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 direction opposite the acceleration, or (c) remain motionless? A: It will move in the direction of the acceleration.

Click here to see a video of the action. Note that the balloon moves to the front when the box is pushed to the right, and to the rear when the box stops due to friction with the table.

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

See Question of the Week #71 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1G20.75) See Demonstration Reference File for a number of articles describing this setup and its uses.

EQUIPMENT: Helium balloon tethered to bottom of plastic box.

SETUP TIME: 5 min.