hey all hope everybody are doing fine and welcome back to class 11 physics series I am with the chapter motion in straight line the most requested chapter and I already completed two videos on it video links are provided in the description you can check it out if you have not it watched and today I am with the most important topic see In today's video, I am going to cover something about deceleration or retardation. It's very important to understand the difference between acceleration and deceleration. Many of them think that deceleration is a negative acceleration. But no. deceleration is not a negative acceleration. So let's understand what is the major difference between deceleration and acceleration.
After that, I will be talking about stopping distance, which is the most important topic and also reaction time. And later, I will come up with One more important topic that is relative velocity in one dimension. So this is the topic students get confused. So wait for the beautiful trick and wait for the easiest way of solving a problem on relative velocity. So watch the video till the end.
Drefinitely you will have maximum benefit from the video. Especially the topic of relative velocity. Clear? Okay.
So let me start with the first topic that is deceleration. So what I meant to tell you is acceleration and deceleration. Both are two different words.
We know what do you mean by acceleration, right? And deceleration, we actually know what do we mean by deceleration also. And most of them think that deceleration is a negative acceleration, right? So, this is how you will think about deceleration. But no, this is not a negative acceleration, okay?
See you. In a simple terms, I can say that acceleration is something related to speeding up. Okay, speeding up.
Say for example, you take a vehicle. Yes, this is some car. Say it started its journey with for example, 50 kilometers per hour. Okay, so after some time the vehicle reaches here and if its speed is 55 km per hour.
So it travels certain distance and at the end if you see its speed, I mean at some point, if you observe speed of around 70 km per hour. So you know what, what you can observe here? say here initially it was 50 now it became 55 and then it became 70. so here speeding up of a car you would observe right and this is what we mean by acceleration okay so acceleration is something related to speeding up whereas deceleration is something related to slowing slowing of object okay Say for example speed was 50 and now it has become 45 and here it has become around 30. So what happened here? The speed of the object or car has become slow. right so this is slowing down slowing of object is what we mean by deceleration so you might be wondering yes we can also call it as negative acceleration but no just observe this situation where i will give you some examples to prove that acceleration and deceleration are different To prove that deceleration is not a negative acceleration, right?
So, just observe this. So, this is you know positive x-axis, negative x-axis. Okay, let it be say four situations I will show for you and this direction I will consider as positive direction. Okay, so when an object is moving in this direction, I consider that direction as positive. We all know this.
We always consider. positive x axis as positive direction right okay imagine you have a car here right and this car has a velocity 10 meter per second initially and It has travelled certain distance and you know what it reached this point and at this point the car's speed was 20 meter per second. Okay, say what you can observe here?
Here you know what speeding up of car, speeding up is what you will observe. Can you tell me what is the sign for acceleration here? Drefinitely we say acceleration is positive, right? So this is positive acceleration. This is positive acceleration.
Okay. So what do we say in this situation? The car is accelerating. Okay.
So car is speeding up. Right. So this is what one situation I have put in front of you.
I am coming to the second situation. Okay. So here a car with initial velocity 10 meter per second.
Okay. And you know what its final velocity if you the moment it reached this point. point here at this point the velocity is 5 meter per second. So here what you can observe here it started with the speed 10 meter per second but it is at this point the speed is 5 meter per second that means object has become slow. So what you observe here is slowing down you have observed right.
So here the direction right or the sign for a is negative you know what this is what we call deceleration this is what we call deceleration it is slowing down right so a is negative it is deceleration but observe this situation. Okay. See, I considered this as positive direction, right? Say now, imagine the car here with 10 meter per second and it is moving and here the final velocity of a car is 20 meter per second.
Okay. So, the car was moving like this. Okay. So, now what is the sign of Y?
Okay, see this direction you took it as a positive direction, right? But the car is moving in this direction, but the car is accelerating. What you have observed here?
Speeding up you have observed here. The car is accelerating. It is accelerating, right?
But what is the sign of A? Sign of A is negative. Sign of A is negative. Can you call it as deceleration? No, because the car is speeding up.
The car is speeding up. Only the sign is negative. So this negative or positive is always with respect to the direction that you consider.
Say I consider this direction as positive, but now the car is is moving in this direction whereas it is speeding up. Okay. But the acceleration is negative.
