Newton's first law

0:00- [Narrator] Suppose you have a puck at rest on a concrete floor and you don't do anything to it, what will happen? It will continue to be at rest, isn't it? But why? Not because there are no forces acting on it. I mean, there are forces acting on it. For example, there is the force of gravity acting downwards, but there's also an upward normal force that the ground or the concrete is putting back on the puck as a result of which the forces are balanced, which means the net force acting on the puck is zero.

0:31So yeah, the puck's motion is not changing, it's at rest, it'll continue to be at rest. But now let's bring a stick and hit it. What's going to happen? Well, we know what's going to happen. If you hit it gently, it might, you know, travel for some distance and come to a stop.

0:48The big question is, why did it stop? Well, let's think about this. Our intuition might say that, "Hey, that's probably because, you know, puck naturally tends to stop. Objects naturally tend to come to a rest." Or maybe you might think that, "Hey, you know, it ran out of the force that, you know, we applied," but that's not true.

1:10Let think about what's really going on. Let's slow down things a little bit, okay? So when we hit the puck, we apply a force. In fact, we apply an unbalanced force on the puck. Because it's an unbalanced force, the puck starts moving, and this force lasts as long as there is contact.

1:28But the moment the puck loses the contact with the stick, the force will disappear, and yet the puck continues to move forward, even though the force disappears immediately after the contact is lost. But now the big question is, why does the puck stop? Well, the reason it stops is not because it's natural tendency is to come to a stop.

1:49No, no, no. But because there are forces that are pushing it in the opposite direction. Which are these forces? Well, there's air that's pushing it in the opposite direction. There's also the force of friction between the concrete and the puck's surface. Because these forces are acting in the opposite direction, it's these forces that make it stop.

2:09So let's look at the animation carefully now. The puck will start moving, and look! Look, there are these resistive forces, we call them, frictional forces, air resistances. These are the forces that make them stop. So let's look at it one more time. You have the stick that hits it, it applies an unbalanced force, it gets the puck moving, but then these resistive forces start applying force in the opposite direction.

2:34They start pushing them in the opposite direction, making it stop. But now here's the question. What if we could somehow reduce those resistive forces, reduce the amount of friction, for example, then we would expect the puck to move much longer before coming to a stop, because the resistive forces are much smaller, isn't it?

2:53And that's exactly what happens. If we were to carry this out on say, ice. The friction between the puck and the ice is very tiny, much smaller compared to that on the concrete. And so now, if you hit it with exactly the same force in both the cases, what will we find?

3:08The puck moves much farther on the ice compared to the concrete, even though the force is the same! Why? Well, that's because the resistive force, that's the one that was smaller over here, because of which it moved a much farther distance before coming to a stop.

3:25And now comes the question, what if we could somehow make that resistive force zero? What if there was absolutely no friction or air resistance or anything like that? What if we carried this out in outer space, deep intergalactic space where there's absolutely no other forces, for example?

3:42Well, now, once you hit the puck, that puck will keep moving forever. This means, look, you don't need an unbalanced force to keep an object in motion. Object which is in motion will just keep moving with that same velocity as long as there is no net force acting on it.

4:03And if this sounds unbelievable, then just think about the moon that's going around the earth, or for example, the earth that's been going around the sun. They've been doing that... They've been at it for billions of years. What's making them move? There are no forces acting in the direction of the motion.

4:19Then why is it moving? They just keep moving because they're already moving, and more importantly, because there's nothing slowing them down, there's no force acting in the opposite direction to slow them down. Okay, we can put it all together in what is called the Newton's First Law, which says that if the net force acting on an object is zero, the object's motion will not change.

4:44This means if you had an object at rest, and again, if there is no net force acting on it, then that object will continue to be at rest. On the other hand, if you have an object that's moving and there are no net forces acting on it, it'll continue to move with that same constant velocity forever.

5:00This is the non-intuitive part, because on earth there are always some resistive forces, always some unbalanced forces that tend to make things stop. But in the absence of them, objects in motion would just continue to move with that same velocity forever. And by the way, whether something is moving at a constant velocity or something's at rest purely depends upon reference frames, right?

5:23For example, if this puck was in a train moving at a constant velocity, then the people inside the train will see the puck to be at rest. But people who are outside on the ground will see the puck moving at a constant velocity. But regardless of which reference point you're dealing with, the fact is, if there are no other forces acting on the puck, then the people who see the puck to be at rest, they will continue to see the puck to be at rest, and the people who see the puck to be in motion, they will continue to see that puck to be moving at a constant velocity.

5:51The fact is, as long as not net force is acting on an object, the object's motion will not change. In other words, the object will not accelerate.