Poinsot's ellipsoid

The law of conservation of energy implies that in the absence of energy dissipation or applied torques, the angular kinetic energy ${\displaystyle T\ }$ is conserved, so ${\displaystyle {\frac {dT}{dt}}=0}$.

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

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

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

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

The law of conservation of angular momentum states that in the absence of applied torques, the angular momentum vector ${\displaystyle \mathbf {L} }$ is conserved in an inertial reference frame, so ${\displaystyle {\frac {d\mathbf {L} }{dt}}=0}$.

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

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

which leads to the equation

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

Since the dot product of ${\displaystyle {\boldsymbol {\omega }}}$ and ${\displaystyle \mathbf {L} }$ is constant, and ${\displaystyle \mathbf {L} }$ itself is constant, the angular velocity vector ${\displaystyle {\boldsymbol {\omega }}}$ has a constant component in the direction of the angular momentum vector ${\displaystyle \mathbf {L} }$. This imposes a second constraint on the vector ${\displaystyle {\boldsymbol {\omega }}}$; in absolute space, it must lie on the invariable plane defined by its dot product with the conserved vector ${\displaystyle \mathbf {L} }$. The normal vector to the invariable plane is aligned with ${\displaystyle \mathbf {L} }$. The path traced out by the angular velocity vector ${\displaystyle {\boldsymbol {\omega }}}$ on the invariable plane is called the herpolhode (coined from Greek roots for "serpentine pole path").

The herpolhode is generally an open curve, which means that the rotation does not perfectly repeat, but the polhode is a closed curve (see below).[1]

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 ${\displaystyle {\boldsymbol {\omega }}}$ equals the angular momentum vector ${\displaystyle \mathbf {L} }$

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

Hence, the normal vector to the kinetic-energy ellipsoid at ${\displaystyle {\boldsymbol {\omega }}}$ is proportional to ${\displaystyle \mathbf {L} }$, 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 ${\displaystyle {\boldsymbol {\omega }}}$ 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.

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 ${\displaystyle L\ }$ of the angular momentum and the kinetic energy ${\displaystyle T\ }$ are both conserved

${\displaystyle L^{2}=L_{1}^{2}+L_{2}^{2}+L_{3}^{2}}$

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

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

These conservation laws are equivalent to two constraints to the three-dimensional angular momentum vector ${\displaystyle \mathbf {L} }$. The kinetic energy constrains ${\displaystyle \mathbf {L} }$ to lie on an ellipsoid, whereas the angular momentum constraint constrains ${\displaystyle \mathbf {L} }$ to lie on a sphere. These two surfaces intersect in two curves shaped like the edge of a taco that define the possible solutions for ${\displaystyle \mathbf {L} }$. This shows that ${\displaystyle \mathbf {L} }$, and the polhode, stay on a closed loop, in the object's moving frame of reference.

If the body is set spinning on its intermediate principal axis, then the intersection of the ellipsoid and the sphere is like two loops that cross at two points, lined up with that axis. ${\displaystyle \mathbf {L} }$ will eventually move off this point along one of the four tracks that depart from this point, and head to the opposite point. This is reflected by ${\displaystyle {\boldsymbol {\omega }}}$ on the Poinsot ellipsoid. See video at right and Tennis racket theorem.

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

In the general case of rotation of an unsymmetric body, which has different values of the moment of inertia about the three principal axes, the rotational motion can be quite complex unless the body is rotating around a principal axis. As described in the tennis racket theorem, rotation of an object around its first or third principal axis is stable, while rotation around its second principal axis (or intermediate axis) is not. The motion is simplified in the case of an axisymmetric body, in which the moment of inertia is the same about two of the principal axes. These cases include rotation of a prolate spheroid (the shape of an American football), or rotation of an oblate spheroid (the shape of a flattened sphere). In this case, the angular velocity describes a cone, and the polhode is a circle. This analysis is applicable, for example, to the axial precession of the rotation of a planet (the case of an oblate spheroid.)

Hyperion (a moon of Saturn), two moons of Pluto and many other small bodies of the Solar System have tumbling rotations.

One of the applications of Poinsot's construction is in visualizing the rotation of a spacecraft in orbit.[2]

• Poinsot (1834) Theorie Nouvelle de la Rotation des Corps, Bachelier, Paris.
• Landau LD and Lifshitz EM (1976) Mechanics, 3rd. ed., Pergamon Press. ISBN 0-08-021022-8 (hardcover) and ISBN 0-08-029141-4 (softcover).
• Goldstein H. (1980) Classical Mechanics, 2nd. ed., Addison-Wesley. ISBN 0-201-02918-9
• Symon KR. (1971) Mechanics, 3rd. ed., Addison-Wesley. ISBN 0-201-07392-7

• Poinsot construction in stereo 3D simulation - online and free.