Materials Included In Kit

Additional Materials Required

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The cups, index cards and ping-pong balls may be stored and used again in future classes. The straws should be disposed of in the trash or properly sanitized before reuse. If the straws are saved, disinfect them to avoid contamination and the spread of germs. The straws can be sanitized with a Lysol^® solution (1¼ oz to 1 gal of water) or a 10% bleach solution. Dip the straws in the sanitizing solution for 10 seconds, rinse thoroughly with water and allow to air dry. Store them in a zipper-lock storage bag.

Lab Hints

Demonstration 1. Bernoulli Balloon 
(Perform as the students do Activity 3. The “Floating” Ball.) 

Materials: Balloon, large*; Hair dryer; String, 10 cm*; Washers, 3* *Materials included in kit.

1. Inflate the balloon to full size, but do not overinflate. Tie a knot in the end of the balloon to close it.
2. Cut a 10-cm length of string.
3. Tie one end of the string to one washer.
4. Tie the free end of the string to the knot in the balloon.
5. Check the weight of the balloon. Turn on the hair dryer and switch it to the high, cool setting (if applicable). Position the hair dryer so the air current travels up. Place the balloon in the air column and release it. Observe the balloon float in the air column of the hair dryer. If the balloon is too light and the hair dryer blows the balloon away, add a second or third washer to the end of the string until the balloon is counterweighted enough (see Figure 6).

6. Once the balloon is properly counterweighted for the hair dryer air current, follow step 5 when demonstrating in front of students.

Demonstration 2. Wind Bag

Materials: Big breath of air; Wind bags, 2* *Materials included in kit.

1. Select a student volunteer. Challenge the student volunteer to a race to see who can inflate the wind bag faster.
2. Stand back to back.
3. After saying “Ready, set, go!” wait patiently for about 10 seconds as the student attempts to blow up the bag. (The student will likely try to inflate the bag by wrapping the bag around his or her mouth like blowing up a paper bag.)
4. As the student is blowing up the bag, extend your bag out straight. Hold the open end of the bag approximately 20 centimeters away from your mouth. Using only one big breath, blow a sharp burst of air into the bag. Remember to stay about 20 cm away from the bag as you blow. Quickly seal the bag with your hand so that none of the air escapes. Twirl the end of the bag to seal it and slide your closed hand down the bag to push the air toward the sealed end of the bag so the bag is completely inflated.
5. Display the inflated bag to the class and to the student still attempting to fill his bag.

Teacher Tips

• This kit contains several Bernoulli-principle activities for students to perform at their desks. As students are working on Activities 1 and 2, prepare the balloon for Demonstration 1. As students perform Activity 3 (The “Floating” Ball), demonstrate the larger Bernoulli demonstrator (Demonstration 1). Perform Demonstration 2 (The Wind Bag) after students have completed the activities, or save for another class period.
• Too much force will cause the ball to fly out of the column of air. A steady stream of air must be created around the ball for Bernoulli’s principle to work. If the air flow is too fast, the friction from the ball’s surface will create too much turbulence at the bottom of the ball and the air will not travel around the ball. If this occurs, Newton’s third law of motion (action—reaction) will take over and the ball will shoot upward out of the column of air.

Sample Data

Answers to Questions

Activity 1

1. Explain why the card did not blow away even with a strong blowing force.

The fast moving air traveling under the card lowered the pressure under the card. The higher atmospheric pressure on top of the card pushed the card down and prevented it from flying away. When blowing on the card from the top, the moving air hits the card directly and pushes the card down.

Activity 2

2. Were you able to blow the ball out of the cup? Use Bernoulli’s principle to explain the “Heavy” Ball.

The ball did not fly out of the cup no matter how hard air was blown through the straw. The ball stayed in the cup because the fast-moving air under the ball resulted in a lower pressure under the ball. Bernoulli’s principle states that faster moving air provides less pressure on the surface that it flows over. Atmospheric pressure above the ball will be higher than the pressure below the ball and the ball will stay inside the cup. The cup also helps to trap the moving air and causes turbulence above the ball which helps to keep the ball inside the cup even when a strong air current is present.

Activity 3

3. Use Bernoulli’s principle to explain why the ball remained in the column of air of the “Bernoulli demonstrator.”

The ball remained in the column of air because the fast-moving air traveling around the outside of the ball reduced the pressure. This is known as Bernoulli’s principle—fast-moving air over the surface of an object provides less pressure than slow-moving (or nonmoving) air. When the ball moves out of the column of air, the higher atmospheric pressure outside the air column pushes the ball back into the column.

Activity 4

4. What caused the water to rise up the straw when air was blown across the open end?

The fast-moving air over the opening of the straw lowers the pressure at the opening of the straw. Atmospheric pressure pushing on the water is higher than the pressure at the opening of the straw so the unbalanced atmospheric pressure pushes down on the water and pushes it up the straw.

Teacher Demonstration

5. Explain the procedure for filling the Wind Bag. Why does blowing on the opening of the Wind Bag from several centimeters away fill the bag faster than cupping the bag around the mouth and blowing directly into the bag?

Blowing at the opening of the bag from several centimeters away causes a reduced pressure, in accordance with Bernoulli’s principle. Creating this low-pressure region just outside the opening of the balloon causes the higher atmospheric pressure air to rush into the low-pressure region (wind travels from areas of high pressure to areas of low pressure). The wind caused by the high-pressure air traveling into the low-pressure region continues to flow into the Wind Bag and causes it to inflate.

