Key Concepts
Physics
Pressure
Bernoulli's principle
Air flow

Introduction
Did you know that airplanes and sound have something in common? Can you guess what it might be? Air pressure! It is fascinating how air—something that is so fluid and invisible—can power an amazing number of phenomena. In this activity, you will use your own breath to blow a small paper ball into an empty bottle. It sounds simple, but is it? Try it out and see for yourself!

Background
Deep space has areas that are entirely empty—void of all matter. Scientists call this absence of matter a vacuum. On Earth a cup or bottle can seem empty, but it is different than in truly empty space. There is not actually a vacuum inside of these containers, but air. Air is invisible to the human eye but is actually made of tiny particles that move around.

When left undisturbed, air (which is a gas) will try to create an equilibrium, meaning it will try to equalize the distribution and movement of its particles. This gets interesting when you create a disturbance, such as the wing of an airplane passing through air or a vibrating drum rhythmically pushing air particles together. In the 18th century the Swiss scientist Daniel Bernoulli noticed that places with fast-moving air had lower air pressure compared with places where the air moves slower. As the air strives to reestablish the equilibrium, air will automatically try to move from the area with higher pressure to the area with lower pressure. This creates the lifting push on the airplane wing, the drumming sound and other more phenomena.

Could a difference in air pressure make blowing a paper ball into an "empty" bottle harder than it looks like it should be? Try the activity to find out!

Materials

• A four-by-four-inch piece of printer paper
• Plastic wide-mouth bottle, roughly 500 to 800 milliliters (about 17 to 27 ounces). (Use a two-by-four-inch piece of paper to create the ball if you only have bottles with a regular opening.)
• Table or other flat surface
• Carboard tube, such as from a paper towel roll
• Helper
• Small balls (optional)
• Other bottles or jars (optional)
• Drinking straw (optional)

Preparation

• Crumple the piece of paper into a tight ball. The ball should easily pass through the opening of the bottle.

Procedures

• Lay the bottle on its side with its mouth facing you. Ask a helper to hold the mouth down so it touches the work surface.
• Place the paper ball in front of the bottle's mouth, about 2.5 centimeters (1 inch) away from the bottle.
• In a moment you will blow the ball into the bottle. How challenging do you expect this to be?
• Try it out! Is it as you expected?
• Switch places with your helper. Can they blow the ball in the bottle?
• Brainstorm ideas that can make blowing the ball in the bottle easier. Try out the ones that sound most promising. If some work, what do you think makes these solutions effective whereas others fail?
• Looking at a similar situation might help explain why it is surprisingly hard to blow a paper ball into a bottle. Try rolling the ball into the bottle. Is that difficult? What is different when you roll a ball compared with when you blow a ball?
• Lay the cardboard tube with an opening facing you. Place the paper ball about 2.5 cm in front of the tube's opening. How challenging do you expect blowing the ball into the tube will be?
• Try it out. Is it as you expected?
• Compare the tube with the bottle. What is different and what is similar? What difference could make it more difficult to blow the ball in the bottle? Can you find ways to test your explanation?
• Extra: Look for other small balls or make them with items you find around the house. Is it easier with a heavier or lighter ball?
• Extra: Switch the plastic bottle for other bottles or jars. Can you blow the ball into those? Why do you think you got those results?
• Extra: Use a drinking straw to direct the air you blow directly onto the ball. Can that help you blow the ball into the bottle?

Observations and Results
It was probably almost impossible to blow the ball into the bottle without using a tool—but easy to blow it into the tube or roll it into the bottle.

Although the bottle and the tube seem empty, both are filled with air. The air in the tube can freely flow out at both ends of the tube, whereas the air in the bottle can only leave through its mouth.

When you blow you create a current of air, and the movement of air can take a light ball with it. When you blow toward the tube the air in front of the tube pushes the air that is already in the tube out on the other end. The ball follows the flow of air and enters the tube. When you blow toward the mouth of a bottle it is as if the air you blow and the ball following this flow of air bounce off the air that is already inside the bottle—because that inside air has nowhere to go. The ball does not enter the bottle.

You can also use Bernoulli's observation to explain why blowing the ball does not push the ball into the bottle. The air inside the bottle is moving slowly, so it is at a higher pressure compared with the fast-moving air in front of the bottle (the air you just blew). Because air always tries to reach equilibrium the air from the bottle (the high-pressure region) will flow out of the bottle toward the low-pressure region and take the ball with it.

When you roll the ball into the bottle air can simultaneously move out of the bottle through the bottle mouth while the ball is rolling in. In order to successfully blow the ball into the bottle, you need to concentrate the air you blow onto the ball—instead of letting the air go around it. A drinking straw can help you do that.

More to Explore
Balloon Magic with Bernoulli's Principle, from Science Buddies
Bernoulli's Principle (pdf), from NASA
Hair Dryer Gravity Defier, from Scientific American
Holes That Do Not Leak!, from Scientific American
STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies