When the stone strikes a target, the kinetic energy becomes heat energy. The law of conservation of energy states that the amount of energy in the universe is always the same. It can neither be increased or lessened. You can get only as much energy out of a machine as you put into it. Usually, you cannot even get this amount of energy out of any machine in a usable form.
Much useful energy is wasted by friction in the form of heat.
Friction will cause any machine to run down eventually. The pendulum is one example of the way energy changes and is conserved. When a pendulum reaches the top of its swing, it has potential energy because of its position. As the pendulum swings through the lowest point of its swing, it has only kinetic energy. This energy becomes potential energy as the pendulum again reaches the top of its swing.