What causes machines to stop work?

Answer 1

Friction, or rather, the dissipation of energy into heat causes machines to stop work. As you know, the law of conservation of energy says that the intrinsic energy of the universe (or in a system such as a machine) is always preserved. It doesn't matter what form that energy is in-- whether it be mechanical, potential, kinetic, or frictional (also called heat energy); mathematically, the Joules of energy in a system after some function has been done must always equal the same Joules of energy it started with. And also as you know, everything in real life experiences friction. Friction (as defined by answers.com) is "a force that resists the relative motion or tendency to such motion of two bodies in contact". A machine contains parts that all move and do work on one another (thereby transferring mechanical energy), so therefore it follows that while these individual parts are in contact and moving, there will be friction. This resistance to movement lowers the overall mechanical system (since the velocity of each part is lowered). But where does that mechanical energy go? The answer is it turns into heat caused by friction caused by the parts of the machine moving against one another. This explanation is the reason why a perpetual motion machine will never work.