Bernoulli Demonstrator

Introduction

What causes the lift that takes a jet airplane off the runway? What makes a baseball curve? An 18th century Swiss scientist and mathematician Daniel Bernoulli (1700–1782) gave some insight into the answer. Bernoulli proposed that the faster a fluid like air or water moves, the less pressure it exerts. Let’s see!

Concepts

• Pressure
• Bernoulli’s principle

Materials

Safety Precautions

Follow standard laboratory precautions. To avoid contamination and spread of germs, do not allow students to share demonstrators. The devices can be sanitized after use with a Lysol^® solution (1¼ oz to 1 gal water) or bleach solution. Dip the demonstrator in the sanitizing solution, rinse thoroughly with water and allow to air dry.

Disposal

The Bernoulli Demonstrator may be sanitized and reused.

Procedure

1. With the ball in the funnel cup of the Bernoulli Demonstrator, blow gently into the mouthpiece end of the demonstrator.
2. Observe what happens to the ball—see Bernoulli’s principle in action! With practice, the ball can be made to levitate above the funnel cup.

Teacher Tips

• Care must be taken when blowing into the Bernoulli Demonstrator because too much force will cause the ball to fly out of the column of air. For Bernoulli’s principle to work for this demonstration, an air column and streamline of air must be created around the ball, which can only be created with a slow, steady flow of air. If the air flow is too fast, the friction from the ball’s surface will create too much turbulence at the base, and a streamline of air will not travel around the ball. The Venturi effect (see Discussion) will not occur and this will simply turn into a Newton’s third law demonstration of action–reaction. The air molecules will strike the base of the ball with a force much greater than the force of gravity pulling down, and the ball will fly out of the Bernoulli Demonstrator.
• Make a giant Bernoulli Demonstrator using a leaf blower and a beach ball! Place the beach ball in a vertical column of air produced by the leaf blower approximately a foot from the end of the nozzle. Make sure the ball is centered in the air column. Then carefully let go of the ball. It should float and possibly rise up and remain suspended in the air column, just like the Bernoulli Demonstrator. This may take a steady hand and a few attempts. Be patient.
• A hair dryer will also work if a leaf blower is not available or is not convenient to use. A blown-up balloon with a counterweight works well. Tie a piece of string around a washer and then tie the free end of the string to the tied end of the blown-up balloon (see Figure 1). Place the balloon into the air current of the hair dryer. If the balloon is not heavy enough, add another washer to the string until the balloon balances in the air current of the hair dryer.
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Discussion

Bernoulli’s principle states when the speed of a fluid increases, the pressure on the surrounding surface drops. This is also known as the Venturi effect and explains why planes fly, why baseballs curve, and why a race car needs a spoiler.

What causes the ball to levitate and remain suspended in the air stream above the funnel cup? The reason for the ball’s levitation can be explained by Bernoulli’s principle as well as Newton’s laws of motion. When the ball is placed into the column of rapidly moving air, the friction from the surface of the ball creates turbulence and slows the air speed down at the surface. This creates high pressure on the bottom of the ball. The ball’s surface also deflects the air column and the air is channeled around the ball in what is known as a streamline. However, as the streamline of air reaches the top of the ball, it wants to continue traveling in a straight line and does not want to follow the spherical surface of the ball. Therefore, small, turbulent air currents are produced at the top surface of the ball, which do not produce as much pressure as there is on the bottom, creating a pocket of low pressure (see Figure 2). Since there is more pressure (force) below the ball than above, the ball will rise up until the net upward force and the force of gravity pulling down are balanced. The ball will remain stable so long as the air velocity remains stable.

Bernoulli’s principle is also responsible for keeping the ball in the column of air. For example, if the ball were to drift slightly to the right, the right side of the ball will move out of the air column, creating more turbulence that will slow the air down on that side. Meanwhile, the left side of the ball has moved into the air column and has become more streamlined with the air column, creating less turbulence, and the air will travel over the surface more quickly. So this situation creates more pressure on the right side (slower air speed) and less pressure on the left side (faster air speed), and the ball is forced back into the column of air until the pressure on both sides becomes equal. This is why the ball will remain in the center of the air column.

References

Special thanks to members of the Weird Science group—Lee Marek, Naperville North High School, Naperville, IL, and Bob Lewis, Downers Grove High School, Downers Grove, IL—for bringing this demonstration to our attention.

Tipler, Paul A. Physics For Scientists and Engineers, Third Edition, Volume 1; Worth Publishers: New York, 1990; pp 346–351.