 Have a look at this video. I'm going to kick this chair, which is at rest right now. And when I'm kicking the chair, I put a force on that chair, which makes it move. Not surprising. But once I let go of my foot, there is no longer any force acting on that chair, right? But still the chair continues to move. Have you thought about that? There's nothing that is pushing that chair forward right now, but still the chair continues to move. Why? To answer that question, we need to look at Newton's first law. So let's look at that. Newton's first law says that objects have a tendency to continue their state of rest or uniform motion until acted upon by a net force. What does it mean? It says that if you have an object that is at rest, it has a natural tendency to continue that, which is normal, which is straightforward, right? Things that are at rest, they naturally continue that state. We understand that. But the most important part is that objects that have a tendency, objects that are in uniform motion, meaning things that are moving at a constant velocity, they also have a natural tendency to continue that motion, meaning you don't need a force to keep things in motion. Things in motion have a natural tendency to stay in motion. This is what we call the Newton's first law. And that's why the chair was continuing to move. Why? Because you don't need a force to keep things in motion. Things in motion stay in motion, Newton's first law. But then you could ask, well, then why did that chair eventually stop? Why did it stop? Hey, it stopped because there was frictional force acting backwards. That's why it stopped. If there was no frictional force, the chair would keep moving forever. So it's the friction that stopped the chair. And so what this means is that you need a net force to start the motion, like we did initially when we kicked it, or to stop the motion, like the friction is doing it. We also need a force to change the direction of the motion or to change the speed of the motion. In other words, you need a net force only to accelerate an object, change its velocity. But you don't need a net force. A net force is not needed to keep a body in uniform motion. This is the most confusing part over here. Now, of course, in our day-to-day life, this is hard to see. We don't see things moving just like that, right? We see all things that are at rest and it feels like you need to keep pushing things to make them move. But that's because there is friction. The reason why we have to push on this chair to keep this chair in motion is because we have to overcome friction. But if friction was not there, then you would see that truly, you wouldn't need any force to keep things in motion. So that's Newton's first law. Now, let's say if you understood this by taking a few examples. Okay, in the first example, we have some stones which are at rest. The question I have for you is, is there a net force acting on this stone? I want you to pause and think about it. All right. We know that this is at rest and from our Newton's first law, we know that things at rest continue to stay at rest as long as there is no net force acting on it. Now, since this is continuing to stay at rest, that means there is no net force acting on it. And when I say no net force, I don't mean there are no forces. Of course there are forces. Gravity is pulling down on it. But this say the bottom rock is pushing up on it. But what the meaning of no net force is that the forces are balanced. If you add up all the forces, the gravitational force is completely balanced by the push of the rock upwards. That's what it means by no net force. Okay, let's consider the second case. We have an aeroplane that is in uniform motion to the right. It's moving to the right, but it's moving at a uniform motion. Is there a net force acting on it? Why don't you pause and think about it? All right. My first instinct would be, yes, something must be pushing to the right. Otherwise it wouldn't be moving to the right, no? But remember, that's not true. From Newton's first law, we learned that things in motion have a natural tendency to stay in uniform motion. You don't need a force. You don't need a net force to keep things in uniform motion. Since this is in uniform motion, even here there is no net force. But you may ask, oh, if you don't, then why are there engines? Why do you need fuel and all of that? Well, you need engines and fuel because there is friction. There's air resistance, not friction, air that is pushing back on this aeroplane. So to ensure to overcome the force of the air, there is engines that pulls the aeroplane forward. But this pulling force and the force from the air that cancels out, making sure net force is zero. The forces are balanced and that ensures that the plane is going in a uniform motion. Okay, let's consider the third case. We have a ball that's moving faster as it rolls down. So in this case, the tennis ball is just rolling down and it is going faster and faster. You may have experiences, right? It just, as it rolls down, it goes faster. Is there a net force acting on this? Why don't you pause and think about it? Okay, this time notice the speed of the ball is increasing. This is not a uniform motion. There is acceleration. From Newton first law, we saw that if there is no net force acting on an object, it would continue to move in uniform motion like it did over here. Or it would stay at rest like over here. But this is moving faster. It is accelerating. If it's accelerating, there must be a force acting on it, right? It's the force that accelerates, a net force accelerates a body. Therefore, there must be a net force acting on this tennis ball. In other words, there must be unbalanced forces. So there must be some force that is unbalanced that is causing the acceleration. If the forces were balanced, it wouldn't be moving faster. It would either be at rest or continue to be in uniform motion.