Trying to blow the bag up by cupping it around one’s mouth will take a long time because each breathe will only add the air contained in the lungs. Using Bernoulli’s principle, the large amount of surrounding air is used to fill the bag with only one breath.

Discussion

The Stubborn Card and “Heavy Ball”

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.

For the stubborn card, as air travels quickly under the “bridge,” the pressure is reduced and the higher atmospheric pressure above the “bridge” pushes down on the card. The unbalanced forces prevent the card from flying away, no matter how fast the air travels under the card. The “Heavy” ball acts in a similar manner. Fast-moving air travels past the bottom of the ball, but the Styrofoam cup interferes with the creation of a streamline. Therefore, “normal” atmospheric pressure still acts on the top and sides of the ball and push the ball towards the low-pressure region at the bottom of the cup. The unbalanced forces again prevent the ball from flying out of the cup just as the card does not fly off the table.

The “Floating Ball”

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 (see Figure 7). 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 7). 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 the ball 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. 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 the reason the ball remains in the center of the air column.

Atomizer

An atomizer (like that of a hand-pump or perfume bottle) also uses the effect of Bernoulli’s principle to spray liquid easily. As the pump handle is squeezed, it forces air into a narrow constriction. This narrow constriction increases the speed of the air (also due to Bernoulli’s principle) which passes over an opening that leads to a reservoir of liquid. The liquid has atmospheric pressure above it, since it is open to the outside, and since the air speed has increased over the surface of the opening, there is a decrease in pressure. The atmospheric pressure pushes down on the liquid and forces it into the region of less pressure and the liquid is forced into the air stream, resulting in a fine spray of droplets. Use a straw and a glass of water to demonstrate this principle.

Wind Bag

The wind bag quickly fills with air as a result of Bernoulli’s principle. As air is blown into the bag, the air pressure around the mouth of the bag drops. Bernoulli observed that whenever air moves, its pressure drops. The faster the air moves, the more the pressure drops. As a result, the air in the atmosphere (high pressure) fills the bag as long as the fast moving air (from your lungs) creates an area of low pressure around the mouth of the bag. In this example, high pressure air moves toward low pressure air and the bag fills (see Figure 8).

References

Physics: For Scientists and Engineers. Tipler, Paul A. Third Edition, Volume 1, Copyright Worth Publishers: 1990.

Introduction

In the 18th century, a Swiss scientist and mathematician named Daniel Bernoulli (1700–1782) proposed that the faster a fluid moves the lower the pressure it exerts. Bernoulli’s principle can be used to explain why an airplane wing produces lift and why a baseball pitcher can throw a curve ball. However, before scientists could understand those complex behaviors, they had to start with the basics. Explore the foundation of Bernoulli’s principle with the following activities.

Concepts

• Bernoulli’s principle
• Newton’s laws of motion

Materials

Safety Precautions

Although these activities are considered non-hazardous, please follow normal laboratory safety guidelines. Do not blow in the direction of other students. Water may spray out of the straw. Clean up water spills with paper towels.

Procedure

Activity 1. The Stubborn Card

1. Obtain a 3" x 5" index card.
2. Fold the ends of the card down as shown in Figure 1.

3. Stand the card on the tabletop as shown in Figure 2.

4. With a strong breath, attempt to blow the card off the table top. Blow on the card from above. Then, blow under the card. What happens to the card? Record the results in the Student Worksheet. 5. Consult with the instructor for the appropriate storage or disposal procedures.

Activity 2. The “Heavy” Ball

1. Obtain a foam cup and a sharp pencil.
2. Using the pencil, poke a hole a little smaller than the width (diameter) of the straw in the center of the bottom of the foam cup.
3. Bend the flexible straw to form an “L” shape.
4. Insert the short end of the straw into the hole in the cup (see Figure 3). Push the straw into the cup about a half centimeter. Make sure the straw and cup remain attached. If necessary, use a small amount of transparent tape to secure the straw and cup together.

5. Hold the straw so the cup is upright and the long end of the straw can be placed in your mouth.
6. Place the Ping-Pong ball into the cup.
7. Blow into the straw with a strong force to try to propel the ball out of the cup. What happens? Record the results in the Student Worksheet.

Activity 3. The “Floating” Ball

1. Bend the flexible elbow of the straw to make an “L” shape.
2. Hold the Ping-Pong ball in one hand and place the long end of the straw into your mouth with the short end of the straw pointing up.
3. Blow strongly and continuously into the straw and hold the ping-pong ball in the air column exiting the short end of the straw. While blowing consistently, let go of the ball in the air column (see Figure 4). What happens? Record observations in the Student Worksheet.

4. With the ball floating in the air column, carefully angle the end of the straw away from vertical. Record the observations in the Student Worksheet.

Activity 4. The Atomizer

1. Fill a foam cup with about 50 mL of water.
2. Use scissors to cut the straw in half.
3. Place one half-length straw into the cup of water.
4. Hold the straw upright and place your mouth about two centimeters from the open end of the straw.
5. Blow across the open end of the straw with a strong breath (see Figure 5). Do not blow into the straw. Observe the water in the straw. Record your observations (including sound) in the Student Worksheet.

6. Repeat step 5 as often as necessary.
7. Use paper towels to wipe up any water that may have sprayed.
8. Consult with the instructor for the appropriate storage or disposal procedures.

Student Worksheet PDF

12993_Student1.pdf