Throughout history, many designs of perpetual machines (using wheels, weights or magnets) have been proposed but none actually work. Why do all real machines eventually slow down and stop? Explain in terms of work and energy.
A perpetual motion machine is proposed to do useful work forever without any fuel or electricity. However, such machines cannot exist in reality.
Energy is the capacity to do work. For any machine to keep operating, it must have energy. The total mechanical energy of a system, which is the sum of its kinetic energy and potential energy, is what drives its motion.
In real machines, moving parts interact, creating friction. As the machine operates, it must do work against friction.
Unlike work done against some internal forces that can be stored as potential energy, work done against friction does not lead to a storage of energy. This energy is essentially lost from the system's usable mechanical energy.
Consequently, the machine's mechanical energy continuously decreases. Since no energy is stored from overcoming friction, and without an external energy source like fuel or electricity to compensate, the machine loses its capacity to do work, causing it to slow down and eventually stop.
Explanation
The textbook context explains that energy is the capacity to do work, and mechanical energy is the sum of kinetic and potential energy. While a perpetual motion machine is a hypothetical concept that could do useful work forever without fuel or electricity, it fails in reality due to friction. The text explicitly states that 'work done against friction does not lead to a storage of energy.' Therefore, in real machines, mechanical energy is constantly dissipated as work is done against friction, and without an external energy input, the system loses its capacity to do work and stops.