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Old 2015-10-20, 06:48 PM   #61
minimalist
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Conservation of angular momentum

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Originally Posted by kamikaze View Post
But it's not effective because you push air.
It's not the air resistance that makes it effective, it is the change in the angular momentum that it causes. By the law of conservation of angular momentum your body must rotate in the opposite direction. You would get the same effect if a vacuum.
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Old 2015-10-20, 06:50 PM   #62
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Quote:
Originally Posted by kamikaze View Post
I don't think the effect is significant. Especially if you're just waving around your arms.
True. While it might be enough to be useful in a zero-g stillstand, there'd be no point in doing such a thing. But give me a pressurized, zero-g environment with enough room to play around, and I'll make myself a pair of wings (and a tail, I guess) and fly all over the place in there!
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They tried to turn a nut with a wrench and found they could not do it. When they tried to turn the nut counter-clockwise their body would rotate clockwise instead. Since then NASA has installed handles that the astronauts can grab to brace themselves.
I'm surprised nobody though of that in advance. My dad was part of the "army of engineers" NASA had in the 60s and was involved with that sort of stuff for the Gemini and Apollo programs.
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Old 2015-10-21, 03:22 AM   #63
LargeEddie
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I was going to gripe about rhetorical physics and the futility of getting anywhere without using numbers or equations. But arm-waving about literal arm-waving... Gotta respect that.
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Old 2015-10-21, 05:38 PM   #64
acl
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Quote:
Originally Posted by kamikaze View Post
Your assumptions only work in zero-G and are about maintaining impulse, not centre of mass.
From what I read, impulse = force * time = effect/change of momentum. So I'm not sure that's useful here...

Quote:
You are not an isolated system, but subject to an external force, gravity.

By lowering your centre of mass you can leverage your potential energy to change your impulse and shift your centre of mass horizontally. If you do that right you can let gravity pull you back over the contact patch. You can straighten your body again over the patch
I kinda like that idea/explanation - you can sort of borrow time by falling, then push yourself back up when you're straight - but I'm not sure how much it explains. I think the later discussion of angular momentum is much more promising....

Quote:
Originally Posted by minimalist
It's not the air resistance that makes (waving your arms) effective, it is the change in the angular momentum that it causes. By the law of conservation of angular momentum your body must rotate in the opposite direction.
That's more like it. So by waving my arms, I make my body rotate. My body would like to rotate about the same center about which my arms are rotating, but that's not possible because it's anchored at the ground. Instead, a horizontal force is exerted on my tyre, which is transmitted to the earth via friction (and thus the Earth moves, infinitesimally); and the equal-and-opposite horizontal force pushes back on me, and yes, my center of mass moves.

It has to be rotation, tho - if I just move my arms (say 5kg) horizontally (say 1m), then the equal-and-opposite force would just move my body (say 50kg) back in the opposite direction (by 10cm); I can't change my center-of-mass this way (tyre friction may stop entire body+unicycle moving by 10cm, and turn it into rotation, plus corresponding counter-rotation of the earth to preserve angular momentum, but I don't think that ends up moving my centre of mass, unless I *rotate* my arms rather than just move them across).

That's now my understanding, anyway. Thanks to all who posted in helping me get to that .
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