All right. Well, welcome to lecture three, forces part one. So, in this session, we'll explore what a force actually is. Okay, the different types of forces and how they affect the motion of objects. Okay, we'll also lay the groundwork for understanding Newton's three laws of motion. Okay, which will explain how forces behave and interact in the world around us. Okay, so let's start with a basic question. Okay, what is a force? Okay, so a force is simply a push or a pull. Okay, it's what causes objects to accelerate or change their motion. Okay, whether you're pushing a door open, pulling on a rope, or dropping a ball, okay, you're seeing forces at work. Some forces are obvious, okay, like a hand pushing on an object, okay? Others, like gravity or friction, okay, are more invisible, okay, but they're always present, okay? There are many different types of forces we'll talk about in this course. Okay. So here's um well here's pretty much we it'll cover all of them uh in this section here. Okay. So the first one is an applied force. Okay. So this is when something is physically pushed or pulled. Okay. The next one is gravity. Okay. That's the force that pulls objects towards the center of Earth. Okay. And then we have normal force. Okay. This is a support force from a surface pushing upward. Okay? Or just pushing on an object. Okay? So like the box there, it has a normal force from the table. Okay? Pushing upward on the box to hold it up. And we also have a spring force. Okay? So this is when a spring is compressed or stretched. Okay? It's trying to return to its original shape. Okay? And we also have resistive forces. Okay? also such as friction. So resistive and friction forces, these are forces that resist motion between surfaces. Okay. So whether it's between surfaces or even a resistive force like air resistance. Okay, like the air pushing on a plane. And then finally, we have a tension force. Okay. This is a pulling force along ropes or cables. Okay. So in a tug-of-war match, there will be what's called a tension force within the rope. Okay. So each of these plays a key role in how we interact with the physical world. Now there's also something important here. Okay? Because force is what's called a vector. Okay? That means it has both a size and a direction. Okay? or has a magnitude and a direction. Okay? So for example, pushing a box to the right, okay, and pulling it to the left involves different directions. And so even if the amount of force is the same, okay, the two equal forces are going in opposite directions. Okay? So they could potentially cancel out. So if they push in the same direction, okay, then they combine like the two people pulling on that uh that train car on the tracks. Okay, so this is going to be crucial when we start analyzing what's called net forces. Okay, so a net force acting on some object. Okay, the net force is the same as the total force acting on the object. Okay. All right. So, let's talk briefly about falling objects. This is something that both like Galileo, which you see him there, and Newton, Isaac Newton studied. Okay. So, first, all objects regardless of their mass fall at the same rate. Okay? About 10 meters per second squared. Okay? And that's if we ignore air resistance. And so that's why a hammer and a feather will fall at the same speed on the moon because there's no air there. So on Earth, a in one second an object will fall about 5 m. Okay. And kind of analyzing that 10 m/ second squared or 10 m/ second per second. That also means that every second an object falls it will increase its speed by 10 meters per second. Okay? So if you release a ball from rest and after one second it will be traveling at 10 meters per second. Okay. After two seconds it'll be traveling at 20 m/s. After 3 seconds it'll be traveling at 30 m/s. You can kind of see how the pattern goes. So something else. Okay. This is actually something Newton showed. Okay. He showed that no force is needed to keep an object moving in a straight line at constant speed. Okay. And this idea was actually revolutionary at the time. Okay. And both Galileo and Newton all their studies and like observations all these reasonings were based on measurements they made. Okay. This is the basis of science. Okay. So remember all science is is um all these reasonings and developments based on measurements made. All right. Well, since we were discussing falling objects, okay, I figure the next thing we should do is well test objects out. Okay. So, um I already told you like briefly about the experiment of dropping like the feather alongside the hammer on the moon. Okay. And we can see those fall at the same rate because the moon doesn't have any air. Okay. But here on Earth, okay, well, instead of a