PURPOSE: To illustrate buoyancy in a paradoxical way.

DESCRIPTION: A ping pong ball is tethered by a spring to the bottom of a water container, which in turn hangs from a spring attached to a fixed point. At rest, the ping pong ball floats near the center of the water tank. Q: How does the ping pong ball move, if at all, when the water tank is raised vertically and released from rest, so that it executes simple harmonic motion? A: The ping pong ball moves out of phase with the motion of the water tank. The system functions as an accelerometer, where the ball moves (with respect to its equilibrium position) in the direction of the acceleration of the tank. The magnitude of the displacement is roughly proportional to the magnitude of the acceleration of the tank.

Click your mouse on the photograph at the left above to see the action.

SUGGESTIONS: Ask your students to discuss this problem before doing the experiment.

See Question of the Week #95 for information on using this demonstration to enhance class involvement.


EQUIPMENT: Tank with ping pong ball system hanging from spring on tall stand, water.

SETUP TIME: 10 minutes.

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