KinexiaLEARN PHYSICS

Bernoulli Ping-Pong Trick

Air can push things together and hold a ball floating in nothing. Two demos, one principle: Bernoulli.

Safety:

Use the hair dryer's cool setting only. Keep the dryer away from water.

Materials

  • Two ping-pong balls
  • String (about 60 cm)
  • Tape
  • A straw
  • A hair dryer with a cool setting (optional)

Procedure

1

Hang two balls

Tape a 30 cm string to each ball and hang them from a doorway or shelf so they dangle side by side, about 3 cm apart.

2

Blow between them

Blow a steady stream through the straw directly between the balls. Watch which way they move and describe it.

3

Blow on one side

Now blow on the outer side of one ball only. Compare the motion — this isolates what the pressure difference does.

4

Levitate a ball

Hold the hair dryer pointing straight up on cool. Drop a ball into the stream from a few centimeters above and let go.

5

Tilt and test stability

Tilt the dryer slowly. How far can you tilt before the ball escapes? Note the self-correction as it wobbles.

What to observe

  • The hanging balls swing toward each other when you blow between them.

  • A ball stays trapped in the airstream, wobbling but not falling.

  • Blowing harder makes the effects stronger, not weaker.

Why it happens

Fast-moving air has lower pressure than still air (Bernoulli's principle: P+12ρv2=constantP + \tfrac{1}{2}\rho v^2 = \text{constant} along a streamline). Blowing between the balls drops the pressure there, so the higher outside pressure squeezes them together. The levitating ball rides the same effect: the fast jet's low pressure pulls it toward the stream's center whenever it drifts, self-correcting its position — the reason a shower curtain billows inward and airplane wings lift.

Try variations

  • Hold a strip of paper under your lower lip and blow across its top — which way does it move?

  • Try levitating a balloon instead of a ball. What changes?