Axis of Figure: Rigid Bodies, Rotation & MOI

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Discussion Overview

The discussion revolves around the concept of the "axis of figure" or "figure axis" in the context of rigid bodies, rotation, and moment of inertia, particularly as it relates to the dynamics of a symmetrical top.

Discussion Character

  • Exploratory, Technical explanation, Conceptual clarification

Main Points Raised

  • One participant seeks clarification on the definition of "axis of figure" in relation to rigid body dynamics.
  • Another participant questions whether the term requires deeper meaning beyond its apparent definition, noting that a figure can have multiple rotational axes.
  • A third participant provides a quote from Sommerfeld's Lectures on Mechanics, suggesting that the term relates to the gravitational torque and its dependence on the angle between the vertical and the axis of figure.
  • A later reply proposes that the axis of figure for a symmetrical top is associated with the unequal moment of inertia.

Areas of Agreement / Disagreement

Participants express varying interpretations of the term "axis of figure," with no consensus reached on its precise definition or implications in the context discussed.

Contextual Notes

There is a lack of visual representation in the referenced material, which may contribute to the ambiguity surrounding the term "axis of figure." Additionally, the discussion does not resolve the mathematical implications of the gravitational torque as described.

becko
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Hello. Can anyone tell me what is the "axis of figure" or "figure axis" ?
This is in the context of rigid bodies, rotation, and moment of inertia.
 
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Can you provide the actual quote? Does there need to be any more meaning to it than the obvious?

The figure in question would have (infinite) axes about which it could rotate i.e. anyone of them could be the rotational axis of the figure.
 
This is from Sommerfeld's Lectures on Mechanics. This is the quote

"For the heavy symmetrical top the fixed point O (point of support in the socket) no longer coincides with the center of mass G (located on the axis of symmetry); call s the distance OG. The magnitude of the gravitational torque is then:

|L|=m*g*s*sin(theta)

where theta is the angle between the vertical and the axis of figure."

I'm pretty sure that theta equals the angle between the vertical and the line OG. And, by the way, there not a single figure (as in picture) in the whole section where this quote is taken from. That's why I guess this term must have some definition.
 
I just found out that the axis of figure of a symmetrical top is the axis corresponding to the unequal moment of inertia.
 

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