# Buoyancy and Perpetual Motion

1. Jan 14, 2009

### myanmar

perhaps you can help me.
on http://www.lhup.edu/~dsimanek/museum/themes/buoyant.htm fundamentally, what is the reason that the picture under the words "buoyant wheels and belts" doesn't work? (Except remove the movement of water between two buckets. replace that with compressing and expanding air)

2. Jan 14, 2009

Staff Emeritus
The last paragraph on that page wasn't clear? What was confusing about it?

3. Jan 14, 2009

### myanmar

That almost sounds like saying
"It wont work because of the conservation of energy".
which I understand.

I just don't see where the flaw is that makes it stop working.

4. Jan 14, 2009

Staff Emeritus
But conservation of energy is the flaw that makes it stop working.

5. Jan 14, 2009

### myanmar

While this is certainly true, we should be able to prove this without relying on the conservation of energy.

That is, there is somewhere in the system where energy is lost or something I have ignored.

6. Jan 14, 2009

### Staff: Mentor

Can you prove you were driving under the speed limit without citing the speed limit?

Conservation of energy isn't just a law that we go back and check to see if it was violated after doing a problem, it is a tool used to do problems. When doing a problem like this, you write a conservation of energy statement: Eout - Ein = 0 ....and see if it works.

Last edited: Jan 14, 2009
7. Jan 14, 2009

Staff Emeritus
The point is that you don't need to lose energy. You show that when you move the machine from one position to the next and you recover the same configuration. The energy must be the same (ignoring dispersive forces), so you can't extract energy out. When you include dispersive forces - any ones, anywhere - it can only act to slow things down.

8. Jan 14, 2009

### vanesch

Staff Emeritus
That said, it can be interesting to analyze in detail how such a machine actually behaves, and where exactly it doesn't behave as its inventor claims. Analysing PMM *can* be fun. I think that was what the OP was asking for.

9. Jan 14, 2009

### Staff: Mentor

There is nothing magical about buoyancy, it is fundamentally no different than rolling a rock down hill. Both cconvert gravitational potential energy into work. When you go to roll the rock back up the hill it takes at least as much energy as you got out. In buoyancy the "rock" is the fluid. Rolling it downhill can give you work (like a water wheel) but as soon as you try to roll it back uphill you lose all of the energy and more.

10. Jan 14, 2009

### Mapes

Of course, the committed (and I mean committed) inventor may just change a few details and say "OK, now it'll work!"

11. Jan 14, 2009

### stewartcs

Irreversibilities.

CS

12. Jan 14, 2009

### LURCH

Perhaps the mechanism you are looking for is the buoyancy of the air that must be forced down from the piston at the top of the belt to the piston at the bottom. This buoyancy can be understood as the difference in water pressure between the top of the belt and the bottom. In order for the topmost piston to push in, the lowest piston must be forced to expand. However, this lowest piston will resist expansion due to this difference in pressure. This is the same resistance encountered any time one tries to push air (or any buoyant object or substance) downward into deeper water.

This resistance will be equal to the rotational force supplied by the overall process. Since the two forces are equal and opposite, the machine remains still.

Is that the sort of answer you are looking for?

13. Jan 14, 2009

### mgb_phys

I did like the fact that the machine in the link was patented - in 1976!
I can understand the patents from 18xx when perhaps the patent office wasn't quite as stocked with greatest minds of it's generation and technical education was a little lacking.
But 1976 - surely they must have had a memo with "Remember no perpetual motion machines" by then!

14. Jan 14, 2009

I think back then we didn't have those volumes of useless patent applications. Today they don't really care if it works, because then there will be no claims either.

15. Jan 15, 2009

### myanmar

LURCH, this is exactly the kind of answer I'm looking for.

However, wouldn't adding more cups (or making them bigger) increase the rotational force and make this irrelvant?

If not, explain.

16. Jan 15, 2009

### Carid

Yes, and by 1902 there was that German chap working in the Berne Federal Office for Intellectual property who obviously couldn't concentrate on the matter in hand.

17. Jan 15, 2009

### Staff: Mentor

What happens here is the same every time: the "committedd inventor" increases the complexity of the device until he/she is no longer able to analyze/evaluate what is happening, and then assumes incorrectly, "OK, now it'll work!"

18. Jan 15, 2009

### Staff: Mentor

Of course not. More or bigger cups means more work is required to push the air down (and lift the water up).

19. Jan 18, 2009

### LURCH

LOL! I know; a lot of these devices look like, "just gimme some time to work on it. If I make the weights heavier, or the air pockets larger, I'm sure I can get it to work." Sadly, however, anything you do to increase the force also increases the counterforce. For example; if you made the cups twice as large, you would double the surface area on which pressure is acting against the cup at the bottom of the belt. This of course would double the buoyant force preventing you from moving air downward (or you could say it doubles the amount of air we're trying to push down into deeper water). Using twice as many cups would have the exact same effect.