# 4: Rigid Body Rotation

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No real solid body is perfectly rigid. A rotating nonrigid body will be distorted by centrifugal force* or by interactions with other bodies. Nevertheless most people will allow that in practice some solids are fairly rigid, are rotating at only a modest speed, and any distortion is small compared with the overall size of the body. No excuses, therefore, are needed or offered for analyzing, to begin with the rotation of a rigid body.

• 4.1: Introduction to Rigid Body Rotation
A full treatment of the rotation of an asymmetric top (whose three principal moments of inertia are unequal) is very lengthy, since there are so many cases to consider. I shall restrict consideration of the motion of an asymmetric top to a qualitative argument that shows that rotation about the principal axis of greatest moment of inertia or about the axis of least moment of inertia is stable, whereas rotation about the intermediate axis is unstable.
• 4.2: Angular Velocity and Eulerian Angles
We are going to examine the motion of a body that is rotating about a non-principal axis. If the body is freely rotating in space with no external torques acting upon it, its angular momentum L will be constant in magnitude and direction.
• 4.3: Kinetic Energy of Rigid Body Rotation
This formula is adequate for simple situations in which a body is rotating about a principal axis, but is not adequate for a body rotating about a non-principal axis.
• 4.4: Lagrange's Equations of Motion
In deriving Euler’s equations, I find it convenient to make use of Lagrange’s equations of motion. This will cause no difficulty to anyone who is already familiar with Lagrangian mechanics. The geometrical description of a mechanical system at some instant of time can be given by specifying a number of coordinates, e.g., if the system consists of just a single particle, you could specify its rectangular coordinates xyz or its cylindrical coordinates ρϕz , or its spherical coordinates rθϕ .
• 4.5: Euler's Equations of Motion
Euler's rotation equations are a vectorial quasilinear first-order ordinary differential equation describing the rotation of a rigid body, using a rotating reference frame with its axes fixed to the body and parallel to the body's principal axes of inertia.
• 4.6: Force-free Motion of a Rigid Asymmetric Top
By “asymmetric top” I mean a body whose three principal moments of inertia are unequal. While we often think of a “top” as a symmetric body spinning on a table, in this section the “top” will not necessarily be symmetric, and it will not be in contact with any table, nor indeed subjected to any external forces or torques.
• 4.7: Nonrigid Rotator
The rotational kinetic energy of a body rotating about a principal axis is 12Iω312Iω3 , where I is the moment of inertia about that principal axis, and the angular momentum is L = Iω. (For rotation about a nonprincipal axis, see section 4.3.) Thus the rotational kinetic energy can be written as L2/(2I).
• 4.8: Force-free Motion of a Rigid Symmetric Top
• 4.9: Centrifugal and Coriolis Forces
We are usually told in elementary books that there is “no such thing” as centrifugal force. When a satellite orbits around Earth, it is not held in equilibrium between two equal and opposite forces, namely gravity acting towards Earth and centrifugal force acting outwards. In reality, we are told, the satellite is accelerating (the centripetal acceleration); there is only one force, namely the gravitational force, which is equal to the mass times the centripetal acceleration.
• 4.10: The Top
We have classified solid bodies technically as symmetric, asymmetric, spherical and linear tops, according to the relative sizes of their principal moments of inertia.
• 4.11: Appendix

Thumbnail: Proper Euler angles geometrical definition. The xyz (fixed) system is shown in blue, the XYZ (rotated) system is shown in red. The line of nodes (N) is shown in green. (CC BY 3.0; Lionel Brits).

This page titled 4: Rigid Body Rotation is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Jeremy Tatum via source content that was edited to the style and standards of the LibreTexts platform.