 So what will happen in a rigid body? Let's take a big object. A big object, a big rock. What kind of motion that can perform? It can simply move forward like that. It can simply move like that. All it can spin. It can just spin like for example a fan is spinning. Okay? Or it can spin as well as move forward. So spinning and ball is spinning and moving forward. That is your rigid body motion. Okay? So when that happens then you'll understand what I'm saying here. That you need to observe everything relative to a point on the rigid body first. The rigid body what will happen is that distance of every point from every other point is fixed. Yes or no? For example if I take a big object like this. Let's take any two points on the object. Any two points. If the distance between these two points is changing, if the object is not fixed. If distance between these two points is fixed, then how these two points can move relative to each other? They can move only in a circle. So when the rigid body is moving, spinning and moving forward, it is nothing but collection of several motions happening. Many many several motions are happening together. Okay? Let's say this is just spinning. It is spinning. Like for example fan. Fan is not a point object. Every point on the fan is moving in different speed. We have discussed that. Okay? So fan is an example of a big object spinning. All the points on the fan, they are moving in a concentric circles. Alright? So in this chapter, what we are going to do is that we are going to only observe one point moving in a circle. But when we go to the rigid body motion chapter, there we will take the entire object and then we will study all the circle motions together. All the points in there. Suppose n particles are there. All the n particles starts to move in a circle motion. How to write kinetic energy of that? How to write potential energy of that? That we will discuss later on. Right now we are taking a very simple scenario. A point is moving in a circle. Okay? And mostly the center is at rest. Why I have told you everything else? Because you should be aware that that is not the only thing that exists. Anyways, let's take the simplistic scenario now. Draw the circle in which a particle is moving in a circular path. Let's say radius is r to the path. Whatever it is, the direction of velocity is always tangent to the path. Okay? Fine. So when I am learning circular motion, I will be studying circular motion in two different ways. Circular motion can be uniform circular motion. It can be uniform as well as non-uniform. What is uniform circular motion? Constant speed. Okay? But please write down an object moving with a constant speed in a circle is a uniform circular motion. An object moving with a constant speed in a circle is an example of uniform circular motion. Okay? Now I am giving you here four points. 8, 8, 9, 8. Can you represent the velocity? Suppose the object is moving like this. Show the velocity. Four points. Point 8 will be like that. Point 8 will be like this. Velocity changing or constant? Constant. Will there be acceleration? Velocity. Acceleration is there. Since velocity is changing with time, acceleration must be present. Acceleration must be present. Now if acceleration is, who creates acceleration? Force. Force is required to create acceleration, right? So there must be a force required. Force is required to create a circular motion. Must be there for motion. If that force is not there, you cannot expect a circular motion. Understood? Okay? Now let us try to derive what is that acceleration, how much it is and what is the direction of acceleration. Clear? Alright? So this is one of those, that is the acceleration. Then you will find out what is the force and then it will be to mass and acceleration to get the acceleration. But here, why are we finding acceleration first? Because given that it is moving in a circle, I need to find out how much force is required to move in a circle. So what I do is I have to move in a circle in acceleration, how much it is. By the way, in the acceleration, have you heard of it? And you are inside the vehicle, you tend to, why? Understood? Inertia. But the vehicle is moving in a circle present for you, so that you can move in a circle. So what do you do? And then normalization from the door creates the provision for that required force so that you move in a circle. When the earth is a circular motion, there must be a force. And what are examples that we have taken? The motion of earth and the stream, that force is towards the center. If the force is not towards the center, but if force is tangential. Suppose tangential force is required to make sure that it is made constant. I am saying that if I apply force, let us say this particle will move in a circle. If I apply force in this direction, will the magnitude of this velocity change or not? Yes. Magnitude changes. But I know the motion. Yes or no? So there should not be any component of force in the direction of velocity. Can you write down? In a uniform circle motion, there should not be any component of force in the direction of velocity. In a uniform circle motion, there should not be any component of force in the direction of velocity, otherwise it will change. Is the option perpendicular? If force is perpendicular, this one or that one? Two. If there is a confusion, you can see that velocity. So the force should be in that direction. So please write down that the force required by the object towards the center. So if you have to find out the direction of force, extended line somewhere on the line, center will line. So the force always, even the direction of force is continuously changing. If the velocity direction changes, immediately the direction of force to make sure that the direction of force is again towards the center. So that the force is perpendicular. That is what is happening when anything moves in a circle. When earth moves towards, when earth is moving in a circle motion around the sun, right now let's say earth is here, this is sun, the force is like that. So even force is fine. So we have found out, in order to find the magnitude of force, first we will find out the magnitude of the acceleration. And then just multiply with mass with the force.