feather, I got just some blank paper. Okay. And here's my hammer. And well, we can already guess what's going to happen when we drop these. Okay. We see the hammer hits the ground first. Okay. And because of air resistance, okay, the paper slowly falls to the ground because air is in the way and it's pushing it back up. Okay. But if we try to reduce this air resistance, okay, say by changing the shape of our paper, there we go. Just like that. Okay. Now, let's go ahead and watch them fall side by side. Okay. And you can see them both hit the ground at the same time. Okay. because we reduced the amount of air resistance that was acting on our paper just by changing the shape. All right. And I have another little experiment here. Okay. So inside, okay, you can see I have a feather and a penny. Okay. They can move freely inside. Okay. They're not stuck to a wall or surface or anything like that. Okay. And just looking carefully. Okay. And you might be able to see the penny just fall pretty much all the way down fairly quickly and the feather just kind of floats down because there's air inside here. Okay, then you see the feather go after it. Okay, but what if we remove the air from this container? Okay, so we'll go ahead and try that next. So I have a little valve on this side connecting to the chamber here. Okay, so I have a little pump down here to remove the air. All right. So, I think we got most of the air removed from our chamber. Okay. And you can still see I got both the feather and the penny inside. Okay. So, now let's go ahead and see what happens when we turn this again. Okay. Well, hopefully you can see that. Okay, it does go pretty quick. But you can pretty much see the feather is falling with the penny. Okay, so again, like they're not stuck together or anything. Okay, but since there's no air, okay, they fall at that same rate. Okay. And then if we allow the air back in we can see it goes back to what we've seen before with that feather falling more slowly because the air's in the way. All right. So let's see what we got next. So now we'll take a look at gravity and normal force. So, first you can imagine or take a look at the picture. Imagine a book resting on a table. Okay? We know gravity is pulling the book down, but it's not falling. Okay? Why? Well, that's because the table pushes up with an equal force. And remember that force is called the normal force. Okay? So, these two forces are balanced. Okay? So, the book doesn't accelerate, okay? Or change its motion. Okay? You'll hear the term normal force quite a lot. Okay? All it means is it's the support force from a surface pushing perpendicular to it. Okay? So that means if we have that perfectly level table like we see in the picture here, okay, that normal force is pushing directly up. Okay? If we had kind of a ramp, okay, with a t like a book resting on kind of a ramped surface, that means the normal force will actually be pushing. It'll always be 90° okay compared to that the surface. Okay, so in this case compared to the ramp it'll be 90° away from the surface. Okay, which means perpendicular. Okay. All right. So now looking at balanced versus unbalanced forces. Okay. So balanced forces don't change an object's motion. Okay. So if something is at rest okay or moving at a constant speed in a straight line okay the forces on it are balanced okay so we can see that in the very first diagram there in uh figure A and there's no force acting on that satellite so we can say it's oh it's moving at a constant speed in a straight line okay the forces are either balanced okay or well forces are balanced if there's no forces acting on it too Okay. And either way, we'll say it's balanced. But if the forces are unbalanced and if one side is stronger than the other, then the object will accelerate. And so it might start moving or it could stop or slow down. Okay? Or it could even change direction. Okay? So in each of these diagrams we show here, we have a satellite looks like moving to the right. Okay? So in figure B, okay, if we have a force acting to the left, okay, in the opposite direction of its motion, then we know the satellite will start slowing down, okay, and could come to a stop or even reverse direction and speed up the other way, okay? So in that case, we have an unbalanced force, okay? Changing the motion of that object. And then in figure C, okay, we have a force that's pushing in the same direction as the motion. Okay, so now we can see that our satellite will be speeding up, okay, or accelerating in that same direction. Okay, so it'll speed up getting faster and faster moving to the right. Okay, in figure D, okay, we have a satellite still moving to the right, but now we have a force that's applied kind of sideways. Okay, it