A pendulum consisting of a rigid body that is free to swing about a horizontal axis. Suppose that the centre of mass of the rigid body is a distance d from the axis, m is the mass of the body, and I is the moment of inertia of the body about the axis. The equation of motion can be shown to give $I\ddot{\theta }=-mgd\text{sin}$θ‎. As with the simple pendulum, this gives approximate simple harmonic motion for small oscillations.