Poinsot's construction

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Poinsot's construction is a geometrical method for visualizing the torque-free motion of a rotating rigid body. It was developed by Louis Poinsot and uses the conservation of kinetic energy and angular momentum as constraints on the angular velocity vector \boldsymbol\omega.

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[edit] Kinetic energy constraint

In the absence of applied torques, the kinetic energy T\ is conserved so \frac{dT}{dt} = 0.

The kinetic energy may be expressed in terms of the moment of inertia tensor \mathbf{I} and the angular momentum vector \boldsymbol\omega

T = \frac{1}{2} \boldsymbol\omega \cdot \mathbf{I} \cdot \boldsymbol\omega = \frac{1}{2} I_{1} \omega_{1}^{2} + \frac{1}{2} I_{2} \omega_{2}^{2} + \frac{1}{2} I_{3} \omega_{3}^{2}

where \omega_{k}\ are the components of the angular velocity vector \boldsymbol\omega along the principal axes, and the I_{k}\ are the principal moments of inertia. Thus, the conservation of kinetic energy imposes a constraint on the three-dimensional angular velocity vector \boldsymbol\omega; in the principal axis frame, it must lie on an ellipsoid

T = \frac{1}{2} I_{1} \omega_{1}^{2} + \frac{1}{2} I_{2} \omega_{2}^{2} + \frac{1}{2} I_{3} \omega_{3}^{2}

The path traced out on this ellipsoid by the angular velocity vector \boldsymbol\omega is called the polhode (Greek for "pole path") and is generally circular or taco-shaped.

[edit] Angular momentum constraint

In the absence of applied torques, the angular momentum vector \mathbf{L} is conserved in an inertial reference frame \frac{d\mathbf{L}}{dt} = 0.

The angular momentum vector \mathbf{L} can also be expressed in terms of the moment of inertia tensor \mathbf{I} and the angular momentum vector \boldsymbol\omega

\mathbf{L} = \mathbf{I} \cdot \boldsymbol\omega

which leads to the equation

T = \frac{1}{2} \boldsymbol\omega \cdot \mathbf{L}

showing that the angular velocity vector \boldsymbol\omega has a constant component in the direction of the angular momentum vector \mathbf{L}. This imposes a second constraint on the vector \boldsymbol\omega; in absolute space, it must lie on an invariable plane defined by its dot product with the conserved vector \mathbf{L}. The normal vector to the invariable plane is aligned with \mathbf{L}. The path traced out by the angular velocity vector \boldsymbol\omega on the invariable plane is called the herpolhode (Greek for "serpentine path"). The etymology of "herpolhode" is obscure, since most herpolhodes are simple circles, not sinusoidal; Poinsot may have been thinking of the ouroboros, the symbol of endless cycling.

[edit] Tangency condition and construction

These two constraints operate in different reference frames; the ellipsoidal constraint holds in the (rotating) principal axis frame, whereas the invariable plane constant operates in absolute space. To relate these constraints, we note that the gradient vector of the kinetic energy with respect to angular velocity vector \boldsymbol\omega equals the angular momentum vector \mathbf{L}

\frac{dT}{d\boldsymbol\omega} = \mathbf{I} \cdot \boldsymbol\omega = \mathbf{L}

Hence, the normal vector to the kinetic-energy ellipsoid at \boldsymbol\omega is proportional to \mathbf{L}, which is also true of the invariable plane. Since their normal vectors point in the same direction, these two surfaces will intersect tangentially.

Taken together, these results show that, in an absolute reference frame, the instantaneous angular velocity vector \boldsymbol\omega is the point of intersection between a fixed invariable plane and a kinetic-energy ellipsoid that is tangent to it and rolls around on it without slipping. This is Poinsot's construction.

[edit] Derivation of the polhodes

In the principal axis frame (which is rotating in absolute space), the angular momentum vector is not conserved even in the absence of applied torques, but varies as described by Euler's equations. However, in the absence of applied torques, the magnitude L\ of the angular momentum and the kinetic energy T\ are both conserved

L^{2} = L_{1}^{2} + L_{2}^{2} + L_{3}^{2}
T = \frac{L_{1}^{2}}{2I_{1}} + \frac{L_{2}^{2}}{2I_{2}} + \frac{L_{3}^{2}}{2I_{3}}

where the L_{k}\ are the components of the angular momentum vector along the principal axes, and the I_{k}\ are the principal moments of inertia.

These conservation laws are equivalent to two constraints to the three-dimensional angular momentum vector \mathbf{L}. The kinetic energy constrains \mathbf{L} to lie on an ellipsoid, whereas the angular momentum constraint constrains \mathbf{L} to lie on a sphere. These two surfaces intersect in taco-shaped curves that define the possible solutions for \mathbf{L}.

This construction differs from Poinsot's construction because it considers the angular momentum vector \mathbf{L} rather than the angular velocity vector \boldsymbol\omega. It appears to have been developed by Jacques Philippe Marie Binet.

[edit] References

  • Poinsot (1834) Theorie Nouvelle de la Rotation des Corps'.

[edit] See also

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