Right. So, since A is negative, can I call it as retardation or deceleration? Never. Right. So, this is where you have to be careful.
Say here also A is negative but this is deceleration. Why I called it as deceleration is because you see, you know what? the slowing of object took place right so whenever you observe that slowing of object if your final velocity is lesser than initial velocity then it is deceleration and a you will get negative okay and and you see one more situation here so this is your car with 10 meter per second back The final velocity of a car here is 5 meter per second. Okay.
So 10 meters has become 5 meter per second. So what you have observed here, here also your object was slowing down. Okay.
But what is this? This is, you know what? See, this direction is positive. Now, this direction, you know, slowing of object took place.
So, here actually A is positive and this A is actually deceleration. This is actually deceleration. It's not that deceleration is always negative. But no, you see in this situation, you got your deceleration positive, isn't it? So, you have to mainly focus on what was initial speed and final speed.
If the spinal... final speed is lesser than the initial speed, then it is deceleration. So, or in other words, okay then then it is called as deceleration okay say you will call it as deceleration or retardation when when the direction of velocity and acceleration is opposite okay you see say the direction of velocity is like this but the direction of acceleration is like this so when you have their directions opposite then it is deceleration right so this is one important point that you have to keep in your mind see uh always have a conception that always have an idea that you know deceleration is not a negative acceleration okay so you see here deceleration can be negative deceleration can be positive it all depends on what direction you have considered as positive okay so i considered this direction as positive so that's the reason actually it's a deceleration but according to my direction i get my a is equal to positive right And you see in this case, actually this is a speeding up of car. It's acceleration. Initial velocity 10 meter per second, final velocity 20 meter per second.
Actually it is speeding up, but a value is negative. It's because this direction I considered as positive, right? Say whenever you see a negative acceleration, it is not deceleration. You just have to decide that based on initial and final velocity, you just observe whether you are observing speeding up of an object or slowing up of an object.
So always object if it is speeding up it is acceleration if it is slowing down it is deceleration. So this is what you have to keep in your mind. Okay so now let us go for stopping distance and reaction time.
See guys stopping distance is something that you can see it in real life situations. So imagine you are driving a car, right? So you are sitting here and you are driving a car and this is your route where you have to drive and you are driving with certain initial velocity u meter second okay and suddenly all of a sudden there is a person on a road who is trying to cross and and this person has not seen the car coming so he is haribari just uh crossing the road and he is on a road right and looking at this man what you think is you will think to put the brake right say definitely you will put the brakes And imagine when you try to put the brake here, I mean at this spot, will the car get stopped immediately?
No, right? See, the moment you put a brake here at this point, car would travel to certain distance and then it will stop, right? The moment you put a brake, car will not stop immediately. It will travel certain distance and then it stops.
How much distance will it travel always depends on with what speed you are driving a car. So if you are driving with high speed then the distance travelled by a car before stopping will be more. In case if your velocity is very less.
then the distance traveled by a car before stopping will become less. So, it always depends on the initial velocity. Okay. So, anyway, and this distance.
So, before stopping the car has traveled certain distance. So, that distance we will call it as stopping distance. Okay. So, it's very simple stopping distance. So we can represent stopping distance as ds.
Okay, so what is the definition you will give when brakes are applied? So I'm just giving you the definition of stopping distance here. When brakes are applied, the distance travelled by a car. travel by a car before stopping before stopping is called stopping distance okay so always there's a little distance traveled by a car before stopping whenever sudden brakes are applied.
So that is called stopping distance. Okay. And this stopping distance is very much necessary to set speed limits. Speed limits.
Okay. Near schools, hospitals, etc. right so it has a very very important application guys okay fine so this is the theory of stopping distance and coming to its formula say it has three different formulas okay depending on the situation you can select a suitable formula to calculate a numerical okay see here so we all know the formula v square minus u square is equal to 2as right fine and here v square final velocity i mean final velocity is the velocity of a car when it is stop say final velocity is zero right initial velocity let it be some u meter per second so final velocity is zero minus u square is equal to 2 as let this s be let this s be stopping distance ds okay so let S is dS. Okay. So, what do you get?
So, your dS is equal to minus u square by 2a. Right. So, this is one formula. And can you tell me what is a is the sign here?
