Key Concepts
Physics
Pressure
Bernoulli's principle
Airflow

Introduction
Have you ever seen pictures or videos of a roof being blown off a house during a hurricane or tornado? You might be surprised to hear that the roof is actually not blown off by the strong winds but instead by the air inside the house! This can be explained by Bernoulli's principle, which states that fast-moving fluids or air, such as strong winds, have lower pressure than slow-moving air. In a hurricane the fast flow of air above the roof's shape generates a low-pressure area. This creates a pressure difference between the air outside and the air inside the house. Eventually the pressure difference becomes large enough that the air inside the house starts pushing on the roof. The roof experiences lift, similar to an airplane wing, and flies away! In this activity you will put Bernoulli's principle to work—but do not worry, your roof will be safe!

Background
Daniel Bernoulli was a Swiss scientist who in the 18th century studied how fluids behave when they are in motion. When experimenting with fluids flowing through an hourglass-shaped tube, he made a discovery. He realized that fast-moving fluids produce less pressure and slow-moving fluids produce greater pressure. His discovery became known as the Bernoulli principle. It is not only true for fluids but also for air because gases—just like fluids—are able to flow and take on different shapes. A simple demonstration of Bernoulli's principle requires floating a ping pong ball in a moving stream of air, for example on top of a fan or hairdryer pointed straight up. Why does the ball not fly off the fan? It is because of Bernoulli's principle. The fast-moving air, which carries the ball into the air, is at a lower pressure than the air surrounding the ball. When the ball starts to fall off the column of air above the fan, the surrounding higher-pressure air pushes the ball back into the area of lower pressure above the fan. As a result the ball stays afloat on top of the fan.

Bernoulli's principle can also explain how lift is generated under an airplane wing. Airplane wings are designed to let the air flowing over the top move faster than the air flowing underneath. This creates a pressure difference in which the pressure on the top of the wing is lower than on the bottom. This higher-pressure air pushes up on the wing and thus creates an upward lifting force (similar to the roof example above). There are many more demonstrations of Bernoulli's principle in the real world. You are going to perform one of them in this activity. It doesn't involve flying or blowing a roof off a house, but it will be just as impressive—in fact it will almost be magical!

Materials

• Two balloons of the same size
• String (about 60 centimeters length)
• Scissors
• Tape
• A door frame
• Paper-towel tube

Preparation

• Inflate both balloons, and tie them off at their ends. Both balloons should be approximately the same size.
• Cut two pieces of string, each about 30 cm in length.
• Tie the end of one string to one of the balloons.
• Tie the end of the other string to the second balloon.
• Use tape to attach the loose end of each of the strings to the underside of the top of the door frame. Space the balloons so that there is a gap of about 15 cm between them.
• Make sure to keep the balloons away from significant air flow (such as a vent or fan).

Procedure

• Step in front of the balloons and hold the paper-towel tube so that you can blow air into the space between the two balloons. What do you expect to happen to the balloons if you blow air in between them?
• Make sure that the balloons are still. Then blow into the paper-towel tube very slowly. Try to produce a steady air flow. What do you notice? Are the balloons moving?
• If the balloons moved, stop their movement and then blow in between them again using the paper -towel tube. This time try to blow through the tube harder than before but still try to maintain a steady air flow. What happens to the balloons this time? Can you explain your observations?
• Repeat the previous step but this time blow through the tube as hard as you can, producing a steady air flow. Do your results change with increasing air flow? Why or why not?
• Extra: Repeat the same tests but vary the size of your paper tube. Do smaller diameter tubes, such as a straw, have the same effect?
• Extra: Find out if you can still make the balloons move if they are farther apart from each other. Change the distance between the balloons, and test if it affects your results. Do you find a maximum or minimum distance beyond which the activity does not work anymore?
• Extra: Instead of balloons try using ping pong balls or other objects in this activity. Do you still see the same effect with larger or smaller or heavier objects?

Observations and Results
Did you notice that both balloons magically moved toward each other without being touched at all? The effect which you observed is a great demonstration of Bernoulli's principle. As long as both balloons just dangle from the doorframe, the air around them in every direction is static. This means the air exerts the same amount of pressure onto every side of the balloon and both balloons are still. When you slowly blew air in between the balloons, they probably did not move much. This is because a very slow air flow does not greatly change the pressure conditions around the balloons.

When you blew through the paper tube more forcefully, however, you should have noticed that the balloons (almost!) magically came together. By blowing air forcefully between the balloons you created an area of low pressure. This is because fast-moving air produces less pressure. The air pressure between the balloons decreased in comparison with the air pressure around the rest of the balloons. Because higher pressure pushes toward lower pressure the balloons were pushed toward each other. You could have made the same observations using ping pong balls instead of balloons. With heavier objects, however, the generated air pressure difference might not be enough to make them move. The spacing between the balloons matters as well. If the gap between the balloons is too wide, the low air pressure area produced by blowing between them will no longer have an effect.

More to Explore
Bernoulli's Principle, from the National Science Teaching Association
Theory of Flight, from the M.I.T. Department of Aeronautics and Astronautics
Suction Science: How to Break a Ruler Using Air Pressure, from Scientific American
Measure Wind Speed with Your Own Wind Meter, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies