A perpetual motion machine is a machine which, given an initial amount of energy initiates its designed motion and continues to move using just that given amount of energy forever.
For example, a simple pendulum can be considered a machine that converts kinetic energy into potential energy and back again. Given an initial displacement of the bob, the bob will swing back and forth for a very long time. However, this motion is not perpetual since (energy) losses due to friction and air resistance will eventually reduce the amplitude of the swing back to rest (after a very, very, very long time.)
A perpetual motion machine violates the 2nd law of thermodynamics.
If the first law says "all energy is accountable -- so you can't win", meaning you can't get more energy out of a system than you put into it, then the 2nd law says "there is a price to pay in any energy conversion -- so you can't break even", meaning that the energy you put in will eventually dissipate, so for ANY machine to continue to operate indefinitely requires a constant input of energy to replace that lost due to the conversion cost (operating inefficiency.)
For example, a simple pendulum can be considered a machine that converts kinetic energy into potential energy and back again. Given an initial displacement of the bob, the bob will swing back and forth for a very long time. However, this motion is not perpetual since (energy) losses due to friction and air resistance will eventually reduce the amplitude of the swing back to rest (after a very, very, very long time.)
A perpetual motion machine violates the 2nd law of thermodynamics.
If the first law says "all energy is accountable -- so you can't win", meaning you can't get more energy out of a system than you put into it, then the 2nd law says "there is a price to pay in any energy conversion -- so you can't break even", meaning that the energy you put in will eventually dissipate, so for ANY machine to continue to operate indefinitely requires a constant input of energy to replace that lost due to the conversion cost (operating inefficiency.)
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Any kind of invention that says "power the motor from a generator on the wheels" or "run your car from a wind turbine on the roof" or "unlimited energy from water" is probably a perpetual motion machine. Some of them violate the first law of thermodynamics, not just the second.