See. The car with a certain speed, it is getting stopped with zero speed, right? Slowing up of car took place, right? So, it is actually a deceleration.
It is actually a deceleration, isn't it? So, here A is negative. So, this A is actually a deceleration.
It is not a acceleration. It is a deceleration, right? So, yes.
So, this is what you have to keep in mind. A value is a negative value. So, obviously at the end, ds value when you calculate, you will get answer in positive answer.
Okay. So, is that clear? Minus u.
Right. So, divided by 2a. A value, you know, acceleration is negative here because it's a deceleration. Right. So, ds value, you will get some positive answer itself.
okay so and so meters you will get when you apply this formula right and we have another formula say i am giving you formula number two this is formula number one and formula number two ds is equal to v square or u square this is initial velocity itself divided by 2 nu g. So, what is v here? v is velocity, velocity of an object or car.
Okay, which velocity? It is an initial velocity only. If you are getting confused, let me write it as u square okay and g is what acceleration due to gravity that is 10 meter per second square or you know actual value is 9.8 meter per second square and so you can take it most of the times for the calculation okay and what is new this is friction coefficient friction coefficient Okay, so actually stopping distance depends on friction coefficient also.
What is friction coefficient? It is the friction between the tires of car and road. Okay, that is friction coefficient. Fine.
And this is the second formula. And the third formula that you have is stopping distance is equal to Ku square. Whereas this K is nothing but proportionality constant.
proportionality constant okay so in a question if they give proportionality constant this is the formula you have to select so in the question if they mention something about uh you know friction coefficient then this formula you have to use then if they have given uh directly some deceleration and initial velocity then this formula you can use right so depending on the content given in a question you have to use a suitable formula here right and one more important point that i would like to tell you and you see here our stopping distance is directly proportional to u square that is initial velocity square right so based on this i can always say that if i increase the initial velocity by 2 times, our stopping distance increases by a factor of 4, right? So, if initial velocity is doubled, this point is very important. Stopping distance ds increases this. increases by a factor of 4. By a factor of 4. Yes or no?
Because you see the relationship. If I double this, ds value will get increased by a factor of 4. 4 times ds value will get increased. If I increase by 2 times, this will get increased by 4 times. Okay? So, the more the initial velocity, obviously.
more is the distance that a car travels before stopping so if i double this this will increases this will increase by four times so that's all about you know the stopping distance and now let us see few questions on this stopping distance look at the first question here a bike moves with a velocity of 15 meter per second and applies brake suddenly brake is applied calculates Calculate its stopping distance if constant of proportionality is 0.6. So constant of proportionality is given u value. So u is equal to 15 meter per second. Constant of proportionality k is equal to 0.6.
So which is the formula you will select to calculate stopping distance? It is ds is equal to k into u square. right so k value is 0.6 u square is 15 square right so 0.6 into 15 square 225 you have to do so you will get your answer 202 meters isn't it right so this is how you can apply this formula for this question it's a very direct question right fine and coming to second question you Anu drives a car at 50 kilometers per hour. So initial velocity is 50 kilometers per hour. She puts a brake when she sees a stop sign.
The coefficient of friction between tires and road is 0.60. Okay, so coefficient of friction, right? So that is nu is equal to 0.60. Yep. What is the stopping distance?
So friction coefficient is given, initial velocity is given. So what is the formula that you will use? Stopping distance is equal to u square divided by 2 g nu.
Right. So u square is how much? 50 kilometers per hour. Convert this into meter per second.
So how do you convert this? 50 into 1000 divided by 1 hour is how many seconds? 3600 seconds.
So you would get the answer here almost around 38 point meter per second. Just check whether this is correct or not. So that one you have to take it in the numerator.
It's square you do. So what is the g value? So, g value is equal to 10 meter per second you take. Okay.
And new value is given that is 0.60. So, on substituting every value, new is this, g is this and new is, you know, this one. So, this is u.
Right. Okay. Dron't get confused.
So, that's the reason formula. In the formula, I will make a change. Instead of U, I will consider it as V. So just because you know U and U both look similar, right?
So this way I just have put V here. So what is V? V is this. Okay.
So you calculate this yourself and see how much is the answer you will get. So this is how some direct questions that can be asked on stopping distance. So remember three important formula here.
