Torsional Oscillation of a Suspended Sphere

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Oscillations Intermediate Simple harmonic oscillation

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.

What is the period of the oscillations that result when the sphere is then released?

[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.

$$T = 2\pi\sqrt{\frac{2(95 \text{ kg})(0.15 \text{ m})^2(0.85 \text{ rad})}{5(0.20 \text{ N} \cdot \text{m})}}$$