PURPOSE: To illustrate the affect of density on acceleration in a counterintuitive way.

DESCRIPTION: A helium balloon is tethered to the bottom of an enclosed plastic box. Q: If the box is accelerated, what will the balloon do: (a) move in the direction of the acceleration, (b) move in the direcrtion opposite the acceleration, or (c) remain motionless? A: It will move in the direction of the acceleration.

SUGGESTIONS: Ask your students to vote before performing the experiment.

REFERENCES: See Demonstration Reference File for a number of articles describing this setup and its uses.

EQUIPMENT: Helium balloon tethered to bottom of box.

SETUP TIME: 5 min.

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