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. [Pg. No. 135]

All real machines eventually slow down and stop because some of their mechanical energy is always lost due to friction and air resistance. During motion, part of the useful energy is converted into heat and sound energy. Since no machine can create energy on its own continuously, the available mechanical energy keeps decreasing, and finally the machine stops.