Okay. and now let us discuss a very small concept that is reaction time okay guys coming to reaction time uh you know what just try to observe this situation uh like the situation i told you for stopping distance so you are driving a car right with certain uh velocity and car say you you will see a person here right and you know what Reaction time is the time taken for the person sitting inside a car to observe, to think and to act. The moment you are traveling on this road, the moment you see this person, you will observe, you will take some time to observe this person and you will take some time to put the brake. So that time they will call it as reaction time.
Okay. So how can I define this? The time in which person observes things. and acts and acts is called reaction time okay see first you will observe that there is someone when you are traveling on a road after observing you will think to put the break so that time that small fraction of time it's a very small fraction of a time right say you know That particular time we are calling it as reaction time.
Okay. And now let us try to derive a formula to calculate reaction time, which is indicated as TR. Okay.
Reaction time. So imagine you have a long scale. Okay.
So you are holding a scale and your friend have to catch that scale. Say for example, this is the long scale. So I am leaving this.
okay and its initial velocity is zero right its initial velocity is zero meter per second so just observe its initial velocity zero meter per second because it's a trust i am holding this and this is uh the hand of your friend okay so i i will leave this right but your friend must act in a proper time to catch this okay so i leave this and your friend catched So that time, that time taken for your frame to catch is called reaction time. Say in this time, the object has moved certain distance. That distance, let it be d.
Okay and let us calculate that reaction time and we all know the formula s is equal to ut plus half at square. Okay so let this t be the reaction time. Okay so let t is reaction time. So what is d? You know that the time in which you know the object has moved.
In the reaction time, where the object has moved is called Dr. Clear? So that is reaction distance. What do we call this as? Reaction distance. So in the reaction time, the object must have moved to a certain distance.
So that is reaction distance. So this S is a reaction distance. Okay?
So let me call this as reaction distance DrR. Okay? So let S be DrR. okay.
So, dr is equal to initial velocity is 0, 0 into 2, right, plus half a, a is nothing but g here in case of like the acceleration is acceleration due to gravity. So, a is nothing but g, okay. So, it is g t square, right.
So, here dr is equal to half g t square. This t is nothing but reaction time. So what is the formula I can get here?
2 dr by g is equal to tr square reaction time. So tr is equal to square root of 2 dr divided by g. okay so this is what the formula which is used to calculate reaction time okay so what is this dr it is in the reaction time the object must have moved to a certain distance no that is dr and what is this g g is acceleration due to gravity so when an object is falling from top to bottom somebody is catching it like that's how you will calculate the reaction time okay so the distance that has moved in that particular time is what your dr and g is acceleration due to gravity. Okay.
So, all g can be acceleration. If you apply to a car that is moving on a road and this g is what your acceleration. Okay. So, that is how you can use reaction time and this is the major formula to calculate that.
Okay. Fine. So now we will go for very very important concept that is relative velocity.
It's very important to understand the concept of relative velocity. I am dealing with relative velocity in one dimension okay and my approach of solving question on relative velocity is very very different. You feel it much easier so don't miss it out try to watch the complete video. See guys, relative velocity is very simple. It is just you have to understand the concept.
That's it. See, actually, relative velocity is nothing but velocity of an object. Velocity of an object with respect to another object.
is called relative velocity. Okay, say for example, there is a walking track and there are two people walking. Okay, so this is person A and this is person B.
Okay, so they are walking with some velocity. So let the velocity of this person A person B V A and velocity of person B let it be V B. Okay, so with some velocity V A, A person is walking with some velocity V B, B person is walking. Okay, say and there is an observer standing on a ground.
So, who is he? He is an observer. Okay, and this observer tries to compare the velocities of A and B and he is the one who can able to judge whether A is walking faster or B is walking faster. Okay, so it is who who decides the you know the speed of these two persons.
isn't it say it is the observer who is talking about who is comparing the velocity of a and b Okay, so this is what we mean by relative velocity. Okay, so it is some third person, okay, who is observing, who is trying to compare the velocity of A and B. Okay, so he can judge about the velocity of A in comparison with B. And it is this person who can judge about the velocity of B in comparison with A. okay see these persons cannot compare their velocity with each other it is the third person why they cannot compare their velocities with each other because both are moving because both are moving right so it is the third person who is standing on the ground and observing he is the one who can talk about velocity of this person in comparison with this person and he is the one who can talk about velocity of this person in comparison with this.
