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Perpetual motion (please just give me a chance, it's just a few questions)

  1. Aug 18, 2011 #1
    I realise just from the title a lot of people are going to shoot me down very quickly, but these are just questions to help my basic understanding. I'm definitely not a physics buff.

    I just want to know a couple of things pertaining to gravity and permanent magnets.

    Technically speaking are these 2 forces not endless in their possibly energy? Presuming a permanent magnet truly is as it says on the label; permanent; then why can't they be used as an endless energy supply in some form or another?

    The same question for gravity?

    I realise that perpetual motion in theory is impossible as energy must be lost somewhere and so it would eventually have to stop. But if there are endless amounts of energy, why can't they be constantly re-used?

    I'm looking for a better understanding of the mechanics of all this. It just doesn't make sense to me as to why these forces cannot be harnessed in a way that makes them viable for some form of a perpetual motion device.

    If it could be explained without someone saying something like, 'Newtons 1st law states....' that would be great.

    I'm looking for an explanation as to why these forces in one way or another can't be used as endless supply of energy or as a form of perpetual motion.

    I'm not trying to claim they can be either, I just want to know why not.

    Like I said just trying to increase my understanding of the matter so any help and advice that could improve my understanding would be greatly appreciated.
  2. jcsd
  3. Aug 18, 2011 #2


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    You might think of it in practical terms rather than a problem of "physics rules."

    Suppose you have gravitational potential energy in some form you can harness, like water above a dam. You can let the water down and extract some energy (using turbines or waterwheels or whatever you like), but when you're done, the water is below the dam. To repeat this cycle you would have to spend energy to raise the water back up again, storing gravitational potential energy once again.

    The same idea would apply for magnets.

    So, no perpetual motion machine.
  4. Aug 18, 2011 #3


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    The problem with perpetual motion machines is that whilst perpetual motion could be technically possible (i.e. a perpetually rotating magnet in a vacuum) this does not produce any energy, it only stores it. The moment you try to extract any energy you do just that and eventually there is none left to tap.
  5. Aug 18, 2011 #4


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    Your basic misunderstanding comes from the fact that force and energy are completely different things. So the question about a force being an endless energy is just nonsensical.
  6. Aug 18, 2011 #5


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  7. Aug 18, 2011 #6
    This much makes complete sense to me. However why can't magnets be used to pull say a ball back up to the top again.

    Is there no way of getting the ball back up to the top whilst still getting gravity to at some point make it drop again?

    Do angles make a difference to the strength of gravity for instance?

    The picture I have in my head is of basically a triangular tubing of sorts, with a metal ball inside.

    A right angled triangle with the right angle being at the top. It doesn't necessarily have to be right angled but it makes it easier to visualise. So the gravity part is used when the ball hits the right angle and would thus drop down.

    Now is there no way of getting the ball from the bottom (where it has just hit after dropping from the right angle) up to the third corner of the triangle along the hypotenuse by using magnets, but still getting the ball to drop when it hits the top, then rolling back down to the right angle and dropping, thus starting again.

    Finding it hard to explain precisely what I mean.

    I understand the ideas of perpetual motion and the reasonings as to why it shouldn't work. But if the ball is able to drop when it hits the top, wouldn't the whole process start again, and then you would have perpetual motion?

    As long as you can get the ball to drop when it hits the top, the process would start all over again, wouldn't it?
  8. Aug 18, 2011 #7


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    Magnets act only over a very short distance, so no.

    The path you are going down here is to construct a scenario jut complex enough that you can't find your mistakes, which will then convince you you've discovered perpetual motion.
  9. Aug 18, 2011 #8
    Why would it have to be a long distance for it to be dragged up a triangular shape?

    As long as there is some mechanism at the top to let the ball drop, everything should start all over again?

    Perhaps the force with which the ball is attracted to the magnet could be enough to trigger a mechanism that would release the ball. And if the magnet is permanent, then the force with which the ball is attracted to it should always be the same?
  10. Aug 18, 2011 #9
    But it really isn't complex, it couldn't really be much simpler....

    As long as you can release the ball the at the top, everything would start again. It would just be a matter of releasing the ball, wouldn't it?

