Physics in the Kitchen: The Dancing Ping Pong Ball
Amaze your friends with the dancing ping pong ball. It’s not magic… It’s science!
What you Need
- A large wine glass with a stem
- a ping pong ball
- your finger
What to Do
- Wash your hands with dish soap and rinse your hands thoroughly.
- Place the ping pong ball in a clean wine glass.
- Dampen the tip of your index finger by dipping it in a bit of water.
- Hold the bottom of the stem of the wine glass with your dry hand.
- Rub your damp finger around the outside rim of the glass.
- If no sound is made, try changing your speed and applying more or less pressure.
- Once you have made the glass make a tone, tilt the glass on its side (just to the point that the ball is about to fall out).
- Rub your damp finger around the rim of the glass and watch as the vibrations make the ping pong ball dance.
What’s Going On?
Glass has a natural frequency where it vibrates, called a resonant frequency or natural frequency. When you rub your damp finger on the outside of the glass your finger sticks to the glass because of friction and then slips because of the water which forms a cushion that reduces friction. When this slip and stick happens in equal intervals it causes the glass to vibrate. The vibration from the glass pushes the air molecules around the glass in equal intervals, causing compression waves to form and ultimately make the sound we hear. The ping pong ball dances because it gets pushed by the vibration of the glass each time the ball comes in contact with edge of the glass. A glass that makes the tone A, just above middle C, vibrates back and forth 440 times per second, which is too fast for the eye to see. Even though you can’t see the vibration, the ping pong ball reacts to it and you can see that.
- Try tilting the glass at different angles. What effect does it have on the dancing ping pong ball?
- Try the experiment with a dry finger. Does it work?
- Try filling the glass with water (don’t tilt it). Does the amount of water effect the pitch?