So that is what we mean by relative velocity. So relative velocity is always with respect to the third person who acts as a observer. observers whose velocity is 0 meter per second. Okay, so the ground observer's velocity is always 0 meter per second.
He is not moving. If he is also moving, well, where is the matter of comparison, right? If he is also moving along with them, you know, there is no concept of relative velocity. Right. So here the third person, the third person's velocity is 0 meter per second.
And he is the one who acting as observer relate and he is the one who is comparing the velocities of these guys. Okay. So that is what the concept of relative velocity. Okay. So here actually, if you want to find out relative velocity of relative relative velocity of A with respect to B.
Okay. So, relative velocity of A with respect to B, if you want to find out, I can write always like this relative velocity of A. A means what? If I write A first, what does this mean?
You are finding the relative velocity of this guy, A guy, A guy with respect to in comparison with B. Okay. It's always found by Velocity of A minus velocity of B.
So if you want to find, if you want to compare the velocity of A in comparison with respect to B. So who is comparing? Third person.
Ground observer is comparing. So if this guy want to compare this person's velocity with respect to B, then you know. it's the formula is V minus V. Let me write relative velocity as RV.
Okay, fine. So, if I have to find relative velocity of B with respect to A. Okay, so whenever I write relative velocity of BA means what?
I am trying to find out the velocity of this guy in comparison with this. Then the formula can be, you know, velocity of B minus velocity. of A.
So here understanding the meanings of these terms is very important. Whenever I write relative velocity of A means what? We are trying to find the velocity of A guy in comparison with B. Who is trying to find out? Ground observer.
In the perspective of ground observer relative velocity of A with respect to B means you can always do velocity of A minus velocity of B. In the perspective of ground observer whose velocity is 0 meter per second, if you want to find out velocity of B in comparison to A, then you can find with the formula velocity of B that is V minus V. So this is the major formulas that you are applying to solve the problems on relative velocity. So now I will be solving three problems where you can understand the entire concept and you will get an idea how to solve a question easily.
So just understanding is very very important here. Okay, so let's go for the problems now. So remember guys, this is really very very important to understand.
So in whose perspective we are calculating relative velocity? In the perspective of the third person, that's a ground observer. Okay, fine. So let's go for problems now.
Look at the problem carefully. Raju sees Geeta running towards him with a speed of 7 meter per second. Okay. So there is a person Raju.
So he is Raju. Okay. So this Raju sees Geeta.
Okay. So she is Geeta. Right.
So Geeta. And this Geeta is running towards Raju. Okay.
So Geeta is running towards Raju with a speed of 7 meter per second. Okay, 7 meter per second. And a dog chasing her with a speed of. Okay, there is a dog.
There is a dog and this dog is chasing whom? It is chasing Geeta with a speed of 4 meter per second. Okay.
What is dog's speed relative to Gita? So they are asking relative velocity of dog with respect to Gita. right?
So, you are getting my point and Raju is a observer. Raju is not moving, right? So, he is a third person.
Always relative velocity is who will compare? Observer will compare, okay? So, you are calculating the velocity of dog with respect to Gita. Okay, so who is observing all this?
So this relative velocities are always spoken with respect to the observer that is Raju. He is just standing, he is not moving anywhere, he is standing and observing. He is the one who is actually has to calculate relative velocity of dog with respect to Gita. So what is the formula?
Relative velocity of dog with respect to Gita. That means velocity of dog minus velocity of Gita you have to do. Okay. So, what is the velocity of a dog?
Velocity of a dog is 4 meter. You know, consider this direction as positive. Any direction you can consider as positive, right?
So, I am considering this direction. Okay. Imagine this direction. The direction towards Raju.
is positive. Okay. So, you know, considering this direction as positive, you know, the dog's velocity is how much? It is 4 meters.
right? So, relative velocity I am calculating 4 meter per second minus velocity of Gita is how much? 7 meter per second, 7 meter per second.
So, what is the answer? Relative velocity of dog, relative velocity of dog with respect to Gita is 4 minus 7 that is minus 3 meter per second is the answer. Okay.
