A torsional pendulum involves the oscillation of a rigid body in which the restoring force is provided by the torsion in the string from which the rigid body is suspended. Ideally, the string should be massless; practically, its mass is much smaller than the rigid body's mass and is neglected.
As long as the rigid body's angular displacement is small, its oscillation can be modeled as a linear angular oscillation. The amplitude of the oscillation is an angle. The role of mass is played by the rigid body's moment of inertia about the point of suspension and the axis passing perpendicular to it.
Using the relationship between torque and angular acceleration, the equation is seen to mimic the equation of the simple harmonic motion of a simple pendulum. This observation allows for the easy determination of the angular frequency of the angular oscillation and its time period.
