Shoutbox

You guys are looking too into it.. this ins't a college problem. It's a simple physics problem.

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Originally posted by CookieRevised
dunno, why not.... does that matter that you connect it? it doesn't add or substract any more 'force' on one side compared to the other side, does it?

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Well yeah it does, in fact it changes the whole system.
Think about it after a sleep.

Close your eyes and count SonicSams

P.S. perpetual motion is possible in theoretical physics where such things as frictionless planes exist.

How does it change the system? You simply add like 10kg to one end and 10kg to the other so to speak... you simply add the same pulling force to both ends, so it would be as if it would never be there as it would cancel out anyways in those equations. It's like saying x/y wouldn't be the same as x+10 / y+10.... (x and y being those formulas or whatever you have in above posts). And come to think of it, that even goes no matter if there is friction or not!

Or take both ends of such a chain in your hands and let yourself hang... you aren't going to move all of sudden, the chain isn't going to pull you up on one side; you would hang there motionless (assuming you would weight the same or less than the chain).

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Originally posted by gif83
P.S. perpetual motion is possible in theoretical physics where such things as frictionless planes exist.

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yeah, but friction or not, the chain wouldn't move, even in theory. I think the frictionless thingie is just to throw you off or something...




blah... going to sleep now before I change my mind again

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Originally posted by gif83
Close your eyes and count SonicSams

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aaaaaarrgggghh noooooooo

EDIT: I just thought of another way of seeing it and prove why it wouldn't move (you get that when you're counting SonicSams ): look at the two limits of the system. In the first limit example, the two planes have an infinite small angle (aka flat): the chain wouldn't move, no matter how long one end is, and friction or not. In the other limit example, the two planes are both 90°, which means both ends of the chain simple hang strait down from a point right in the middle of the chain, again, no movement, no matter if there is friction or not.... back to counting SonicSams now...

With no friction a wheel would continue turning in a vacuum indefinitely (even though there's gravity). A chain (or rather a smooth string with no ridges) on a spindle would do the same in the same conditions. Unintuitive things happen when you eliminate fundamental principles of physics, doesn't necessarily make the model useless, just limited.

Lol cookie, perpetual motion is possible in a frictionless environment, the problem is in real life there isn't such a thing Though, supercooled materials are pretty interesting in that area.  (plus, there are many other factors which make perpetual motion inconceivable)

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Originally posted by Volv
With no friction a wheel would continue turning in a vacuum indefinitely (even though there's gravity).

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But only if there is a force which starts the motion though, and that's only to begin with. With the chain system depicted above, there isn't such an 'starting' force.

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Originally posted by vaccination
Lol cookie, perpetual motion is possible in a frictionless environment,

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Perpetual motion machines aren't possible, not even in theory, not even in a frictionless environment since there should always be an equilibrium of energy. And a perpetual motion machine is exactly based on the nonequilibrium of energy (being it motion energy, heat energy or whatever energy): it produces more energy than you put into it, that's the whole definition of such machines.

The fact that it is said that a perpetual motion machine is not possible in practice is not _just_ because of friction, but mostly because you can not get more energy out of something than you put into it (while preserving its mass). friction is only a tiny part of the problem of perpetual motion machines. Cancelling that out and you would still have the same problems as before: you can not get more energy out of it than you have put into it.

So, forgetting the fact that it isn't possible to completely rule out friction (even with supercooled, superconductors or whatever), thus in a theoretical friction free world, the moment you want to extract energy out of such a perpetual motion machine means that you will eventually loose the stored energy in that machine. And therefore it isn't a real perpetual motion machine to begin with as it will eventually run out.

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There is no more (or less) force pulling on the chain on the left side than there is on the right side, ergo, there is nothing to start movement, ergo there is no movement, even in a theoretical frictionless system, no matter what angle or length the two slopes are, as long as both ends are on the same horizontal plane. The system is always in equilibrium.

I'm (almost) sure that if you take those equations from above and work them further out you would see they are equal (you can in theory because there is always a direct relation between the various elements of the two sides (mass, length, angle, etc), by using ratios instead of real numbers)). The more I think about it the more I am convinced of that.

I've got a practical experiment for you to try out.

1) Put a long (30cm for example) thin chain (the smaller the links the better) on a very steep ramp which it can slide down easily and watch it slide down.
2) Repeat, this time placing 5mm of the chain over the top of the ramp, observe and record results.
3) Repeat with 1cm over the top of the ramp.

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Originally posted by Volv
I've got a practical experiment for you to try out.

1) Put a long (30cm for example) thin chain (the smaller the links the better) on a very steep ramp which it can slide down easily and watch it slide down.
2) Repeat, this time placing 5mm of the chain over the top of the ramp, observe and record results.
3) Repeat with 1cm over the top of the ramp.

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That practice example is not the same as the system above (if you always keep the same angle of the ramps)! Both the ends of the chain must be at the same horizontal plane, see picture from SonicSam.

Which means that in your step 2 the long ramp would be almost flat.
In your step 3, the long ramp would be slightly steeper than in step 2. Etc...

Doing that, at any time, the chain will not move at all.

Similar: in your step 1: the long ramp would be completely flat. Again the chain will (in this case obviously) not move. See my previous post where I explained the limits of the system.

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EDIT: a pic to make my ramblings a bit more clear (also for myself )... can't sleep anyways... note: I suck at drawing a sloped chain with just a laptop mousepad...



The first three are very obvious (limit) cases of SonicSam's picture. In those three cases it is very obvious that there is no movement:

1) Extreme limit case, planes are both flat.
2) 'Center' case, both planes are of equal length and thus equally steep
3) Other extreme limit case, planes are both at 90°
4) SonicSam's picture.

The green line is what I talked about in this post:
You simply hang a second chain of the same type as the one you had on both the ends. This does _not_ change anything in the system since the chain would pull equally hard on both sides; aka the two opposite forces (one on the left end, the other on the right end) cancel eachother out. But, it is now a lot easier to see in pic4 why there wouldn't be a movement, not even in a frictionless environment; it's just a closed chain hanging on a irregular object; it is never going to move out of itself.

Or to put it in another way: say that it does move counterclockwise by some freak thing in nature. That would mean that part of the green part of the chain is now on the right slope and part of the red left-sloped chain is now hanging from the left slope below it. In that case there hasn't actually been any change in mass or whatever in any point of the system; we would still have the same amount of chain hanging below and the same amount of chain hanging over the triangle. So why would it have moved the first time if there wasn't any change to begin with? It wouldn't.

Or, by definition and in the opposite way: if it did move by that freak thing in nature, it would keep on moving because after the initial move the system is again exactly the same as before the move. Aka if it moved a first time, it should also move a second time, and a third time, etc, untill infinity. (= what I meant to say to toddy here).

If that was a parameter of SonicSam's question then yes, you are absolutely right

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Originally posted by CookieRevised
You simply hang a second chain of the same type as the one you had on both the ends. This does _not_ change anything in the system since the chain would pull equally hard on both sides; aka the two opposite forces (one on the left end, the other on the right end) cancel eachother out. But, it is now a lot easier to see in pic4 why there wouldn't be a movement, not even in a frictionless environment; it's just a closed chain hanging on a irregular object; it is never going to move out of itself.

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It would actually, you move the centerpoint of the mass of the chain...