Not-So-Simple Pendulum

Adding detail to the model of the pendulum.

Solution

System: A model of a pendulum, simply supported and free to swing without friction about a supporting axis under the torque due to gravity.

Interactions: torque due to gravity and the upward force exerted against gravity by the axis.

Model: Angular Momentum and External Torque about a Single Axis

Approach:

Diagrammatic Representation

We consider first the usual Simple Model of a Pendulum

And then a slightly more detailed model

With two variations.

Mathematical Representation

The simple model has the virtue that it is extremely simple to calculate themoment of inertia, I, of the pendulum about the axis of rotation. We assume a massless stick of length L and a point mass m at the end. The moment of inertia is simply

If we pull the pendulum away from its vertical equilibrium position by an angle θ, then the restoring force F_res is given by

And the natural frequency, as noted in the vocabulary entry on pendulum, is given by

Now consider the second diagram above. Now, instead pf a point particle, the mass of the pendulum ihas real extent. It is a disc of radius r. We can calculate the real moment of inertia by using the parallel axis theorem. According to that, we can calculate the moment of inertia about a point other than the center of mass (about an axis parallel to one running through the center of mass) by simply adding md² to it, where d is the distance between the center of mass and the axis of rotation. For a uniform disc, rotating about its center, the moment of inertia is