times the lever arm of that force)is equal to the rate of change of the angular momentum of that particle, and that the angular momentum of the ith particle its momentum times its momentum lever arm. ow suppose we add the torques Ti for all the particles and call it the total torque T. Then this will be the rate of change of the sum of the angular momenta of all the particles Li, and that defines a new quantity which we call the total angular momentum L. Just as the total momentum of an object is the sum of the momenta of all the parts, so the angular momentum is the sum of the angular momenta of all the parts. Then the rate of change of the total L is the total torque ∑r;=∑ dL:_亚 Now it might seem that the total torque is a complicated thing. There are all those internal forces and all the outside forces to be considered. But, if we take Newton's law of action and reaction to say, not simply that the action and reaction are equal, but also that they are directed exactly oppositely along the same line Newton may or may not actually have said this, but he tacitly assumed it), then the two torques on the reacting objects, due to their mutual interaction, will be ite because the lever arms for any axis are equal. Therefore the internal torques balance out pair by pair, and so we have the remarkable theorem that the rate of change of the total angular momentum about any axis is equal to the external torque about that axis. dL/dr Thus we have a very powerful theorem concerning the motion of large collections of particles, which permits us to study the over-all motion without having to look at the detailed machinery inside. This theorem is true for any collection of objects, hether they form a rigid One extremely important case of the above theorem is the law of conservation of angular momentum: if no external torques act upon a system of particles, the angular momentum remains constant. A special case of great importance is that of a rigid body, that is, an object of a definite shape that is just turning around. Consider an object that is fixed in its geometrical dimensions, and which is rotating about a fixed axis. Various parts of the object bear the same relationship to one another at all times. Now let us try to find the total angular momentum of this object. If the mass of one of its particles is mi, and its position or location is at (xi, yi), then the problem is to find the angular momentum of that particle, because the total angular momentum is the sum of the angular momenta of all such particles in the body. For an object going around in a circle, the angular momentum, of course, is the mass times the velocity times the distance from the axis, and the velocity is equal to the angular velocity times the distance from the axis (18.20) or, summing over all the particles i, we get L=Ia where This is the analog of the law that the momentum is mass times velocit Velocity is replaced by angular velocity, and that the m replaced by a new thing which we call the moment of inertia I, which is analogous to the mass Equations(18. 21)and (18. 22) say that a body has inertia for turning which depends not just on the masses, but on how far away they are from the axis. So if we hav two objects of the same mass, when we put the masses farther away from the axis, the inertia for turning will be higher. This is easily demonstrated by the apparatus