= Solution
The <torsional wave equation in a circular shaft> has speed $\sqrt{G/\rho}$. The given shaft has torque $100\,\mathrm{N\,m}$, twist $9.95\times10^{-3}\,\mathrm{rad}$, strain energy $0.497\,\mathrm J$ and kinetic energy $181\,\mathrm J$. The respective transit-time estimates are $786\,\mathrm W$ and $286\,\mathrm{kW}$; these must not be confused with independent bounds on the actual $30\,\mathrm{kW}$ stress-work flux.
Full derivation: <past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2010/paper-2a/section-c/c12/solution>.
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