looks like it's pushing upward on the satellite. Okay, so this is where we'll have a change in direction. Okay, we know it'll keep moving to the right, but it's also going to start accelerating upward at the same time. Okay, almost well, if you can see my cursor here, it'll start kind of moving and then maybe kind of increasing its speed going upward almost in that uh different kind of trajectory. Okay, that increasing motion moving upward. Okay. So in figure D, that's a good example of when a force causes a change in direction. Okay. So in all unbalanced force cases, okay, we'll always have some kind of acceleration. Okay, and all acceleration is is it means is there's a change in motion. Okay, that could be a change in speed or a change in direction. So now here's some common misconceptions that I'd want to kind of clear up for us. Okay. So first, okay, motion does not require a continuous force. Okay, you may think it needs a continuous force for it to keep moving. Okay, but that is incorrect. Motion again, motion does not require a continuous force. Okay, once something is moving, it stays moving unless something stops it. Okay, that could be some kind of resistive force, could be friction, or even just like a solid wall like stopping the motion. Okay, it could even be gravity. Okay, pulling it back down to earth and then friction taking over to stop the ball. Okay, and now second, okay, another misconception is that heavier objects fall faster. Okay, that is not true. Heavier objects don't fall faster. Okay. Unless air resistance is involved, everything will fall at the same rate. Okay. And third, okay, another misconception is that all forces cause motion. Okay. So that's not necessarily true. Not all forces can cause motion. Okay? Because forces can cancel each other out like we saw in the last slide. Okay? If we have balanced forces, they cancel each other out. So they won't cause any change in motion. Okay? Just like the book resting on a table, okay? It has the force of gravity pulling it down, but also the normal force pushing up, canceling each other out. So the book remains motionless. All right. So now looking at force in everyday life. Okay. So when you say lean on a wall, okay, the wall pushes back with an equal force. Okay. It's like you're pushing on the wall, the wall pushes you back. So, you're stationary. Okay? Or when you're when you're walking, okay, your feet will push backwards on the ground and the ground pushes you forward. Okay? Well, that's Newton's third law, which we'll get to in a minute here. Okay? Or, uh, car tires will push backwards on the road and the road pushes the car forward. Okay? That's how you accelerate. Okay? Now, let's look go ahead and start looking at Newton's laws of motion. Okay? Okay, so Newton's first law is also called the law of inertia. Okay, so it states that every object remains at rest or in its state of uniform straight line motion unless acted upon by an unbalanced force. Okay, so inertia is an object's resistance to change in motion. Okay, that's why you feel thrown backward when a car suddenly accelerates. Okay, the car is moving forward, but your body is wanting to stay in place. Okay, and you can see Newton's first law in action in the little gif here. Okay, so you see the leaves are initially just at rest. Okay, and then we pull out that net from beneath it. They're trying to stay at rest, but then gravity, okay, the force of gravity starts acting on it and then they slowly fall down to the ground. Okay. At that rate of 10 meters/s squared. Okay. All right. So now let's go ahead and take a look at a few demonstrations showing Newton's first law. All right. So next thing we're going to be looking at is the law of inertia or Newton's first law. Okay. So we're gonna kind of see this in action. Okay. So here we have uh the demo demo or demonstration I've shown you before. Okay, but let's go ahead and take a look at this through the lens of Newton's first law. Okay, so let's go ahead and get our golf ball out. Okay, we can get this set up. So again, I have my stick. This will kind of give our golf ball some elevation. Okay. And then we'll split our golf ball directly on the T. Okay. So again, like Newton's first law. Okay. This tells us that an object at rest tends to stay at rest. Object in motion tends to stay in motion unless acted upon by an outside force or a net force. Okay. So right now, okay, my golf ball is at rest. Okay. It has two forces acting on it. Okay. So the force of gravity pulling it downward and the normal force from the T holding it upward. Okay, forces are balanced. But then if we remove the T, okay, that we remove the normal force holding it upward and then we have a net force. Okay, just the force of gravity