Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Quantumcat on 23/08/2004 12:10:54
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If you take a ring from your finger and spin it so that it spins really fast by itself on the table - you'll notice that the heaviest part, after a second or so, will move to the top. It doesn't matter where the heavy part is when you start it spinning - on the bottom, on the side - it always moves to the top. Why is this????? Please tell me, I desperately want to know! I thought it might have just been that the ring wanted the mass to be symmetrical but then the heavy bit would stay on the bottom, and it'd be easier to stay on the bottom anyway since you'd have to do work to get it up at the top - obviously it's a lot more complicated than just being symmetrical. We're doing angular movement now in physics - maybe I might find the answer there, maybe. If anyone knows why please please say! Thanks in advance.
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Although I have absolutely no qualifications or such to understand my brain is something, its just a matter of trying to put it into intelligible words.
Perhaps it is something along the lines of the heaviest part of the ring is going to need the most energy to move it. Because this spinning motion doesn't really have anything other than the table to push against the force is doing just that and exerting the maximum force on the heaviest part of the ring which pushes that section away from the table to the top where it can't get any higher.
probably way off track but it’s a thought.
Tim
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I'm not sure of my answer either, but i'll give it a shot. Basically, the ring wants to be as stable as possible. In order to be most stable, it has to be travelling at the slowest possible speed. If the heaviest part of the ring is highest, then its centre of gravity will move upwards, maximising its gravitational potential energy, and its speed will be reduced (due to k.e. being converted to g.p.e.)
Thats the only explanation that i can think of. This implies that being stationary at a higher point is more stable than spinning at a lower point?
"I have great faith in fools; self-confidence my friends call it."
-Edgar Allan Poe
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There are two stable positions of the heavy part of the ring, 1) at the top, and 2) at the bottom, both aligned with the spin-axis-moment vector. You probably cannot spin the ring perfectly enough to produce a stable bottom position, but maybe if you practice enough. If the heavy part of the ring departs from the spin axis, then precessional torque will begin acting on it. There is friction force applied against this where the ring is in contact with the table. The rules of spinning masses are that the precessional torque always moves at right angles to the applied force, and will align itself with the force (called "chasing the force"). This results in the heavy mass on the ring aligning itself with the spin axis, opposite the frictional force applied. Thus the spinning moment of inertia is minimized and the friction no longer causes a precession, since it is aligned with the spin axis, and only reduces the rate of spin. In the process, the heavy part of the ring is raised against the force of gravity. Now, gravity will cause the ring to precess about the spin axis.
In order for this to work as you have seen, there has to be sufficient friction between the table and the bottom of the ring to overcome the force of gravity acting on the massive end of the ring. Apparently this is true for common rings and tables.
There was a detailed solution to this problem presented only a few years ago. It is rather complicated. I hope I have summarized it correctly for you.