Why can't a perpetual motion machine exist?

[jonlin]

I'm bad at physics.
Is it just energy or something?

 

[kcl]

The short answer is that energy cannot be created or destroyed, so there is no source for the extra kinetic energy necessary for perpetual motion.

Think of it like this:
You have a U shaped ramp, with equal height on both sides. (like a half pipe) You roll a ball down one side. Because the ball only gained x amount of kinetic energy while rolling down the first half, it cannot exert more than x amount while rolling up the second half.

If the ball could somehow gain more than x kinetic energy on the way up the ramp, it could be turned into perpetual motion.

Hopefully that jibberish makes some sense, I'm way too tired to be answering this lol

 

[IRNjuggle28]

The short answer is that energy cannot be created or destroyed, so there is no source for the extra kinetic energy necessary for perpetual motion.

Think of it like this:
You have a U shaped ramp, with equal height on both sides. (like a half pipe) You roll a ball down one side. Because the ball only gained x amount of kinetic energy while rolling down the first half, it cannot exert more than x amount while rolling up the second half.

If the ball could somehow gain more than x kinetic energy on the way up the ramp, it could be turned into perpetual motion.

Hopefully that jibberish makes some sense, I'm way too tired to be answering this lol

disclaimer: I know nothing about physics.

Why would energy need to be created in order for motion to be perpetual? As long as it wasn't destroyed or transfered, an initial imput of energy could last indefinitely.

(I believe you that a perpetual motion machine can't exist, I just don't understand your explanation for why)

 

[Dene]

Wikipedia is helpful sometimes.

 

[kcl]

disclaimer: I know nothing about physics.

Why would energy need to be created in order for motion to be perpetual? As long as it wasn't destroyed or transfered, an initial imput of energy could last indefinitely.

(I believe you that a perpetual motion machine can't exist, I just don't understand your explanation for why)

The energy is always misplaced in some form. For example if the ball makes noise, that's energy. If the ball creates heat, that's energy.

 

[goodatthis]

It's basically because energy needed for the machine to run is slowly dissipated over time, in some way or another. An example is a pendulum, some pendulums can run for a very long time, but if you have x amount of newtons of air pressure that is subtracted from the amount of energy the upwards swinging pendulum is creating, you have less and less energy until it is almost nothing. If you roll a ball down a 100 foot ramp, and then let it roll on a completely smooth level surface for as long as it will go, it will eventually stop because the potential energy the ball had at the top of the ramp is transformed into kinetic energy which is lost by friction or air resistance.

Also thus concept is why you physically cannot have a completely self contained energy source, that is, something like a generator which produces energy which turns the arm of another generator, feeding back into the original, and slowly creating more power from the excess energy. This would be an example of creating energy, which cannot happen.

 

[JediJupiter]

In a perfect environment, a barrel could go up and down the ramp that kclejeune described indefinitely, but that kind of environment is currently impossible for us to create. Even if we did, we couldn't use it to get infinite energy because we would then be taking energy from it and slowing it down.

 

[Deathranger999]

Well theoretically it could. In a vacuum, unaffected by other forces such as gravity. The main problem is friction. As an object moves around, it touches other things, whether other objects, liquids, or just air. Whatever it touches will take some of its kinetic energy. Eventually the system will stop moving as that kinetic energy gets dispersed to elsewhere.

 

[cyberbillp]

Waste heat is your answer.

Heat is energy and when it's radiated, it's gone from the system. Heat will come from friction of the bearings or even friction with the air etc.