Paper 4, Section I, E

Classical Dynamics | Part II, 2019

(a) The angular momentum of a rigid body about its centre of mass is conserved.

Derive Euler's equations,

I1ω˙1=(I2I3)ω2ω3I2ω˙2=(I3I1)ω3ω1I3ω˙3=(I1I2)ω1ω2\begin{aligned} &I_{1} \dot{\omega}_{1}=\left(I_{2}-I_{3}\right) \omega_{2} \omega_{3} \\ &I_{2} \dot{\omega}_{2}=\left(I_{3}-I_{1}\right) \omega_{3} \omega_{1} \\ &I_{3} \dot{\omega}_{3}=\left(I_{1}-I_{2}\right) \omega_{1} \omega_{2} \end{aligned}

explaining the meaning of the quantities appearing in the equations.

(b) Show that there are two independent conserved quantities that are quadratic functions of ω=(ω1,ω2,ω3)\boldsymbol{\omega}=\left(\omega_{1}, \omega_{2}, \omega_{3}\right), and give a physical interpretation of them.

(c) Derive a linear approximation to Euler's equations that applies when ω1ω3\left|\omega_{1}\right| \ll\left|\omega_{3}\right| and ω2ω3\left|\omega_{2}\right| \ll\left|\omega_{3}\right|. Use this to determine the stability of rotation about each of the three principal axes of an asymmetric top.

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