In Fig. 8.23b, the kinetic energy of the Ping-Pong ball is larger after its interaction with the bowling ball than before. From where does the extra energy come? Describe the event in terms of conservation of energy.

The sum of kinetic, potential and internal energies is always conserved.

$\mathbf{∆}\mathit{K}\mathbf{+}\mathbf{∆}\mathit{U}\mathbf{+}{\mathit{U}}_{\mathbf{i}\mathbf{n}\mathbf{t}}\mathbf{=}\mathbf{0}$

Figure 8.23b

From the conservation of energy, we know the sum of the total energy of the system is always conserved. The velocity of the bowling ball before and after the collision is almost equal but it is exactly not equal, so there must be a small change in the kinetic energy of the bowling ball due to this change of velocity.

From the conservation of energy, this kinetic energy is gained by the ping-pong ball. Since the mass of the ping-pong ball is less, so small change in the kinetic energy of the ping-pong ball cause a large change in the velocity of this ball.

Hence, we can conclude that this extra kinetic energy of the ping pong ball has come from the bowling ball.