Group size
Individual students

Time required
15 minutes


Instructional Goal National standards (9-12) addressed

National standards (5-8) addressed

Student Objectives
Student will:

Prerequisite Knowledge
Concept of centripetal acceleration and circular motion.

Advance Preparation Time
Get the materials necessary to conduct the activity.

Teacher tips

Background Information

 The velocity is always perpendicular to the radius. The acceleration is due to the change of direction of velocity even though the speed of the rubber stopper is constant. The force is exerted by the fingers that is holding the string and transmitted to the rubber stopper through the string. The acceleration is called centripetal (center seeking) acceleration.

F = mac = mv2/r (m=mass, v=velocity, F=force, ac = acceleration)

Activity Procedure

Step 1
Everyone in the classroom put on goggles for eye protection.

Step 2
Tie a 1.0 m length of a string onto a one-hole stopper.

Step 3
Swing the stopper around your head in a horizontal circle.

Step 4
Release the string from your hand when the string is lined up with a spot on the wall.

Step 5
Did the stopper travel toward the spot on the wall? What does this indicate about the direction of the velocity compared to the orientation of the string?

Step 6
Repeat the activity until the stopper flies toward the spot on the wall.

Enrichment Activities
For physics students:
Determine the mass the rubber ball. Measure the length of the string. With a timer record the time it takes for one rotation. Calculate magnitude of velocity, centripetal acceleration, and force.

Author: Tugrul Sezen

Curator: Al Globus
If you find any errors on this page contact Al Globus.
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