Hair Dryer Levitation

Contributed by Alec Clark

Introduction:

If you drop a table tennis ball on a powered hair dryer and tilt that hair dryer to the side, you might expect the ball to drop or fly away. However, could you imagine the ball levitating in place?

Materials:

• A table tennis ball
• A hairdryer (be careful, these can get hot! Make sure to not leave it running for too long, parental supervision is recommended)

Procedure:

1. Plug the hair dryer into a working power outlet.
2. Hold the hairdryer, pointing it straight upwards.
3. Turn on the hairdryer.
4. Drop a table tennis ball right above the hairdryer, so it floats.
5. Slowly tilt the hair dryer to the side: does it fall, and when?
6. These next steps are optional, but they help to see why the table tennis ball floats: repeat step 4, then tap the table tennis from the top, and observe what happens.
7. Tap the table tennis ball from the side, again observing what happens.
   

Physics Concepts and Questions:

• You should have noticed that even when tilting the hair dryer to the side, the ball will stay floating: why is this? … Before answering this, it is helpful to consider the two optional steps.
• You should have observed with the 6th step that tapping the table tennis ball from the top made it bob up and down in place: why does this happen?
  • The hair dryer is blowing air upwards, and that air hits the ball, knocking it upwards. Air closer to the hairdryer is moving faster (note: the speed air is moving is related to the temperature. This is why it gets so hot close to the dryer!), so it pushes the ball up higher, making it bounce in place. When the ball is floating and not bouncing at all, it is said to be in “equilibrium”.
• You should have also seen with the 7th step that the ball either bobbed from side to side, or fell: why does this happen?
  • This might have surprised you, since it seems like the ball should fall off, even if it was tapped lightly. Air generates “pressure”, which pushes on everything it contacts. Air also follows “Bernoulli’s Principle”, one result of which is that faster moving air generates less pressure. Since the air from the hairdryer is moving faster than the air around it, it generates less pressure than it: there is essentially a wall of air of higher pressure keeping the ball from falling off the sides! This does have its limits though, if the ball is pushed too hard, it will fall.
• So, why does the ball float when the hair dryer is tilted?
  • This is actually a combination of the two previous observations. The air from the hair dryer knocks the ball upwards, which is why it floats. Bernoulli’s Principle leads to the wall of air of higher pressure, stopping the ball from falling off the sides. Although, this wall of air has its limits, so if the hairdryer is tilted too far, the ball will fall.

Conclusions and Further Investigations:

You can try out various different actions with this set-up. For example, what if you move the hair dryer up and down, or side to side? What if you cup your hand over the top of the airflow? If the hair dryer has high and low settings, does changing them affect anything?