    And I didn't state I had discovered perpetual motion, in fact I'm positive I haven't. I just want to figure out why.
  11. Aug 18, 2011 #10


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    I don't mean to be rude here, but is there anything we can say to convince you that what you are looking for does not exist or do you just intend to give us an endless series of examples to debunk?
  12. Aug 18, 2011 #11


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    How would you release the ball though? The real reason it won't work is because just like gravity, magnetic fields are conservative. So, it will always take just as much energy to remove something from a magnet that it is stuck to as was gained when the object was initially attracted. No matter what combination of gravity and magnetic fields you use, the end result will be the same.
  13. Aug 18, 2011 #12


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    How long is the sloped side of the triangle?

    Is the magnet really pulling the ball or just holding it to something else that pulls it?
  14. Aug 18, 2011 #13


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    There is. we can walk him through it until he sees for himself that the energy needed to be put into the system is always at least as much as is gotten out of the system. He needs to see that chain of logic himself.

    He's not demanding he's right, he's simply asking where he's gone wrong.
  15. Aug 18, 2011 #14


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    I'm sure if we explain it to trebe in a manner fit for his education he will see.

    The problem is you will never have a system where the forces don't balance out. Consider this idea, we have a metal ball in a tube with a magnet at the top. We drop the ball that falls because of gravity, the magnet is there to pull it back up so that gravity can pull it down again. But this will never work, the reason is that either the magnet or gravity will be stronger or they will be equal. If they are equal then the ball wont move.

    Do you see now? (p.s don't be disheartened, everyone learns at some point)
  16. Aug 18, 2011 #15
    It did come across as slightly rude. I only gave one example as far as I'm aware, my bad if it came across differently. It was a genuine quest to increase my knowledge. I'm not trying to claim I'm right I want to understand why I am wrong, the best way of doing that is to question answers in my opinion.

    Edit: dave said it for me, thanks you.

    Thanks, that pretty much sums it up I think.

    So basically it would always take at least the same amount of energy to release the magnet as it would to get the ball attracted to it?

    Does gravity have a large effect on a magnet. In the sense that if magnet is attracting a ball along an incline of say 45 degrees or 75 degrees but is travelling the same distance is there any difference in the pull of the magnet and how it is effected by gravity?

    So if a ball is being magnetically attracted 2cm up a 45 degree angle or a ball being attracted 2cm up a 75 degree angle (so they are travelling the same distance along a surface just at different slopes), is there any difference in how gravity plays a part?

    Will the ball be attracted exactly the same, so they would both travel the same speed or would the one with the steeper angle have more difficulty attracting the ball due to gravity?

    Second edit: could people please not belittle my education standards either. I did a maths degree at a good university. These are just ponderings I'm having as I find the subject interesting.

    I understand the basics to a level, there are just some aspects that have me wondering. I personally think thoughtful musings are important. Even if they aren't up to some peoples standards.
    Last edited: Aug 18, 2011
  17. Aug 18, 2011 #16


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    Yes. I'd refer you to my post directly above. In this kind of situation there are only three options;

    Force A is stronger than Force B
    Force B is stronger than Force A
    Force A and Force B are equal

    In the first two circumstances the same amount of energy is required to reset the system (with a hypothetical perfect system with no losses this reset will bring you back up to zero energy gained and lost. In reality no system is perfect so you will have a net loss of energy if you try this). In the latter circumstance no energy is produced, everything is static.
    Last edited: Aug 18, 2011
  18. Aug 18, 2011 #17
    If the magnet is strong enough to pull the ball to the top of the slope then it will hold it in place.There's always a catch.
  19. Aug 18, 2011 #18


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    There has already been too much talk of your specific ideas. This is strictly aginst forum policy.

    At this point, I would like to see you stop "talking" and start building. When you have a working system then you can laugh at us. Until then the disscussion is over.

    Well understood and proven theory says it cannot work. Sooo...Do your best to prove us wrong, not with words but action.

    Thread locked.
  20. Aug 19, 2011 #19


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    Just to add to what Integral said, in the list of banned subjects (in the Rules link at the top of the page), we try to list some example locked threads where the now-banned subject has been discussed previously. This can help PF users to see what some of the typical discussion points are, and can help users to understand why the subject is banned.

    I've updated the part of the Rules about PMMs to include some links to recent locked discussion threads (including this one):

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