So, 3 meter per second is the answer. So, 3 meter per second is the answer. per second is the velocity of dog with respect to Gita. Okay. So, you see the sign it is negative.
That means the dog is not moving in this direction. The dog is moving in this direction. Okay. So, that means the dog must be chasing.
Okay, the dog must be chasing like this. Geeta is moving towards Raju but the dog is chasing Geeta in the opposite direction. Okay, so with what is the speed?
With 4 meter per second. Okay, so with this much speed, you know, actually the dog is chasing, you know, Geeta. Right?
And what is the difference between their velocity? It is this much. So this is the relative velocity of dark with respect to Gita. Negative sign indicates the direction of dark, you know, the way it is changing Gita. Okay, Geeta is moving towards Raju that I considered as positive.
So answer you got negative means dog is changing in a opposite direction. Okay, so dog is actually moving from Raju's side to Geeta to chase. Clear? So, you know, this is how you will have to think of the answer. Right?
So that's all about the problem. So it is very simple. Just what is given and what they asked you to calculate.
Velocity of Gita is given. Velocity of dog is given. Velocity of dog with respect to. What is dog's speed?
Drog's velocity relative to Gita. So like this you have to write relative velocity of dog. They asked you to calculate dog with respect to Gita. So Dr you have written first to knows velocity of b minus velocity of gita okay so this is how so the sign indicates the direction in which the dog is actually chasing gita okay fine so that's all about the problem let's go for the second problem see this question guys it is from your ncrt book itself two parallel rail tracks run From north to south.
Okay. Train A moves. See, this is our north direction and this is our south direction.
Right. Okay. Fine.
So, train A moves towards north. So, there is a A train which is moving towards north. Okay.
Fine. And with a speed of 54 kilometers per hour. So.
So, A's speed is how much? 54 kilometers per hour. And let me consider this direction as a positive direction.
Right? So, velocity of A is how much? Velocity of train A is 54 kilometer per hour. Okay?
So if you convert this into meter per second, how do you convert? 54 into 1000 divided by 3600, right? So you will get around 15 meter per second.
So V is equal to plus, plus 15 meter per second, right? so hope you understood i i wrote every information about strain a train a is moving towards north so i am considering going towards north as positive direction so train a's velocity is va it is 54 kilometers per hour i converted this into meter per second it is plus 50 meter per second okay fine and now there is another train Okay, so train B. Train B moves south with a speed of 90 kilometers.
Okay, and there is another train B and this is moving towards south guys. What is its velocity? given? Its velocity given was 90 kilometer per hour.
Okay. So now velocity of B. Okay.
I'm calculating V. Velocity of B train is how much? Minus 90 kilometer per hour. Right.
Because when I considered this direction as positive, when the train B is moving downwards, then its velocity is minus. right so when you convert this into meter per second how how much do you get uh so um you know minus 90 into 1000 divided by 3600 so when you do it velocity of b is equal to minus 25 meter per second i get okay fine so now we got velocity of b This much. It's minus 25 meter per second. Velocity of A this much. Okay.
See the question. First question. So what is our first question?
Find velocity of B with respect to A. So you have to find relative velocity of B with respect to A. So what is the formula? B A. So V, V minus V. So, what is V? Minus 25, right?
Minus V is how much? 15, 15, right? So, minus 25 minus 15 is how much guys? So, it is around 40, right?
Minus 40 meter per second. So, relative velocity of B with respect to A is minus 40 meter per second. Okay. So we have calculated our first question.
And coming to second question. Okay. So what is our second question? Velocity of ground with respect to B.
Okay, so you have to calculate relative velocity of ground with respect to B. So ground means what? Observer who is standing on a ground. He is what?
Comparing the velocities of these guys. Okay, so and I always told you. Observer's velocity is always 0 meter per second right. So when you have to calculate relative velocity of ground with respect to B then velocity of ground minus velocity of B.
So velocity of ground is 0 minus velocity of B is how much? It is minus 25 meter per second. So what is your answer?
It is 25 meter per second is the answer for Relative velocity of ground with respect to B. Okay. Now let's go for the third question.
