# Physics Question:

Situation:
Two sandbags are attached by a rope to a spindle suspended off the ground 15 feet. When one sandbag is raised to the top and dropped it propels the other sandbag up. When the second sandbag reaches the top it falls and pulls the first sandbag back up. Assume the rope is short enough so that neither sandbag touchest the ground. The process continues, back and forth

Questions:

1) Can the sand bags be made of a certain weight so that the force generated by one falling, pulling the other up and vis versa be continued, to infinity, all other things being equal.

2) If yes to questions 1, what is the weight of the weight of the sandbags which will allow this?

I appreciate any help on this. Thanks.

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Doc Al
Mentor
I don't understand the setup. Do you have a diagram? I'm picturing two sandbags tied together by a short rope with the rope hung over a pulley. Is this correct?
When the second sandbag reaches the top it falls and pulls the first sandbag back up.
Why would it do this?
The process continues, back and forth
Huh?

If this is a problem that was given to you, please state the complete problem exactly as given.

The rope attached to the sandbags is wrapped around the spindle, as one sandbag falls it turns the spindle one direction. When that sandbag reachest the top, gravity pulls the sandbag down, propelling the other to the top and turning the spindle in the other direction.

I'm trying to develop a self propelled windmill where the spindle turns the blades of the windmill. There would be a ratchet like mechanism connecting the spindle to the windmill blades so that when the second sandbag falls and reverses the directions of the spindle the blades continue to turn in the first direction.

Does this clarify the questions?

mgb_phys
Homework Helper
The falling first sandbag pulls the second one up and then the falling second one pulls the first one back up?
Are you trying to build a perpertual motion machine - be warned those things take 'like forever' to test !

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Doc Al
Mentor
Does this clarify the questions?
No, it just repeats what you said before. Why would the sandbags fall in one direction, unless one of them is heavier? Why would they reverse direction?

Sounds to me like you're trying to come up with a perpetual motion machine. Not going to happen.

(And what would be the point of a self-propelled windmill anyway? The usual idea is to have the wind providing the energy, not the other way around.)

yes, sounds like perpetual motion is the correct name for it. Thanks! The purpose is to generate wind to circulate air in a vineyard to reduce the risk of frost.

Think of the spindle as a pully, one bag goes up, the other comes down due to gravity. Is there a weight where the force of the falling bag is enough to propell the other bag completely back to the top where the process is continued?

Doc Al
Mentor
Lots of luck getting that to work! Let us know how it turns out.

Can you explain why it won't work? Or possibly how long it will work before the energy dissipates. I don't need true perpetual motion, just about 8 hours of it to get through the frost hours of the night.

Doc Al
Mentor
You haven't explained why it would work at all, never mind forever. What mechanism will make the sandbags reverse direction?

It won't work forever since it violates both conservation of energy (the first law of thermodynamics) as well as the second law of thermodynamics.

Unless we're picturing something differently than you, one bag's gonna fall and the other's gonna go up, and then they're gonna sit there. For eternity

Unless you stab the lower sandbag and leak out some sand until it's lighter than the higher one, then they'll switch. But that's like saying I can make a windmill that's perpetual, so long as I stand there and perpetually turn the blades myself

Ok, I think i see what you're saying; since they are the same weight the pull of gavity applied to the higher bag is not enough to overcome the pull of gravity to the lower bag, right??

Doc Al
Mentor
Ok, I think i see what you're saying; since they are the same weight the pull of gavity applied to the higher bag is not enough to overcome the pull of gravity to the lower bag, right??
True. But it's even worse than that, since the energy gained by one bag falling is immediately spent in raising the other bag. There's nothing left over to turn the windmill and blow air around. (If you got it moving at all, it would quickly grind to a halt.)

Thanks; looks like I should have taken physics in highschool rather than anatomy. I appreciate the help.