# Perpetual Motion Machine Question

## Main Question or Discussion Point

Say you had a spring with a magnet on top(stuck to the spring). And when the spring is pushed down, in which it will go back up, it will go near another magnet, pushing it down. Pushing it down makes it go back up. Would this work?

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LeonhardEuler
Gold Member
The problem is that there is air resistance and some inelasticity in the spring (tendency to turn some of the potential energy of the spring into heat rather than kinetic energy when it springs back). At some level of compression, the upward force of the spring exactly cancels the downward force of the other magnet, creating 0 force. The contraption will eventually settle down into this state.

:grumpy: You are kidding. I have the spring long enough so when the magnet pushes it down, it wont go immediately up.

You may want to have a bit more patience, 6 minutes isn't a very long time to expect a response in.

Basically everything LE said.

I'll add that even if there were no losses what so ever (no resistance at all) the best you would have is a 100% efficient system - which would keep itself going forever. The moment you try to use it to drive something (adding resistance) efficiency drops, causing the machine to eventually stop.

This topic is banned on PF. That should tell you all you need to know.

:grumpy: You are kidding. I have the spring long enough so when the magnet pushes it down, it wont go immediately up.
So what?

There will be a 'sweet spot' on any system like this where magnet force downwards = spring force upwards. Eventually, thanks to losses the system will come to rest at this point.

LeonhardEuler
Gold Member
Not kidding. Basically, there is potential energy, kinetic energy, and thermal energy.
The second law of thermodynamics essentially means that if you create any thermal energy at all, there is no way to convert it back to another form of energy in a cyclic way with no outside intervention.

To convert thermal energy to work you will either need some source of energy from the outside, some object at a lower temperature than your system to take in heat, or some kind of 'fuel' in your system that will run out eventually.

Why is this topic banned?

Why is this topic banned?
Perpetual Motion Machines are impossible under the laws of thermodynamics - specifically the 2nd law.

Drakkith
Staff Emeritus
Why is this topic banned?
In the current understanding of science, perpetual motion is not possible. Physics Forums rules state that only mainstream science is allowed to be discussed in the forums unless you go to the Skeptecism and Debunking area. If this rule isn't followed, the forum would quickly be swamped in posts that have nothing to do with normal science.

Edit: Just FYI, it isn't that the 2nd law itself prohibits perpetual motion, it's that it simply doesn't work. The 2nd law is merely an observation on how the universe works. WE aren't keeping it from working, the laws or rules or whatever of the universe are.

Your definition of impossible is another mans definition of challenging. Everything is impossible until someone does it. Impossible is a big term built by small people that just want to come up with an explanation of things they can't come up with.

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Your definition of impossible is another mans definition of challenging. Everything is impossible until someone does it. Impossible is a big term built by small people that just want to come up with an explanation of things they can't come up with.
No, in this case impossible is impossible.

Even if you could get rid of all losses you would have a 100% efficient machine - this would only sustain itself, it wouldn't be able to drive anything - because that would be a resistance loss.

To sustain itself and drive another device would require > 100% efficiency. That would machine the machine would have to create energy - that is impossible.

Impossible is impossible? You have no actual proof behind your statement. There are NO impossibilities. String theory allows for endless possibilities.

Impossible is impossible? You have no actual proof behind your statement. There are NO impossibilities. String theory allows for endless possibilities.
Remove my heart and lungs, provide me no external life support and survival my friend, is impossible.

So now we've clarified impossible, let's drop the rubbish.

Your posts are the reason PMM are banned on PF. It is impossible whether you like it or not and my previous post explains why that is so.

But just to restate it, energy cannot be created, therefore you cannot have a device work at > 100% efficiency.

Drakkith
Staff Emeritus
Impossible is impossible? You have no actual proof behind your statement. There are NO impossibilities. String theory allows for endless possibilities.
This is why we have that rule. Under the view of mainstream science, perpetual motion is NOT possible. We have never ever observed anything that would make us believe that it is and have observed literally hundreds of thousands of things that make us think that it is NOT possible. That's the key here. If perpetual motion were possible, it would violate how the universe works as we have observed it. He have zero reason to think it is possible and every reason to think that it is not possible.

Define survival. Your brain would still be working, and if you smash the brain the atoms and molecules still remain. And how do you know it not possible? Have your heart and lungs been removed?

Drakkith
Staff Emeritus
Define survival. Your brain would still be working, and if you smash the brain the atoms and molecules still remain. And how do you know it not possible? Have your heart and lungs been removed?
It isn't possible because of the thousands of people we have witnessed to die after having heart or lung injuries.

berkeman
Mentor
Why is this topic banned?
This is why it is banned. Do you see how much time you've wasted for all of these people? They were trying to help you, but you wouldn't listen. Waste of time.