This week we’re taking a look at a popular demonstration of simple harmonic motion, G1-15. This demonstration consists of a pair of pendula, with one four times the length of the other. You can see them in action in this video with PhD student Subhayan Sahu.

We can see that as the pendulum oscillates, its period is proportional to the square root of its length. The fact that a simple pendulum’s period is dependent only on its length and on the force of gravity is very handy for other purposes, too! Mechanical clocks are built around a pendulum for this reason, and if you have no clock at all you can make a simple time-measuring device by just making a pendulum and counting its oscillations. Very sensitive measurements of the motion of a pendulum have even been used to measure minute differences in the force of gravity, letting us map Earth’s gravitational field.

 You can also experiment with this at home, with any heavy object and some string. (But advice: ask before using somebody else’s shoelaces for this, they might need them!) If you’re short on string, or if you want to try adjusting different variables at the same time to see how the pendulum’s motion changes, you can also check out this simulation at The Physics Aviary: