Spinning Top Dancer: Moment of Inertia?

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Discussion Overview

The discussion revolves around the stability of a spinning dancer and the role of moment of inertia in maintaining that stability. Participants explore whether having the center of mass above the pivot point is sufficient for stable rotation, and if the shape of the dancer or spinning top affects this stability.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that the center of mass must be directly over the pivot point for stability during spinning.
  • Others argue that the concentration of mass near the axis of rotation can influence the speed of spinning for a given amount of energy.
  • A participant questions whether the shape of a spinning top can be arbitrary, suggesting that as long as the spin axis passes through the center of mass, it may not matter.
  • It is noted that a lower center of gravity can enhance stability by reducing the tendency of small oscillations to displace the center of gravity over the pivot point.
  • Some participants mention that lighter objects may experience less friction at the pivot point, potentially leading to less energy loss.
  • There is a suggestion that aerodynamic shaping could minimize energy loss during rotation.

Areas of Agreement / Disagreement

Participants express differing views on the importance of shape and mass distribution for stable rotation, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

Participants discuss the implications of center of mass, shape, and mass distribution on stability, but do not resolve the specific conditions under which certain shapes may or may not work for stable rotation.

Gerenuk
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If a dancer who balances on one leg starts spinning, is having the center of mass above the point of contact enough, or does he need to assume a special shape?
I mean is the moment of inertia important provided the point of contact (feet) are not slipping?
 
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The center of mass has to be over the pivot point. The more the mass is concentrated near the axis, the faster the spinning for a given starting energy.
 
So wasn't so much the question.
I already assume the the COM is above the pivot point. The question is whether this is enough for a stable rotation.
I mean in free space there are only two special axis of rotation about which a stable rotation is possible. What about a spinning top? Can it's shape be arbitrary?
 
Figure skaters are able to assume various positions while spinning, but the point of contact with the ice translates across the ice somewhat during a spin.
 
OK, but that's also not the question. The question is whether there are positions that don't work.
 
Gerenuk said:
What about a spinning top? Can it's shape be arbitrary?

Yes, I would think so, provided its spin axis passes through the center of mass.
 
Gerenuk said:
OK, but that's also not the question. The question is whether there are positions that don't work.

1. The lower the center of gravity, the more stable it will be. The reason is that small oscillations, whether intrinsic or external will not have as much tendency to displace the center of gravity over the pivot point, if it very low. In this sense, the shape will have an effect.

2. Also, the lighter the object, the less there will be friction at the pivot point, so the less energy it will waste.

3. You could also minimize energy loss by making it aerodynamic in the relevant direction.
 

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