Physics Demo Number: 109

Approximate Run Time: 5 min

Uniform Circular Motion for use on Overhead

Demo Description

Two PVC rings are available to put on the overhead projector stage, or on the document camera stage.  A ball bearing can be launched tangentially along the inside of either ring.

The bearing will roll at constant speed inside the gray ring, since its structure provides a continuous normal radial contact force on the ball.

The white ring supports the rolling in a circle at constant speed mode only until the ball reaches the three inches of arc length cutout from the ring.

 

Scientific Principles

A continuous central force is necessary for motion in a circle at constant speed.

Equipment

  • Two PVC Rings

  • Two ball bearings

  • Overhead Projector


Equipment Location

  • The rings and ball bearings are in Kit (109) on [E-4-6].

  • The Overhead Projector is in the Lecturehall.

Instructions

 




The pictured apparatus shows two approximately 6.5 inch diameter circular rings. The rings have been cut out of PVC pipes with their heights around 1.5 inches.

One ring was left intact as a section of a hollow right circular cylinder. The other ring has a portion of its wall of about 3 inches in arc length removed.

If one places either ring on the overhead projector stage and starts a 1 inch or smaller diameter steel ball rolling in circular motion around the ring's inside wall, the normal force between the wall and the ball will provide the necessary centripetal force for uniform circular motion of the ball (at least for reasonable initial values of the ball's kinetic energy).

One can let the ball make several complete revolutions around the inside of the intact ring. However the ball started into motion inside the cut ring will only maintain its circular motion until reaching the gap.

In this last case , one clearly sees the ball continue on in a straight line motion along the appropriate tangent to the circle upon reaching the gap. Obviously the normal force between ball and wall vanishes when the wall vanishes.

This technique is clearly easier than trying to terminate a centripetal force provided by a string by attempting to instantaneously cut the string in a smooth fashion. The rings and balls live in Kit(109) on [E-4-6].

Writeup created by David A. Burba

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