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Old 07-06-06, 12:08 AM   #1
Karldar
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Help out the Karldar #9. Physics and stuff.

I know it's a bit late, but I forgot to post this on or around my birthday. This is not a homework question or anything important, just something I've been curious about for a while.

If I have a turntable with some(let's say 20) one kg weights on it, that I spin at a constant speed(not regulated by a machine), what will happen if I can remove a weight without influencing(i.e. putting pressure on) the turntable? Will it speed up because there's less weight, slow down because there's less momentum or continue to spin at the same speed? I know that's probably not enough info, but it is foo, after all.
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Old 07-06-06, 12:12 AM   #2
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well since X=3/4ths pie over the sqaure root of the pathagorian something...........

it will tip over and fall on your foot
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Old 07-06-06, 01:17 AM   #3
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No instantaneous change in rotational rate. Consider the following thought experiment:

Imagine a turntable with all the weights plus one being forced around an axis at the given rate and a lone free weight also being forced around the same axis at the same given rate. You bring the two together. What happens? Nothing. (They were already going around and around at the exact same rate. They continue to do so.) You reverse the process. Again, nothing of interest happens.

That said, depending on the location of the weight you remove and of the other weights, you may affect how well balanced the turntable is. Changing the balance is likely to change the magnitude of parasitic forces and change the rate at which the turntable slows. This, however, is a secondary effect that would appear to be outside the scope of the original question.


DISCLAIMER: It's past midnight. I do not stand by this answer for any purposes where anything of substance is at stake!
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Old 07-06-06, 02:02 AM   #4
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Hmm, let's see. A skater spinning on the ice brings in her arms to
increase the rate of rotation. If your moved the weights inward
towards the center of the turntable, wouldn't it speed up? And
if you moved the weights toward the edge of the turntable,
wouldn't it slow down? And, finally, if the weights rolled outward
completely off the turntable, wouldn't it slow down even more?
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Old 07-06-06, 06:24 AM   #5
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As long as you are not trying to accelerate the turntable in any way, there will be no change.

Rotational kinetic energy is given by:

E = 1/2 I w^2 where

E = energy in [length] * [force] / [time]
I = mass moment of inertia in [length] * [mass] / [time]^2
w = angular velocity. [radians] / [time]

If you had two plates on the same axis, but different axles, spinning at the same speed, the total kinetic energy of that system would be calculated as the sum of the two individual kinetic energies. That's the energy it took to accelerate the plates to that particular angular velocity. We will, as always, for simple systems, neglect friction, wind resistance, and relativistic effects )

If you put the plates into contact, no acceleration took place, so the energy of the system is the same.

Popaspoke, you are correct, but the system you describe is slightly different. As the skater brings in her arms, she is decreasing her mass moment of inertia by pulling parts of her mass closer to her axis of rotation. I'm not going to try to put the general equation here so: http://en.wikipedia.org/wiki/Moment_of_inertia.

Therefore, for the energy of her system to remain constant, her angular velocity must increase.
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Old 07-06-06, 07:37 AM   #6
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Has anyone tried to figure out WHY the Karldar asked such a question?
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Old 07-06-06, 07:46 AM   #7
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Ummm, Miss Hathaway, I don't understand anything you just said.
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Old 07-06-06, 01:58 PM   #8
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I think I'll go with eubi's explanation...since I don't know any better. In my(poorly developed) example, the weights are fit into molds on the table to keep them in place while it is spinning. Also, the turntable's shaft would be strong enough to counter all but the most extreme redistributions of weight. Perhaps not the changes in friction, however. I assumed that friction(in the form of gravity's effect on the table) would have an effect, but(from what I have read here) it would be negligible given an even distribution of weight and removal/replacement of only one part at a time.
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Old 07-06-06, 02:30 PM   #9
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Quote:
Originally Posted by jyossarian
Has anyone tried to figure out WHY the Karldar asked such a question?
Um... Maybe he's been hangin' with Merton too much?
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Old 07-06-06, 08:22 PM   #10
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Less weight should in theory allow the platter to reach playback speed sooner, or play with the balance, that's about it.
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Old 07-08-06, 10:23 AM   #11
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Hey, I LIKE questions like this.

It forces me to remember the physics I learned 25 years ago!

Thanks Karldar!
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Old 07-08-06, 08:00 PM   #12
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Quote:
Originally Posted by eubi
Hey, I LIKE questions like this.

It forces me to remember the physics I learned 25 years ago!

Thanks Karldar!
You're quite welcome. It's hard for me to remember the physics I never learned in the first place...except for the lessons I learned the hard way, of course.
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Old 07-08-06, 08:14 PM   #13
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Quote:
Originally Posted by jyossarian
Has anyone tried to figure out WHY the Karldar asked such a question?
World domination ?
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Old 07-08-06, 08:22 PM   #14
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hes making an evil turn table!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
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