pulling it downward. Okay, so let's go ahead and see this again. So you saw that golf ball stay in position until that teeth was removed out from underneath it and then it fell straight down into our so again this is Newton's first law. Okay, the golf ball is wanting to stay still. Law of inertia and it has mass. So it takes some kind of force for it to accelerate. In this case, force due to gravity. Okay, let's see what we got next. Okay, so here's our next demo. Okay, um it'll still be looking at Newton's first law. Okay, but you see how we have this set up. Okay, we have just an empty wine bottle down here. Hey, I have a wooden hoop. Hey, balanced on top of the wine bottle. And then at the very top I have a nail standing on end. Okay. So right now okay all the forces are balanced. Okay. Acting on both nail the hoop and the bottle. Okay. So it's not going anywhere. It's not accelerating. The motion of it is not changing. Okay. But looking just at the nail. Okay. Just like the golf ball we looked at a moment before here. The nail it has both a force due to gravity. Okay. And the normal force acting upward on it. Okay. But once we remove that normal force, okay, remove that hoop. Okay. As long as I do it correctly, we should see that nail fall straight down. Okay. So, let's go ahead and try this out. Hopefully, we don't need more than one take, but we'll see. So, what I'm going to do is I'm going to pull that hoop. Okay. Try to pull it just directly horizontally, removing it out from underneath the nail. I have to be very quick. Okay. And I don't want to accidentally go upward. Okay. Or even downward and moving the bottle. Okay. But let's see if we can. Let's go ahead and give this a try. Three, two, hoop. And you saw the nail fall straight downward and into our bottle. [Music] So again, this reflects Newton's first law. Okay. The object at rest is tending to stay at rest until it was acted on by gravity, fell straight down into our wine bottle. Okay. All right. The next thing, okay, actually I'll end up kind of posing question to you is you'll have to try to predict what's going to happen. Okay. So, let's go ahead and check it out. All right. So, here, okay, all I have is a piece of PVC. Okay. And then a wooden block. Okay. Well, you can see I I screwed two blocks together so it fits around the piece of PVC. Okay. If I kind of twist the PVC, I can move the block, okay, along it, okay, from one end to the other. It does take some force to be able to do that. Okay. But thinking about Newton's first law of motion. Okay. What I want you to do is think about here. Let's go ahead and I'll position the block kind of almost in the middle. Okay. And we'll say that's close enough. Okay. What we're going to do, hey, I'm just going to hold the PVC on this end. Okay. You already saw. Okay. We can kind of shimmy the block up and downward. But the question for you, hey, is what's going to happen to the block if I take a hammer, okay? And only hit the PVC. Okay? I'm going to hit it at the very top here. Okay? I'm going to keep a firm grip on the PVC down here so the PVC doesn't slide through my fingers one way or the other. Okay? So, I'll have a firm grip here. Okay? And I'm going to be hammering on the PVC. Okay? The question is what's going to happen to the block? Is the block going to stay in the same position or is the block going to move towards my hand or is it going to end up moving away from my hand? Okay, so I'll give you a moment to decide or actually well better yet just go ahead and pause the video. You can think about it and see if your prediction is correct. So hopefully you had uh a little time to think hey about what's going to happen to this block. Let's go ahead and test it out. Okay. So, again, I got a firm hand, okay, or a firm grip on the PVC. I'm going to go ahead and hammer to the very top of the PVC. Okay. And well, you can see it actually moved further away from my hand. Okay. So, why was that? Okay. So, thinking about Newton's first law, okay, the block has inertia, okay, or it has mass and it's not really wanting to move unless it's acted upon by some outside force. I wasn't hitting directly on the block, though. And I was hitting on the PVC. Okay? So, if I'm holding it firmly here and I hit the PVC, PVC pipe is trying to go say downward. Okay? I'm producing a force acting downward on it. So it's almost like the PVC is moving downward. Okay. While the block is trying to stay in the same position, okay, because it's not being acted upon by the force from the hammer. Okay? So as we move the PVC downward, it appears the block moves upward on the PVC. Okay? And we can do the same kind of thing. Okay? kind of in the reverse. If I have a firm grip on the top part now, okay, and if I hit