Poinsot's ellipsoid
In classical mechanics, Poinsot's construction is a geometrical method for visualizing the torque-free motion of a rotating rigid body, that is, the motion of a rigid body on which no external forces are acting. This motion has four constants: the kinetic energy of the body and the three components of the angular momentum, expressed with respect to an inertial laboratory frame. The angular velocity vector of the rigid rotor is not constant, but satisfies Euler's equations. Without explicitly solving these equations, Louis Poinsot was able to visualize the motion of the endpoint of the angular velocity vector. To this end he used the conservation of kinetic energy and angular momentum as constraints on the motion of the angular velocity vector. If the rigid rotor is symmetric, the vector describes a cone. This is the torque-free precession of the rotation axis of the rotor.
Angular kinetic energy constraint
The law of conservation of energy implies that in the absence of energy dissipation or applied torques, the angular kinetic energy is conserved, so.The angular kinetic energy may be expressed in terms of the moment of inertia tensor and the angular velocity vector
where are the components of the angular velocity vector along the principal axes, and the are the principal moments of inertia. Thus, the conservation of kinetic energy imposes a constraint on the three-dimensional angular velocity vector ; in the principal axis frame, it must lie on an ellipsoid, called inertia ellipsoid.
The ellipsoid axes values are the half of the principal moments of inertia. The path traced out on this ellipsoid by the angular velocity vector is called the polhode and is generally circular or taco-shaped.
Angular momentum constraint
The law of conservation of angular momentum states that in the absence of applied torques, the angular momentum vector is conserved in an inertial reference frame, soThe angular momentum vector can be expressed
in terms of the moment of inertia tensor and the angular velocity vector
which leads to the equation
Since the dot product of and is constant, and itself is constant, the angular velocity vector
has a constant component in the direction of the angular momentum vector
. This imposes a second constraint on the vector ; in absolute space, it must lie on the invariable plane defined by its dot product with the conserved vector. The normal vector to the invariable plane is aligned with. The path traced out by the angular velocity vector on the invariable plane is called the herpolhode.
The herpolhode is generally an open curve, which means that the rotation does not perfectly repeat, but the polhode is a closed curve.
Tangency condition and construction
These two constraints operate in different reference frames; the ellipsoidal constraint holds in the 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 equals the angular momentum vectorHence, the normal vector to the kinetic-energy ellipsoid at is
proportional to, 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 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.
Derivation of the polhodes in the body frame
In the principal axis frame, 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 of the angular momentum and the kinetic energy are both conservedwhere the are the components of the angular momentum vector along the principal axes, and the are the principal moments of inertia.
These conservation laws are equivalent to two constraints to the three-dimensional angular momentum vector.
The kinetic energy constrains to lie on an
ellipsoid, whereas the angular momentum constraint constrains
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. This shows that, and the polhode, stay on a closed loop, in the object's moving frame of reference.
, NASA.
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. 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 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 rather than the angular velocity vector. It appears to have been developed by Jacques Philippe Marie Binet.
Special case
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 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, or rotation of an oblate spheroid. 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 planetHyperion, two moons of Pluto and many other small bodies of the Solar System have tumbling rotations.