Why are perpetual motion machines impossible?
In: Physics
The other answer is excellent, but just jumping in for a ELI5 answer.
Energy cannot be created or destroyed.
Let’s say that to run the machine, you need to put in 100 units of energy. To do anything useful, (like using the machine to power your lights or whatever) you would need 100 units of energy to make the machine go, plus 20 units for the lights.
So. 100 in. 120 out.
Where’s the extra 20 come from? Nowhere. It can’t come from anywhere. There’s no other source of energy.
So that doesn’t work as a generator.
So what about just as an experiment. You want to run a machine just to power itself and nothing else. Pure perpetual motion.
So. 100 units of energy in. Take the 100 units that you get out and use them to keep running the machine. Right?
Wrong.
No energy transfer is perfect. Moving parts lose some of their energy, mostly as heat to the air or with friction between the parts (rub you hands, they’ll heat up). Can you hear the machine running? That’s some of the energy coming out as sound too.
Even with a great, highly efficient engine you’ll only get 80 units out for every 100 you put in.
So. 100 in. 80 out. Not enough to run the machine. It stops.
Some people have built pretend perpetual motion machines that can actually run for a little while by doing this. But not forever. They always stop.
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