the PPC, okay, which way will the block go? Okay, let's go ahead and test. Okay. And you can see it moved towards my hand. Okay. Still moving upward because I was putting a force. Okay. are acting a force on the PVC which was downward. Okay, so it's like the PVC is moving downward because the force is downward and the block was trying to stay in the same position. Okay, because again the block has inertia or it has mass. Hey, it's not really wanting to move. Okay, it takes some kind of force for it to move. And well, that's the mala and block demo. All right. So, let's go ahead and see what we have next. Okay. So, now Newton's second law tells us that forces cause acceleration. Okay. The bigger the force, the more acceleration you get. Okay? The more mass something has, the harder it is to accelerate. Okay? And the units that we use for a force are called Newtons. Okay? symbolized by that capital N. Okay. So something pushes can push with oh five newtons of force. Okay. It's similar to even like pounds. Okay. Pounds is also a unit of force. Okay. And force is equal to mass of an object times acceleration. Okay. So again force is measured in Newtons. Okay. And that's going to be the same as kilograms meters/ second squared. Okay. And then we have the symbols for that down below here too. Okay. We have Newtons is equal to the mass in kilogram times the acceleration in meters/s squared. Okay. So if you apply the same force a to two carts, the one with less mass will accelerate more. Okay? And so don't forget, okay, Newton's second law is really that relation between force, mass, and acceleration. Okay, the shorthand for it is F= ma or force equals mass times acceleration. Just don't forget, we want those units to be newtons for force, kilogram for mass, okay? And meters/s squared for acceleration. All right. So now, okay, let's go ahead and look at just a couple examples utilizing Newton's second law. All right, so next we'll look at Newton's second law. Okay, so that's where we we're looking at the force acting on an object equals the mass times acceleration. Okay. So, let's go ahead and write that down. Okay. or in shorthand. Okay, f= m a so let's just look at a couple kind of quick examples okay and we'll make them like well we'll try to make them easy so the first one okay is let's just say we have a ball a ball is resting on the ground and So here we know when it's at rest, okay, there's a force of gravity acting downward and a normal force from the ground acting upward. Forces are balanced. But now, okay, we're going to come over and we're going to kick that ball. Okay, we're going to kick it directly to the right. And we'll say we're going to kick it with a force of let's make it 30 newtons. Force of 30 newtons. Okay. And something else. Okay. Let's go ahead and make an estimation for the mass of this ball. Okay. We're just going to make the mass five. kilogram. Okay, so we have the ball, okay, has a mass of 5 kilograms and we have a force acting on it of 30 newtons. Okay, so let's go ahead and figure out what the acceleration of this ball is. So for that, we'll use our equation of Newton's second law. We have a force equals mass time acceleration. Okay, so we know the force is 30 newtons and we know the mass is 5 kilograms and then that'll be multiplied by the unknown acceleration. So times we'll say a. Okay. And now we just need to solve for what that acceleration is. Okay. So what we want to do, okay, is we want the a or acceleration to be by itself. So we need to move the five over to the other side. Since they're multiplied together, they will want to divide both sides by five. So I'm going to go ahead and put this divided by five just like a fraction on the right. And we have to do the same thing to the left. Okay? So on the right 5 / 5, we know that's one. So those will cancel out. And we'll just have acceleration. Okay. And then 30 / 5. Well, we know that's six. Okay. So this tells us that my acceleration is six. Okay. But six what? Well, if you remember the units we use for force, mass and acceleration is forces in Newtons. Check. Mass is in kilograms. Check. And acceleration would be in meters/s squared. And so we say our acceleration is equal to 6 m/s squared. Just like that. Good. So, we use Newton's second law for the first time. Woohoo. All right. So, let's go ahead and look at uh we'll just look at one more example for now. Okay. And then we'll move on to the next thing. So, in that example, let's go ahead and look at a car accelerating. So for cars or vehicles, okay, we're going to say, well, this one we're going to say mass is 1,500 kilogram. and acceleration we're going to make go kind of low. Okay, it's going to be accelerating in this direction. Okay, and we're say it's