The Idea
In this project, you get a pair of ping-pong balls circulating rapidly in a beaker with a blow dryer. The balls continue racing in a frantic high-speed circular path long after the blow dryer is removed. This is a fun, attention-getting demonstration that explores various aspects of circular motion, including angular velocity, centripetal force, and the effect of friction.
What You Need
- 250 mL glass beaker or plastic container roughly 5 inches (12 cm) in diameter
- 2 ping-pong balls
- blow dryer
Method
- Place the ping-pong balls in the beaker.
- With one hand, hold the bottom of the beaker. The other hand holds the blow dryer. (No heat is needed.)
- Direct the air from the blow dryer to rapidly circulate the air flow in a circular horizontal pattern inside the beaker.
- The blow dryer should get the ping-pong balls to rapidly spin inside the glass container.
- As soon as the ping-pong balls are spinning rapidly, quickly turn the beaker upside down and (carefully) place it on the table.
Expected Results
The ping-pong balls continue to revolve around the inner walls of the container. While spinning, they appear to defy gravity. They also tend to move as far away from each other as they can, especially as they slow down.
Why It Works
The rapidly moving air gives the ping-pong balls kinetic energy. The inner walls of the beaker provide centripetal force that keeps the balls moving in a circular path. The force between the rotating balls and the sidewall of the beaker results in a frictional force that is large enough to hold the balls suspended above the table as they rotate. The balls have enough angular momentum to keep going until frictional forces between the ball and the walls of the container cause them to slow down, resulting in the balls continuing to rotate more slowly and drop to the surface of the table.
The rapid rotation causes friction between the balls and the side of the beaker. This can cause the ping pong-balls to become charged, resulting in (minor) attraction to the walls of the container and repulsion from each other.
Other Things to Try
A similar effect can be achieved by vigorously rotating a pair of marbles in an inverted glass or beaker.
The Point
The ping-pong balls are given kinetic energy by the blow dryer. Like all rotating objects, their inertia tends to keep them moving in a straight line. The inside walls of the beaker apply centripetal force, which causes the path to be circular. The balls continue to move until the kinetic energy is converted into friction.
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