Paper 4, Section I, B

Classical Dynamics
Part II, 2013

The Lagrangian for a heavy symmetric top of mass MM, pinned at point OO which is a distance ll from the centre of mass, is

L=12I1(θ˙2+ϕ˙2sin2θ)+12I3(ψ˙+ϕ˙cosθ)2MglcosθL=\frac{1}{2} I_{1}\left(\dot{\theta}^{2}+\dot{\phi}^{2} \sin ^{2} \theta\right)+\frac{1}{2} I_{3}(\dot{\psi}+\dot{\phi} \cos \theta)^{2}-M g l \cos \theta

(i) Starting with the fixed space frame (e~1,e~2,e~3)\left(\tilde{\mathbf{e}}_{\mathbf{1}}, \tilde{\mathbf{e}}_{2}, \tilde{\mathbf{e}}_{3}\right) and choosing OO at its origin, sketch the top with embedded body frame axis e3\mathbf{e}_{3} being the symmetry axis. Clearly identify the Euler angles (θ,ϕ,ψ)(\theta, \phi, \psi).

(ii) Obtain the momenta pθ,pϕp_{\theta}, p_{\phi} and pψp_{\psi} and the Hamiltonian H(θ,ϕ,ψ,pθ,pϕ,pψ)H\left(\theta, \phi, \psi, p_{\theta}, p_{\phi}, p_{\psi}\right). Derive Hamilton's equations. Identify the three conserved quantities.