PURPOSE: To demonstrate circular standing waves in an interesting way.

DESCRIPTION: A teacup can be tapped with a spoon to excite standing waves around its rim, exactly like the standing waves in a glass beaker. The standing wave consists of four alternating nodes and antinodes spaced at 90 degrees around the teacup. If the handle is at an antinode, the resonant frequency is lower than if the handle is at a nodal point, because the vibrating mass is greater but the restoring force is the same. Tap the rim of the teacup moving around the rim at intervals of 45 degrees to get alternating higher and lower frequencies. Click on the image above to see a short video of this demonstration in action.

SUGGESTIONS: We use this with the beaker breaker, H3-61, to give the folks something to talk about and to experiment with when they go out for dessert and coffee after a physics demonstration program.

See Question of the Week #214 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Teacup and spoon.

SETUP TIME: None.