going to be accelerating at do four meters/s squared. Okay, so every second, okay, the car is going to be increasing its speed by 4 meters/ second. So here, let's go ahead and figure out the force that must be acting on this car, okay, for it to accelerate at 4 meters/s squared. Okay, and the force we're going to be finding is also referred to as the net force, okay, or the total force. So, let's go ahead and we're going to apply Newton's second law again, except now we're going to be finding the force acting on the car. So, we have our force equals our mass. Okay, so our mass of the car is 1500 kg. Okay, and then we want to multiply by acceleration. And and our acceleration of the car is at 4 m/s squared. Don't need that for now. Okay. So, pretty much have our force is equal to 1500* 4. Okay. So, you can punch that in your calculator or you can do it in your head. Uh that's fine, too. Okay. So, 1 1500* 4. Okay. That'll be 6,000. Okay. So, our force acting on the car must be 6,000 what? 6,000 newtons. Okay. So, we'll use that capital N. And there's the force acting on our vehicle. Okay. To get it to accelerate at 4 m/s squared. All right. Well, I think that's enough calculations for now. Okay, we'll go ahead and move on to Newton's third law. Okay, so finally, Newton's third law. Okay, this relates forces between objects. Okay, you can even look at this as a contact force if they're physically pushing on one another. Okay, so whenever two objects interact, okay, the force exerted on one object is equal in size and opposite in direction to the force exerted on the other object. Okay. So if you push on a wall, it pushes back with the same force. Okay. If a rocket pushes gas downward, okay, the gas pushes the rocket upward. Okay. So, um, what we can look at with Newton's third law. Okay, I'll just show you a like a kind of a few demonstrations just physically showing you what we mean by Newton's third law here. Okay. All right. So, Newton's third law a is um well, a little harder to demonstrate, okay? But I'll have some nice videos uh after the this lecture and everything to show you too. But for now, I'm just going to go ahead and show you kind of a few examples and just kind of get you thinking about Newton's third law. Okay? So, the first one is just walking around. Okay? So when you're walking, okay, you notice when you walk, you're actually pushing on the ground, okay? But the ground's not moving backwards, okay? The ground's also pushing back. Okay? So when I push off the ground, the ground pushes me back with equal and opposite force. Okay? The harder I push, hey, the more accelerated accelerated I am moving forward. Okay? Or if you say jump or push really hard, okay, that'll accelerate you forward even more. So something else too, okay, is even if you oh push on a wall, okay, so I'm standing on a wall. If I push on it, okay, I get pushed this way, the opposite direction of what I'm pushing. That's because as I push on the wall, okay, the wall pushes back. Okay. And this is especially seen um if you've ever been on like oh two skateboards and you kind of push off of uh each other or something or two skateboards. If you just been on a skateboard near a wall, you push off the wall, you'll move backwards, okay? Or if you've been ice skating, okay, push off the wall or push off of a partner you're skating with, okay, you'll move backward, okay, in opposite directions. And those are really good examples of Newton's third law. Okay. All right. Well, otherwise, hey, um I will have like uh some videos uh specifically um well that NASA made. Hey, so we're going to be looking at Newton's laws of motion in space, okay, on the International Space Station. Okay, so I hope you like otherwise we'll get to looking at the homework next. All right, so here's your homework for this lecture. Okay, so number one, hey, just answering these questions. What is a force? Hey, what is a normal force? What is a net force? And is friction considered a force? I'd like you to provide like relative relevant like images or drawings, that's up to you. Uh that help explain your answers. And then two, what are Newton's three laws of motion? Okay. So, write down Newton's three laws of motion, providing like a an either an example or an image or drawing for each of those laws. Okay. Number three, why do you feel pushed back into your seat when a car accelerates forward? Okay. So, which of Newton's laws does this reflect? Okay. And then finally, if two carts are pushed with the same force, okay, but one has more mass, what happens? Why? So here you want to think about the acceleration of each of the carts, okay? How do those accelerations compare? Otherwise, that's going to be it for this lecture. Okay? Until I see you next time.