So it's very important to understand the third question. You have to calculate velocity of monkey running on train A against its motion. Okay. See there is a monkey. There is a monkey.
okay, which is sitting on a train A and it is moving in opposite direction, they told. Its velocity you must calculate, okay, its velocity you must calculate. You see something is given in a question. They gave, see what they gave, they gave velocity of monkey okay with respect to a that means velocity of monkey with respect to a relative velocity of monkey with respect to a is given that is 18 kilometer per hour okay and and they said it's moving in opposite direction that means the monkey is moving to south direction when we considered this as positive then direction of monkey is opposite this x so it is minus right so they gave relative velocity of monkey with respect to a is this much so if you convert this into meter per second how much do you get 18 into 1000 divided by 3600 if you calculate you will get around minus 5 meter per second okay you So I converted this into meter per second. So relative velocity of monkey with respect to train A is given this much.
They asked you to calculate velocity of monkey. Right. So we can write the formula relative velocity. of monkey with respect to train is equal to velocity of monkey minus velocity of A.
This is given. This is how much? This is minus 5 is equal to velocity of monkey question mark.
We have to calculate. Minus velocity of train A is how much? It is 15. meter per second.
So, how much is V? So, how do you calculate guys? So, minus 5, this one if you bring it this side, it will be plus 15 is equal to velocity of monkey. So, 10 meter per second is the velocity of monkey, right?
See, they gave you velocity of monkey with respect to A. Relative velocity of monkey with respect to A they gave. That is this much. Okay. They asked you to calculate velocity of monkey.
Right. So we have calculated this way. So hope you got your concept clear with this particular problem.
Right. Let's go for one more problem. See this problem guys try to understand it carefully.
Okay draw a diagram so that you will understand. Car moving at 12 meter per second and accelerated 1 meter per second square. Truck moving at 4 meter per second and accelerated at 3 meter per second square. After 5 seconds find velocity of truck related to car. okay see here just let me draw the diagram you have a car right so there is a car and this car has a velocity 12 meter per second and it has moved to certain distance okay so in five minutes let me in sorry five seconds Okay, so within 5 seconds from here to here the car has moved.
With the acceleration what is this here? This is 1 meter per second square. Right? And there is a truck.
Right? And there is a truck. And this truck's speed initially was 4 meter per second.
And you know after 5 seconds. Okay, it is there here. It is there here.
And you know what? Its acceleration, its acceleration is 3 meter per second square, right? Yeah, this is a given information. See, initial velocity of car is this much, this is the acceleration, after 5 seconds, it is there.
Initial velocity of truck is this much, acceleration is this much, after 5 seconds, it is there, okay? So, this is final velocity, V, V of a car and this is final velocity v of a truck you know what they asked you to find out after five seconds after five seconds so i considered these velocity after five seconds velocity is vc and vt after five seconds find velocity of truck that means relative velocity of track with respect to car so what is your formula velocity of track minus velocity of car. But which velocity?
Will you take these velocities? No, you have to consider these velocity. But you don't know these velocities. You have to calculate.
So what is our formula? V is equal to u plus 80. Isn't it? So V of a car is how much? Initial velocity is 12. So, 12 plus acceleration is 1, 1 into time is 5 seconds.
So, 12 plus 5 velocity of a car after 5 seconds is 17 meter per second. So, with this information and this information, I calculated velocity of car after 5 seconds. Similarly, with this information and this information, information and disinformation. Calculate velocity of truck after 5 seconds. That is the same formula v is equal to u plus 80. So u is how much?
4 plus a is 3 into time is 5. So that is 4 plus 3 5 is 15. Isn't it? So all together 19. Velocity of a truck after After 5 minutes is 19 meter per second. Right? And now you got B and B. Can't you calculate relative velocity of truck?
with respect to car yes velocity of truck is how much 19 meter per second velocity of car is how much 17 meter per second so 19 minus 17 it is 2 meter per second right so relative velocity of with respect to car is 2 meter per second right so that's all about the concept of relative velocity right so with this i have actually completed the concepts of a chapter in a detailed way and the remaining aspects of the chapter is all about the graphs right so you have xt graphs you have vt graphs you have at graphs you and let's study about all these graphs in one single video right so hope you understood about the concept of relative velocity stopping distance and etc so with this i am completing the chapter motion in straight line still a video on graph is going to come okay so do watch the videos share with your friends and subscribe our channel to learn the concepts in a easiest way and in a detailed way thank you so much you