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- [Narrator] If I were to ask you, how do you keep things in motion? What would you say? You might say, we have to keep pushing or pulling on that object. A push or a pull is called a force. So, in other words, we need to keep applying a force on the object. If I asked you, why do you think so? You might say, this is from your daily experience. For example, this chair is not moving. If I have to make it move, I have to keep on pulling on it, or I have to keep pushing on it. If I stop pushing or pulling on it, look, it stops moving. And there you go. A force is needed to keep this chair in motion. Without a force, this chair naturally comes to a stop. Right. Well, what if I told you that this was wrong? Would you believe me? I'm guessing, no. Because you just proved it with an experiment. And so I guess the goal of this video, is to convince you that this is wrong. Or at least to make you rethink about this statement. And we do that by using the example of carrom. Suppose we want to make this striker move on this carrom board. What should we do? Well again, you might say we need to keep pushing it. You need to push it to move it. But let's say, instead of pushing it gently, we give it a strike. You know what happens, the striker will move some distance, and will come to a stop. Let's look at that again. This time, in slow motion. What we see is, when our finger touches that striker, it pushes that striker, making it move. But as the striker loses contact from my finger, I stop pushing. There's no longer a push anymore, and what we're seeing after that, is that the striker slows down, slows down, and eventually comes to a stop. And at this point, you might say, ha, I told you so. You have to keep pushing it in order to keep it moving. If you stop pushing it, then it will come to a stop. Things have a natural tendency to come to a stop. Okay, but let's do something. Let's add some powder to the surface. And repeat this. Again, you might know what happens in this game, right? Now if I strike it with pretty much the same force as before, it goes much further before coming to a stop. And if I ask why it traveled farther this time? You might say, well because we added powder, the surface became smoother, and things slide farther on smooth surfaces. But what if I asked you, why? Why do things slide further on smooth surfaces? I mean, if the natural tendency of an object is to come to rest, why does that depend on how smooth the surface is? Huh? Think about that. Why does the surface matter? This is where a man Galileo Galilei came up with a crazy idea. He thought that maybe this piece is not stopping because of its natural state. He thought maybe this rough surface is forcing it to stop. Okay, here's what I mean. If we go back to before adding the powder, once I strike this blue piece, it gets, it sets in motion. And now, Galileo's thinking, maybe the surface itself starts pushing this blue piece in the opposite direction. Opposing its motion. And maybe it's this force that slows it down and eventually makes it stop. This is just like how, when there's an uncontrollable train moving. Superman comes in, pushes the train the opposite direction. Slows it down. Makes it stop. And saves the day. Similarly, Galileo thought it is this force, that's opposing it's motion and makes it stop. And you might even know the name of this force. Today we call it friction. And if we understand how friction works, maybe we can explain this entire scenario. So let's take a look at how friction works. To figure out friction, we need to look carefully at the surface, where the striker meets the board. You see, although these surfaces look very smooth to our eyes, at the microscopic levels, they aren't smooth at all. So if you could zoom in over here, the surfaces might look somewhat like this. These mountains and valleys are just too small for our eyes to make out. And therefore it looks smooth to us. But as this striker moves on the board, notice because of the unevenness, it causes obstruction. And it is this obstruction which we call friction. Okay, but what happens when we add powder? You see, powder particles are so small they can fill in these gaps and smooth it out. Ahh, now because the surfaces are much smoother than before, the striker can slide with much lesser obstruction. And that means friction reduces. So, if we come back to our board, according to Galileo, when I strike this coin, it is friction that is opposing the motion and stopping it. Since friction is the culprit for stopping this coin, when we add powder to the surface, the surface gets smoother, and it's the friction that decreases. That's the change. That's the effect of smoothing the surfaces. And since the friction decreases, the opposing force decreases, this means now it is harder to stop the coin, and as a result the coin travels farther before coming to a stop. So this means, logically, as I make the surface smoother, and smoother, our coin would travel farther, and farther, before stopping. Given that our carrom board is say, big enough. And now, this is the big moment. This is the big, big, Galileo moment, what if we made the surface perfectly smooth? What happens then? Imagine we, somehow, made this board perfectly smooth and gave it tap. What happens now? Well, now that our board is perfectly smooth, that coin will never stop. It will keep moving forever. Of course, provided that our board is super, super big. And now Galileo would look at this and say, look, look, things in motion stay in motion. So to keep things in motion, you don't need to push or pull on them. They have the natural tendency to stay in motion. And why don't we see these in our daily lives? Because of friction. Because friction always acts in the opposite direction of the motion and makes it stop. So if the friction is the culprit, that makes everything stop. Even the chair, the example we saw earlier. Same case. If there was no friction, and if you stop pushing on it, that chair would keep moving forever. But it's the friction that opposes things and makes everything come to a stop. So natural state of moving things is to keep moving. Now you may want to defend this and you might say, wait, wait, wait, wait, wait. But initially the striker was at rest. And you had to put a force on it to start making it move, right? So that way, force is needed to start motion. Yeah, but now I can also say a force is needed to stop the motion. And the only reason why things around us are at always, most of the time, at rest is because of friction. Because friction opposes motion and makes them come to a stop. In the absence of friction, there is no reason for things to be at rest. Things might as well be in motion. And so the natural state of things in the absence of forces is either rest or in motion. And if you find this hard to digest, and if you find this a little confusing, then you are not alone my friend. Because humanity took thousands of years to figure this out. So please take your time. And I'll tell you what helps me, is to think about celestials, like the planet or the stars or the galaxies. They are in perpetual motion. Yes, their motion is a little complicated because there are forces acting on them, but they are always moving. Why don't they stop? Well, because things in motion tend to stay in motion. And a final question that you might have is, what does a force do? I mean we just saw a force doesn't keep things in motion, then what does it do? Well, like we saw, a force can start motion. In other words, a force can speed up things. And like in the case of friction, force can also slow down things, and stop their motion. Turns out that force can also change direction of motion, but don't worry too much about that. So in general, we can say force can accelerate things. That's right. In the absence of forces, if objects are moving, they will move with a constant speed in a straight line. But if you want to accelerate that body then you need to put a force on it. And so what did we learn in this video? We saw that a man named Galileo looked at simple experiments, well not carrom, maybe, but other things. He looked at them carefully and came up with a revolutionary idea. That things in motion have a natural tendency to stay in motion. Don't need to keep pushing or pulling on them to keep them in motion. And what does a push or pull do? What does a force do? The force, in general, speeds up or slows down things by in general, it accelerates things.