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Faster Than Sound

Classroom Activity

Objective
To see a demonstration of the Bernoulli Effect.

• copy of "Lift Off!" student handout (PDF or HTML)
• table tennis ball
• hair dryer with a setting for cool air
• assorted sizes of small, round balloons—15 cm to 20 cm (6 in. to 8 in.) circumference when blown up

1. Divide students into small groups, and distribute materials and the "Lift Off" student handout.

2. Have students use the concepts from the demonstrations to explain how a table tennis ball is able to fly over a stream of air from a hair dryer.

Safety Alert
Caution students to use only the cool air setting on the hair dryer. Most hair dryers with two-speed settings work well.

Demonstrations

1. Air has weight and exerts pressure.

   

   Place a ruler on a table so that it extends about 8 cm (3 in.) over the edge, and hit the end of the ruler. Reposition the ruler on the table, and cover it with a flat sheet of newsprint. Hit the end of the ruler again, and observe what happens.

2. Faster-moving air exerts less pressure than slower-moving air.

   

   Suspend two table tennis balls from string as shown with about 1 cm (0.5 in.) space between them. Blow a stream of air directly between the table tennis balls, and observe what happens.

3. A difference in air pressure above and below an object causes lift.

   

   Hold a strip of paper with your hand just below your mouth. Blow air across the top of the paper, and observe what happens.

Demonstration Explanations

1. Air exerts a pressure which is 14.7 pounds per square inch (psi) at sea level. When air pushes evenly from all directions, the pressure isn't noticeable. The first time the ruler is struck, it flips easily because the amount of air pushing down on it is relatively small or is equal to the surface area of ruler. When a sheet of paper is placed on top of the ruler, the amount of air pushing down increases relative to the area of the paper. It takes more force to push back against this larger amount of air pressure.

2. Blowing air between the balls reduces the air pressure below 14.7 psi. The air pressure around and below the balls remains about the same, pushing the balls together.

3. Blowing air across the top of the strip of paper reduces the air pressure below 14.7 psi. The air pressure below the paper, however, remains about the same, pushing the paper up. The net effect is called aerodynamic lift, which is what makes the paper fly.

Lift Off!
The table tennis ball is suspended, or flies, above the air stream emitted from the hair dryer. The initial force of the moving air from the hair dryer pushes the ball up. Once in the air, gravity pulls the table tennis ball back to Earth. The fast-moving air below and around the ball, however, creates areas of high and low pressure that give the ball lift. Airplane wings are designed to achieve lift. Air flows faster over the curved top part of wing than the straighter part below the wing. The faster moving air creates reduced air pressure above the wing, allowing the wing to lift (the Bernoulli effect).

In the case of the table tennis ball, lift and gravity balance each other, enabling the ball to remain suspended. It may take a few tries to get the table tennis ball to fly successfully. Students can try to explain why sometimes the ball remains suspended in the air stream and why sometimes it shoots out of the air stream.

When students increase the speed of the hair dryer, the table tennis ball's height above the hair dryer increases due to the additional force and lower pressure of the faster-moving air. When a balloon is placed in the air stream, it will be suspended at a higher distance than the table tennis ball due to its lighter weight. As students attempt to fly a balloon and a table tennis ball at the same time, they'll discover that this works only if the ball is below the balloon. If the ball is above the balloon, there is so much turbulence that there's not enough organized lift to keep the table tennis ball suspended.