Why perpetual motion machine wouldn't work.

Main Question or Discussion Point

I'm trying to explain to my friend why the following would not work, but embarrassingly, I'm having trouble.

You have a ramp that allows a ball bearing to roll up, and a strong magnet at the top which is able to pull the ball up the ramp. But there is a hole halfway up the ramp, and so the ball falls through and goes back to the start.

Obviously this wouldn't really be a perpetual motion machine, but I can't explain why not. I can say thinks like "friction" or "magnets run out", but I don't feel like they are very rigorous or satisfactory answers.

Related Other Physics Topics News on Phys.org
If the magnet is strong enough to pull the ball up, it does so with or without the ramp and thus a hole in the ramp is no obstacle.

DrClaude
Mentor
Quick guess: when the ball falls through the hole, it is still attracted by the magnet. Therefore, it will go down slower than if it was just rolling down a slope without a magnet. It will have less speed going down and therefore will not go up as much as if the magnet was not there. As it loses energy to friction and air resistance, it will not be able to reach the hole anymore.

In summary: I think that wath the magnet giveth, the magnet taketh away.

Edit: in light of what voko wrote, I have to clarify that I was thinking that the ball is already rolling and that the magnet is only there to give it a little extra tug.

A.T.
If the magnet is strong enough to pull the ball up, it does so with or without the ramp and thus a hole in the ramp is no obstacle.
Wrong. You need less force to pull something up an incline, than pulling it up directly without the incline. That is why humans use ramps for thousands of years.

A.T.
You have a ramp that allows a ball bearing to roll up, and a strong magnet at the top which is able to pull the ball up the ramp. But there is a hole halfway up the ramp, and so the ball falls through and goes back to the start.
It can fall through the hole, but cannot go back to the start. Depending on the details of the design it will get stuck somewhere, where the sum of gravitational and magnetic potential is minimal.

UltrafastPED
Gold Member
Magnets don't run out, though it is possible to ruin them by banging or heating, etc.

If the magnet is strong enough to pull the ball up the ramp, then it has already lifted it against gravity to where the hole is. Of course it is pulling up the ramp, so the force of gravity is reduced (draw the force diagrams!).

Now the ball reaches the hole; if the magnet is really strong the ball is going so fast that it hops across with a slight bump. But suppose the magnet is weak enough (or the ball heavy enough) that it will fall down the hole (draw the force diagrams required for this to be true!).

Now when the ball reaches the bottom of the hole it is supposed to fall down a second ramp which should lead it to the first one (I'm imagining something like Mousetrap, the game - but you must draw this in some detail).

I am here interested in this second ramp: it clearly has a slope that is gentler than the up-bound ramp, because it starts at the bottom of the hole.

Now draw the force diagram for the ball at the top of this second ramp. Since the downward force for gravity is the same everywhere, the effective lift required to move against it can be determined from the slope of the ramp.

You can also say something about the force from the magnet: it has to be large enough at the bottom of the first ramp to get the ball rolling, and then increases (1/r^2, approximately, from Coulomb's law for magnets). You also know that the magnetic force at the top of the hole is insufficient to keep the ball going, and is not sufficient to keep it from falling down the hole (by assumption!) ... so once again you have a limit on the force vector from the magnet, in this case in the vertical component at the top of the hole.

Having analyzed all of the parts, and established the limits of the magnetic force at various points along the way, for your final step you must draw the force diagram for the ball rolling down the lower ramp. Since at the end it must come out level with the up-bound ramp, or be higher up, but with less slope, it seems to me that the magnet will impede the downward rolling motion of the ball ... and it will freeze up on the way down.

You should be able to show this with the force diagrams, no matter what the design of the contraption is.

You can also construct a working model, and let your friend try different sized ball bearings, and reposition a very strong magnet as required to get the ball moving.

Note that we don't rely on friction or failing magnets here.

@A.T. that would be because a ramp or hill of any kind and any angle still provides some " ground" to sit on while there being nothing provides nothing and the uplifting force has to entirely support the object it is trying to get up.

@OP ,Well normally the way you describe it it wouldn't work that' s for sure.Things might be different if you could some how shield the magnetic force in different parts of the mechanism and switch the magnet itself on and off when needed.
But controlling a magnet in this way would make it an electromagnet as you cannot (sadly) switch on or off permanent ones.
An electromagnet uses current flow when switched on and current flow takes power , certain amount depending on the setup etc.So no longer a PMM.

