Why wouldn't this device work?

[DaveC426913]

Now, that would violate energy conservation wouldn't it? Where did its initial energy go?

If I drop a rock on an ice rink, and it stays there by gravity, does that violate conservation of energy? The energy goes into heating and deforming the ball and ground when it comes to a sudden stop.

I'm trying to figure out why anyone thinks the ball would go back up the slide. It would just sit at the bottom.

 

[rcgldr]

Even discounting friction, this device will not go anywhere. If the magnet is weak enough to let the ball fall through the hole when it's closest, then there is no way it's going to be strong enough when it's farthest from the magnet to act upon it. The ball will drop to the bottom by gravity and stay there.

To make this simple, replace the mechanism with a circular loop, like a pair of circular rods that the ball rolls on.

Start off with this system free-falling in outer space, with no friction in the system. Then with any speed, the ball will travel along the path of the loop. Now add a force, like gravity, if the ball's initial position and speed aren't enough, the ball ends up rolling back and forth, but with enough initial total energy, the ball will travel along the path of the loop. Add a magnet on top, and if the magnet isn't too strong, you reduce the amount of initial total energy required for the ball to travel around the loop. If the magnet is too strong, then the initial energy required to travel the loop will be higher.

What you end up with is a ball "orbiting" within the loop, and opposing forces varying the energy between potential (position) and kinetic energy (speed), but without changing the total energy of the ball. Any path that doesn't reduce the energy of the ball with collisions or friction will work in the ideal case where there is no friction or other sources of energy loss.

 

[rcgldr]

I'm trying to figure out why anyone thinks the ball would go back up the slide. It would just sit at the bottom.

As pictured in the diagram it wouldn't go back up, but with a curved lip at the bottom right to re-direct the path of the ball back up the ramp, then it would be the balls momentum that would cause it to initially follow back up the upper ramp.

If you changed the mechanism so the bottom ramp was a half circle, and the top ramp was a horizontal line, then it might be easier to see.

 

[Gokul43201]

If I drop a rock on an ice rink, and it stays there by gravity, does that violate conservation of energy? The energy goes into heating and deforming the ball and ground when it comes to a sudden stop.

Okay, so you too are counting on dissipative forces to cause the machine to fail. I used the term 'friction' to refer to all non-conservative forces (as I explained a little further down in that post).

But there's no reason to design the loop in a way that requires such an inelastic collision, is there?

 

[GotMex?]

Hey, thanks for that explanation Gokul43201. I asked my dad a few days ago and he told me to look up magnetic forces and to take note that it is a conservative force like you say. I did some reading and I was arriving towards the conclusion you give, but you make it very clear now.

My original explanation was that the magnet would be too powerful and would overcome gravity at the top. However, I did not like this explanation for a perpetual motion device because it seems to imply that it is an engineering deficiency, not a violation of the laws of physics.

Explaining it this way shows that even if we were to find a magnet and an arrangement so that the intended cycle was possible, the prescence of friction would still prohibit this machine from working perpetually.

Thanks to all who replied.

 

[Chronos]

What is this, MOND magnetism - magnetic fields get stronger as distance increases? Hilarious. I admit I'm not a fan of any theory that violates the laws of thermodynamics.