 Now, tell me if the net torque is 0. If the net torque is 0, will the object ever translate? Yes, it can translate. No, it can't translate. It can translate? Yeah. Okay, example? So if I have something and I push it from both sides, if I have a digit body and I push it from both sides. Okay, one second. Similarly, suppose there is one rod like that, you are pushing from like what Param is saying. Suppose and push it from both sides with f. What will be the acceleration of center of mass? 2f by m. 2f is the net force on this system. 2f is equal to m into Acm. So Acm will be 2f by m. Simple? Okay, what about the torque about this point, about this center? How much is the torque? It will be 0. Why? Both of the forces are at the same time. This force creates a rotating torque in this way. So that creates in opposite way. So their rotating effect gets cancelled away. So net torque is 0. So just because the net torque is 0 doesn't mean that net force is 0. Getting it? Sir, this is what perpendicular distance is for right. Perpendicular distance from here into the drop of perpendicular on this line of force. So this is the perpendicular distance itself. It is already perpendicular. Okay, clear? Now what if net force is 0? Can the object or digit body rotate? No, it can. So I can flip that force to that side. Then it will get a net torque where net force is 0. I have to understand what he is saying? Yeah. Okay, so suppose you have a rod like this. One force from top like this. One force from below. Equal opposite force? Will this object rotate? Yes. Will it rotate? When you open the... There also you apply equal opposite force. You don't want center of the car to move. So excess of center of mass is 0. Center of mass is not moving. But entire object is rotating. There is some rotation or there is an angular acceleration. Force causes acceleration and angularization. Please write it down. Cause of acceleration is force. Cause of angular acceleration is torque. If net torque is 0, acceleration will be 0. If net torque is 0, angular acceleration will be 0. Is there any doubt? Even if net torque is 0, it will be rotating with uniform angular velocity. Yes. If the net torque is 0, the angular velocity is either 0 or uniform. Because angular acceleration is 0. Omega can still be there. Constant omega. Just like if force is not 0. So if force is 0, there can be constant velocity. So if net torque is 0, angular acceleration is 0. So this point is in equilibrium. Let's say particle is in equilibrium. What we say? Net force is 0. Translational equilibrium. Net force is 0, ACM is 0. Translation won't happen. But can rotation be still happening? Is it possible? Yes, we have just seen it is still possible to have rotation. Are you getting it? So in order to have a rigid body in equilibrium, not only net force should be 0, but also net torque also should be 0. Then only it will not have acceleration of center of mass, nor it will have angular acceleration. So if angular acceleration of center of mass is alpha, what will be the angular acceleration of the tip of the circle? Alpha. Alpha only. It's a rigid body. So alpha of all the points will be same. Now I mean just one type. There is one special type of torque happens. Torque depends on the distance about which you are finding the torque. Two kilometer away from this point, it will be very large compared to the torque of this, which is nearby force from me. Now there is this special scenario with forces, which we have drawn actually, these form couple. So they are called couple. They are special kind of torque. I am not exactly sure spelling is this only, but let's keep it like that. Forces are applied on this three colors I have. Total length is to find the torque center of mass. First find out how much is the torque about the center. I will write it down. Could you unbolo? I will. Properly do it. How about the total torque? No, about the center which is the center. How about the center? Total torque will be at the total. What about the center? Wait. Is it at the center? No, no, no. Is it at the center? It is at the center. But the center is not rotating. About the center, I am finding the torque. But finding the distance of the center from this point is what? What? What? L by 2. Torque because of this force, about this point is how much? To which way? Like this. For that force, about this point is f L by 2. Which direction? It's up to you. Like I said, one direction you can assume positive, one negative. So actually speaking, it should be minus f L. What direction taking positive? Or you can write f L clockwise. Like that you can write. Find out torque which is at a distance of x from here. Torque because of this force is what? Fx. Fx. About that, this comes out to be f 8 to L. Okay, now find out about, what is this point? What is it? Whose phone is it? Whose alarm? How much time did you take? Torque about P. It's going to be f L, like this time, and clockwise. Properly do it. Properly do it. No nonsense approach. Yeah, it's positive. It can't be anything other than the same. What is the torque because of this force? About this point? Minus fx. It will be f into L this way? Sorry, f into x. Okay, so f into x, like this. What about that force torque is? L plus x. But in which direction? It will be like this. So these two are in opposite direction, sense of rotation about this point. So till now this force was causing like that, sense of rotation. But that was any axis that side. About this axis it causes the rotation this way. That is why this becomes in opposite, this is now in opposite direction of that. So you need to subtract this and from that you will get f L only. So you can take any point about any point. You can take here also, anywhere. The torque will be f into L only. So a couple, if net force is zero, will have the same torque about any axis, anything. Is this free for any body? Yeah, why not? Because couples are always in opposite directions. A couple is? Always in opposite directions. Net force should be zero. The definition of couple is two equal to opposite forces. So please write down condition. By center of mass expression should be zero. It should not translate with any expression. Neither it should rotate with any angular expression. Simple conditions. So basically, net force along x axis is zero. Along y axis should be zero. And talk about which axis should be zero. Talk about any axis. Talk about any axis should be zero. Fine? Any axis you take. Now, going forward we are going to study dynamics of the rotational thing. In fact, moment definition was a part of rotation dynamics only. Here, we need to be very careful with how force is applied. What is its distance from whatever axis you are considering or whatever point you are considering and what is the angle it is making with the distance. So when you draw the freeway through whichever point you want as long as direction is fine. You need to not only take care of the direction but also take care of about which point it is applied and how it is applied. Because you need to find the angle. So going forward freeway diagram becomes little bit more involved. And one more thing. When you write net force zero, all the forces will feature in the equation. All the forces will be there. But it will become zero if you are finding about the point through which that force is applied. I am trying to say suppose this is the thing I am solving by the way so suppose I am finding torque about this point and equating it to zero come in the equation. Yes or no? Because torque is to normal actually many. You can make your life simple by choosing an appropriate point about which you equate net torque to be zero. The variable itself, suppose there are three variables two variables are not coming in the torque equation itself. The answer of that variable by writing the torque equation. So net force equal to zero there is no greater. You just write it down. When you write torque equal to zero appropriately choose about which point you are writing the torque equal to zero. And you can write it about any point. Answer will not change. But the amount of time you take will change.