The force between the bat and ball compresses the spring, converting the kinetic energy of motion into potential energy. At the point of maximum compression, the ball momentarily comes to rest. Then, the spring expands, converting potential energy back into kinetic energy. Because the collision is highly inelastic, not all of the kinetic energy is restored, however. A significant fraction of the energy is lost as frictional heat.
Energy is lost to the ball in other ways, as well.
While the bat does deform slightly under the impact, it takes time for that pulse of energy to travel down the length of the bat and back up again, Nathan said. By the time the pulse has returned to the site of impact, the ball is long gone. The hand and muscle actions of the batter at the moment of impact do not even come into play. It may seem counterintuitive, but if the batter could let go of the bat just prior to hitting the ball, there would be no noticeable effect. According to the Professor, whether the collision is as simple as a bat hitting a ball, or more complex like a high-energy subatomic particle bombarding a nucleus, the same physical principles apply.