Key Concepts
Gravity
Air pressure
Fluid dynamics

Introduction
Flying is thrilling. Do you ever make paper airplanes to watch them soar—or marvel at hot air balloons as they float through the sky? There are many ways of keeping objects aloft. Planes use lift by accelerating to high speeds; hot air balloons heat air so that it "floats" above ambient air. In this activity you'll explore a different form of keeping an object aloft—and learn about gravity at the same time.

Background
Getting an object to fly is an amazing feat. With this activity you'll explore balance of an airstream's push and gravity's pull. When you try to pull the ball out of the airstream, you can feel a force pulling it back in. You can also feel the stream of air is being deflected by the ball. These are all clues to understanding one of the forces that give airplanes lift.

Materials

• Hair dryer (One with a "cool" setting will be helpful for this activity, but isn't necessary.)
• Ping-Pong ball
• Sheet of tissue paper

Preparation
• Be safe and save energy; set the hair dryer to "cool" if possible.
• Have an adult help you use the hair dryer; a helper can also hold the dryer while you manipulate the ball

Procedure
• Blow a stream of air straight up, and carefully balance the ball above the airstream. Release it and move your hand away. What happens to the ball? Why do you think the ball is able to do that?
• Pull the ball slowly out of the flow. What happens as the ball leaves the flow?
• Pause while you're pulling the ball when the ball is half in and half out of the airstream. What sensation do you feel on the ball? Does the ball feel like it is being drawn back in?
• Let go of the ball. What happens now?
• With one hand, pull the ball partially out of the airstream again. With your other hand, dangle a piece of tissue paper and search for the airstream above the ball. What happens to the tissue paper? Does it move? How does the tissue paper react when you move the ball?
• Try tilting the airstream to one side. Can the ball still be suspended at an angle?
• To make the ball fly higher, move the blower and the ball toward a wall (try the corner of a room) and notice the great increase in height of the suspended ball. Why do you think that is?

Observations and results
When the ball is suspended in the airstream, the air flowing upward hits the bottom of the ball and slows down, generating a region of higher pressure. The high-pressure region of air under the ball holds the ball up against the pull of gravity.

When you pull the ball partially out of the airstream, the air flows around the curve of the ball that is nearest the center of the airstream. Air rushes in an arc around the top of the ball and then continues outward above the ball.

This outward-flowing air exerts an inward force on the ball, just like the downward flow of air beneath a helicopter exerts an upward force on the blades of the helicopter. This explanation is based on Newton's law of action and reaction.

When you approach a wall with the balanced ball, the high-pressure region under the ball becomes a region of even higher pressure. The air that hits the bottom of the ball can no longer expand outward in the direction of the wall. The higher pressure drives the ball to a greater height.

More to explore
Lift, from Aviation for Kids
How Do Helicopters Fly?, from Decode Science
Wind Tubes (pdf), from Exploratorium
Bernoulli Levitator, from Exploratorium

Hair Dryer Gravity Defier was developed by Exploratorium, and is on page 62 of Exploralab. Created by Exploratorium, Exploralab: 150 Ways to Investigate the Amazing Science All Around You is a book that takes curious kid scientists, ages eight–12, through 24 hours' worth of household investigations, experiments and discoveries.

This activity brought to you in partnership with Exploratorium