Stopped Pendulum

1M40.15

Brief Description

This demo is a classic physics experiment used to illustrate the Law of Conservation of Energy. The setup consists of a simple pendulum—a mass on a string—and a horizontal "stop" or peg placed partway down the pendulum's path. When the pendulum is released and the string strikes the peg, the pivot point effectively shifts, causing the bob to swing in a tighter, faster arc. Despite this sudden change in the radius of motion, the pendulum bob will always rise to the same vertical height from which it was originally released (minus minor losses to friction). This demonstrates that the total mechanical energy (E_total =K+U) remains constant; the potential energy at the release height must be recovered at the end of the swing, regardless of the path taken.

Materials Needed

1 pendulum, two rods (one meter plus), 4 small rods (about 30 cm), two chalk tray spacers, two rubber spacers, two table clamps, six 90° rod clamps, one meter stick.

Set-up

Build a structure that will hold the pendulum using the two long rods and three of the smaller rods. Two of those rods are used as cross braces for the vertical rods, and one of them is used as the pivot for the pendulum. This structure is the attached to the chalk tray in the class room, using the table clamps and the chalk tray spacers. Place the final shorter rod on the vertical rod that the pendulum is not attached to; this rod acts to “stop” the pendulum. The rod that supports the pendulum and the one that acts as the stop should be supported against the chalk board with the rubber spacer between the end of the rod and the chalk board itself. Mark the chalk board with two horizontal lines, which indicate two different heights that the pendulum will be dropped from.

Setup for Stopped Pendulum Demo

Performing the Demo

First drop the pendulum without the bar in the way the will “stop” the pendulum, that will show the students how a pendulum acts under normal circumstances. Then move the bar into place and ask the students what will happen, after some discussion drop the pendulum and watch the results.

A ball hanging from a rod creating a pendulum that is swinging at a certain height designated by a line on the chalk board.

A ball hanging from a rod creating a pendulum. The string is stopped by a rod and the ball still makes it to the line indicated by the chalkboard line.