This is because each ball has lost the same net PE, so has gained the same KE and thus the same speed. No, the energy isn t lost. It just isn t all transformed into a useful form of energy. Often the wasted energy is turned into heat. It is merely transformed from one kind of energy to another.
Nuclear bonds store a tremendous amount of energy.
The speeds will be the same. Both balls start with the same potential and the same kinetic energies. Thus the total energy is the same and must remain the same. Thus it will be able to do twice the work on the mud. We assume the force of the mud on the stone is constant.