PURPOSE: To show that forces are required to create circular motion.

DESCRIPTION: The apparatus pictured is positioned on a horizontal surface. A small ball bearing (in container at lower left of picture) is blown through the spiral hose, emerging at the right side and moving downward (in the picture), toward one of the five aluminum tube targets. Which of the targets will it hit, and why?

SUGGESTIONS: Suggest (1) that the ball might hit one of the targets at the right (#4 or 5), due to "centrifugal force" acting on the ball after it leaves the spiral, or (2) that the ball might hit one of the targets at the left (#1 or 2), due to its "angular momentum," or (3) it might possibly just go straight after leaving the spiral and hit the target in the middle. Then let your students vote before performing the experiment. Click your mouse on the photograph above to see an mpeg video of the action.

See Question of the Week #44 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA 1D55.10) See Demonstration Reference File for paper describing the use of this apparatus with children to develop their physical intuition regarding Newton's laws of motion. Simpler versions are Demonstration D1-32: TRAJECTORY FROM CIRCULAR ORBIT, or D1-30: TRAJECTORY FROM CIRCULAR ORBIT - OVERHEAD PROJECTOR.

EQUIPMENT: Spiral hose apparatus pre-mounted as photographed, with target tubes.

SETUP TIME: 5 min.