Materials
- 24 inches of string per student
- plastic cup, one per student
- hand held one hole punch
- water
Procedure
- Before the students arrive, punch two holes near the top of a plastic cup with the hole punch. Have the holes facing each other.
- Have the students tie one end of the string through a hole in the cup and finish with a square knot, or some kind of knot that won’t slip.
- Place the other end of the string through the other hole and tie that off as well.
- Fill the cup half way with water.
- Have the students swing the cup of water around, first in a horizontal circle and then in a vertical circle.
- With practice the water remains in the cup, even when upside down.
Explanation
As the water begins to swing around, it is moving and wants to continue moving in a straight line due to inertia. Since the water and cup are moving in a circle, the direction of the cup and water is always changing the water can not go in a straight line. The water pushes on the bottom of the cup, trying to go straight. Alternately the bottom of the cup pushes inward on the water, causing the water to undergo circular motion.
Reinforcement Activities
Have students use some math and their knowledge of physics to analyze the motion of a toy plane flying in a circle. A spring scale is set up in the middle of the apparatus to measure force. Students can use free body diagrams, and some physics math in order to predict the reading on the spring scale.
Students can investigate the different variables effecting rate of rotation of a rubber cork. Have students explore how mass, force, and string length effect the speed of an object undergoing uniform circular motion. An excellent inquiry activity! For more accurate measurements students can use the Quantitative Centripetal Force Apparatus to investigate the same activities.
