This is an example problem I am studying from a classical mechanics textbook. Wrap a light, flexible cable around a solid cylinder with mass M and radius R. The cylinder rotates with negligible friction about a stationary horizontal axis. Tie the free end of the cable to a block of mass m and release the object with no initial velocity at a distance h above the floor. As the block falls, the cable unwinds without stretching or slipping, turning the cylinder. Find the speed of the falling block and the angular speed of the cylinder just as the block strikes the floor. The solution starts as follows: The cable doesn't slip and friction does no work. The cable does no net work; at its upper end the force and displacement are in the same direction, and at its lower end they are in opposite directions. Thus the total work done by the two ends of the cable is zero. Hence, only gravity does work, and so mechanical energy is conserved. ... ... ... I am having trouble with the underlined part. The solution considers only the tension forces exerted by the upper and lower ends of the cable. Is this because the internal tension forces exerted by the inner parts of the cable cancel each other out (i.e. the work done by those forces equals zero)?