This week, doctoral student Subhayan Sahu returns with another pendulum demonstration – this time, Galileo’s Pendulum. Different presentations of this device can be found in our demonstration directory as C8-03 and G1-20. See it in action in Subhayan’s video below:

A pendulum is mounted on a backboard with two pegs on it. The pendulum is hung from the upper peg, with the lower peg interrupting its swing at the midpoint.

This is a fun way to challenge students to think about conservation of energy in a pendulum. The potential energy of a stationary pendulum at the top of its swing is dependent entirely on its height, not its length. So when the peg effectively changes the pendulum length, the bob still has the same energy and so reaches the same height, but the period changes!

 You can explore a similar effect with this simulation in the PhET Collection: The Pendulum Lab lets you start a pendulum moving with any given height; then you can change the length, mass, and even the force of gravity. Try it out and see what happens!