Source: Principles of Physics
Problem
A 95 kg solid sphere with a 15 cm radius is suspended by a vertical wire. A torque of 0.20 N·m is required to rotate the sphere through an angle of 0.85 rad and then maintain that orientation.
[Q1] T = 12 s
The system is a torsion pendulum. The period of oscillation is $T = 2\pi\sqrt{I/\kappa}$, where $I$ is the moment of inertia and $\kappa$ is the torsional constant of the wire.
The torsional constant $\kappa$ is found from the static equilibrium condition where the applied torque $\tau$ equals the restoring torque:
$$\kappa = \frac{\tau}{\theta}$$The moment of inertia for a solid sphere about its center is:
$$I = \frac{2}{5}mR^2$$Substituting these into the period formula:
$$T = 2\pi\sqrt{\frac{I}{\kappa}} = 2\pi\sqrt{\frac{(2/5)mR^2}{\tau/\theta}} = 2\pi\sqrt{\frac{2mR^2\theta}{5\tau}}$$Substitute the given values: $m = 95$ kg, $R = 0.15$ m, $\tau = 0.20$ N·m, and $\theta = 0.85$ rad.
$$T = 2\pi\sqrt{\frac{2(95 \text{ kg})(0.15 \text{ m})^2(0.85 \text{ rad})}{5(0.20 \text{ N} \cdot \text{m})}}$$