Even if you could do all the actions I mentioned with a permanent magnet and even if this all setup would provide some motion without extra energy added , still in real terms nothing is perpetual , elements tend to wear out and break down etc.

@UltrafastPED the only way something like this could work is if you would be able to totally shield off the magnetic force after the ball rolls into the entrance on it' s way down , then it falls down hit all the other balls and pushes the first ball slightly upwards so that the magnetic force takes over and starts to pull it upwards , as there has to be a point at which the magnet is close enough to overcome the gravitational attraction of earth which tried to keep the ball down.
If this setup could even work it would have to do with very precise angles and lengths from one component to the other.Ofcourse without the possibility to extract any useful power.
But I'm not so sure it sounds to me that there is something missing in what I just said.

Well the one thing that makes anyone with a reasonable mind say they have some fatal flaws is that none of them have been brought up to the major public and approved to be working , ofcourse then the " free energy agenda" kicks in to tell you that they have been surpressed by the FBI, NSA, Bill Clinton and James Bond personally. :D:D

Wrong. You need less force to pull something up an incline, than pulling it up directly without the incline. That is why humans use ramps for thousands of years.
I agree with the principle. The principle, however, is that you need less force if the force is aligned with the ramp, compared to a force that acts vertically. I initially understood the contraption had the magnet arranged over the ramp. If the ramp leads to the magnet, then indeed a magnet could set the ball in motion even if it is not strong enough to move it vertically.

As explained by others, the magnet will both accelerate and decelerate the ball, so there will not be a net gain in energy. As such, the system is equivalent to a system without the magnet, whose initial velocity is just enough to reach the hole; without friction, such a system would have the ball in perpetual motion (but it would not do any useful work).

UltrafastPED
Gold Member
@UltrafastPED the only way something like this could work is if you would be able to totally shield off the magnetic force after the ball rolls into the entrance on it' s way down , then it falls down hit all the other balls and pushes the first ball slightly upwards so that the magnetic force takes over and starts to pull it upwards , as there has to be a point at which the magnet is close enough to overcome the gravitational attraction of earth which tried to keep the ball down

Your suggestion reminds me of the title page for Simon Stevin's "Mechanics"

See the image at http://www.lhup.edu/~dsimanek/museum/unwork.htm#stevinprob

Simon Stevin explained that this "ball chain" system was unchanged when you advance each ball by one position ... the first example of the "Principle of Virtual Work" ... if there is no change in the system, then there was no work done or extracted.

For fun see: http://www.lhup.edu/~DSimanek/museum/models/build-pm.htm
and http://www.lhup.edu/~dsimanek/museum/unwork.htm

sophiecentaur
Gold Member
@A.T. that would be because a ramp or hill of any kind and any angle still provides some " ground" to sit on while there being nothing provides nothing and the uplifting force has to entirely support the object it is trying to get up.

@OP ,Well normally the way you describe it it wouldn't work that' s for sure.Things might be different if you could some how shield the magnetic force in different parts of the mechanism and switch the magnet itself on and off when needed.
But controlling a magnet in this way would make it an electromagnet as you cannot (sadly) switch on or off permanent ones.
An electromagnet uses current flow when switched on and current flow takes power , certain amount depending on the setup etc.So no longer a PMM.

Even if you could do all the actions I mentioned with a permanent magnet and even if this all setup would provide some motion without extra energy added , still in real terms nothing is perpetual , elements tend to wear out and break down etc.

That power can be made arbitrarily small and doesn't affect the PMM argument any more than the other 'friction' losses. But what does relate to the PMM idea is that setting up the magnetgic field, in the presence of the steel ball involves a different amount of energy from setting it up without the ball. That extra energy is 'just' what's needed to make this thought machine work.
I must say that I fully support the PF attitude to PMMs. They are really not worth discussing, yet so many people seem to think they have found a 'loophole' and that their particular version will work. We really shouldn't encourage them.

Thanks everyone. I agree that they are not really worth discussing, but explaining why they don't work to my friend is useful I think.

Sophie could you say a little more about the extra energy needed to set up, as this seems to be the crux?

sophiecentaur