story? Hungry crow. There was a crow, sorry, thirsty crow. There was a crow which was thirsty. He was thirsty. But the water was very low. What did the crow do? The crow started putting pebbles in the water. What happened? The water level started to rise. And when the level reached over here, the crow was able to drink the water. Right? Yes? Correct? Right? Hungry, thirsty crow. Okay? Which means, if you have water filled till the top, if you have water filled till the top, and when you put a stone in the water, water will come out, water will be displaced. Jitna volume of water will be displaced, that is the volume of the stone. Clear hui baat? Right? So, if a body is fully submerged, the amount of fluid displaced is the volume of the body. Yes? The crow was clever. Okay, clear? So, if I say, if a body is fully submerged, can I say that volume of body is equal to volume of liquid? Tell me. If a body is fully submerged, can I say the volume of body is equal to the volume of liquid displaced? Yes or no? Yes or no? Come on. Yes. No? Where is it? Yes, right? Piyush, I'll tell you, kid. Now, what was the formula derived for Thurston? Sir, we derived density of liquid multiplied by the value of g multiplied by the volume of body. If volume of body is equal to volume of liquid displaced, it means, here it is, this is your formula. Density of liquid multiplied by the volume of liquid displaced into g is equal to the buoyant force. It is easy. We have derived it properly. Yes. Right? Yes. Otherwise, that I have to do. Don't worry. Correct? Right? So that's why the buoyant force is depending on Sir, the density of liquid. How much volume of liquid is displaced multiplied by the value of G. Even after this, if anyone has a doubt, watch the concept bite. In that, I have given you one level deeper. I have given you one more level deeper. Right? Right? Easy. Story? Which story? I'm forgetting. Let's do the question first. Question, question, question, question. A toy submarine of volume 120 cm3 is immersed into an aquarium as shown. Calculate the buoyant force acting on the submarine and take the density of water as 1 gm per cm3. Yeah, you remember the density. Density of water is 1 gm per cm3 or 1 kg per m3. Yes? Yes, numericals, Gaurav, here it is. Time table depends on me. I go to play at the weekend. After that I study a little. Rest of the weekdays we come at office at 10. After that we do preparation. What to teach, how to teach, when to teach, what to do, we do all this. Yes. Eureka means I found it. I enjoyed it. Yes. Very good, 1.2 Newton, Prisha is saying. Shaurya is saying B, 1.2 Newton. Anybody else? Padhai? Padhai yaar, jo jitne bhi Chandrayaan kar rahe hain. I am not working in Israel na, padhna padhta hain mirko. Tanush is saying B. Set 2? Aa, set 2 aal kar vahunga tum, don't worry. Priya is saying B. Alright, dekho, everybody listen to me. Twice submarine of volume itna. is immersed into an aquarium as shown. If you look at this, submarine is fully submerged. Right? Means, volume of submarine is equal to volume of water displaced. Right? Correct? Very good. Calculate buoyant force acting. Buoyant force will be what? Rho L into volume of liquid displaced into G. If the body is fully submerged, volume of liquid displaced is equal to volume of body. Right? So, Rho L will be 1 gram per centimeter cube multiplied by volume of liquid displaced is equal to volume of body. 120 centimeter cube into value of G. How much is G? What is the value of G? How much is it? Assume 10. If I tell you 10. So, how much is it? Is it cut? I am getting, sir, 1. 1200, right, 10 Newton per meter square. So, I am getting what, sir? 1200, right, correct. This is 1 gram per centimeter cube. Right? So how much did we get? Think about it. I got 120 grams multiplied by 10 Newton per meter square. Dude, this thing is in grams. This is in grams. This is in Newton. There is one in CGS, there is one in SI. Can you multiply? No. So you have to convert gram into kg first. So, upon 1000. So what do you get? 1.2 Newton. Right? Value of G. Yes? Haa, very good. So that's why, correct answer is 1.2 Newton. Clear hai? Right? Yes, Shaurya, you know, the timetable is on the sessions and channels. Right? So, everybody listen to me. To just simply tell you which body will float and which body will sink, we have a very simple funda. First, you check the density of body compared with the density of the liquid. If the body's density is less than the liquid, if the density of body is less than the density of liquid, it will float. If density of body is equal to density of liquid, it is a suspended body. Means, you keep it here, it will stay here. You keep it here, it will stay here inside the liquid. If the density of body is more than the density of liquid, it will sink. Right? Thank you, thank you Aishwarya. Purvi, today it has been recorded, it will take a week. Very good. Why does a ship float? Because it displaces a lot of water. What's the difference between a ship and a nail? We empty the ship from inside. That's it. Yeah, I took GS10. Yes, you're right. Practice. 1.25 B.E.C. Fine? If the density of the body is less, it means it will float. If it is equal, it means it will submerge. If it is more, it will sink. Right? And, there is one more concept called as Relative Density. Relative is nothing. RD that is relative density is same. The only difference is that in this you have one single word or one single quantity, one single numerical value which will tell you that the body will sink or float. Why? Do you know what RD means? It is the ratio of density of liquid to the density of water. Now take oil and water. Oil floats, right? So density of the liquid upon density of water is what we call as relative density of liquids. Right? So for example mass of 100 cm3 of water is equal to 100 grams. There is one more Kerosene. Mass of 100 cc is equal to 80 grams. Means, what is the density of this? Everybody! Density of kerosene will be Sir 80 upon 100. Density of water will be Sir 100 grams upon 100 cc. Related density of kerosene will be how much? Sir density of kerosene upon density of water is equal to 80 by 100 upon 100 by 100. If you solve it, you will get 0.8. 0.8 aagaya? Just looking at this value of 0.8, you can say, Yaar, yeh toh lighter hai, yeh toh float karega. Kerosene oil will float in water. Why? Because Rd is less than 1. Right? If you have mercury, mercury for a 100 cubic centimeters of volume has a mass of 1360 grams. So, its Rd, that is, relative density, how much does it come? 13.6. More than 1. Means, it will sink. Easy. Right? Yes. Correct. Absolutely. There is a timetable, which comes on the community post and on telegram also. Right? So with this, relative density, I want to tell you that gravitation for class 9th is only this much. Today we have done few questions. But a lot of theory, but that was important. So anybody who has any problem in gravitation, watch today's session very sincerely. Apart from this, I have given you special concept bites. If you have any particular problem in any topic, watch those small small sessions. Your doubts will be cleared. You have everything. Rest, now it's your job. Right? Our job is to show the way. To reach the destination is your job. It's a dialogue from Oh My God, I've just copied it. So, it's your job to study. I could only teach you. I have less work to do. My job is to make sure that you practice more questions. We will do this, but before that, read it and come. Otherwise, you won't enjoy it and I won't enjoy it. So, go ahead. Read this chapter properly. Understand this chapter properly. Whatever doubts you have, tell me in the comment section. As far as notes are concerned, so, for notes, a proper sequence has been made. And yes, I know, you might not be getting... You will get full chapter notes, not session wise. You will get notes of this on Telegram. Relax completely. Right? Yes. 2-3 questions. Are you sure? No. One homework for you. If iron, if the density of iron is more than water, why do ships float? You answer that. Answer this and the rest is fine. This is the homework question for you. Fine? Alright, Tishen, it will be done by 7th, very soon. So everybody, with this, the Gravitation chapter is done. You studied properly because we are going to do good good questions in the exam itself. Next session will be on question solving. Before that, if you study, it will be very nice. You will feel very empowered. Otherwise, you will face problems. Study well. Watch the concert bite. Let's meet in the next one. Let's do the questions. It will be fun. All the best. See you next time. Bye-bye. Thank you.