In this experiment, students see the forces produced by the Bernoulli effect. As the students blow between the hanging balloons, they speed up the air, creating an area of lower air pressure. The balloons move together since the surrounding air, having a higher pressure, pushes the balloons together.

Bernoulli's Principle states that as the speed of air increases, its pressure decreases.

Airfoils create lift using Bernoulli effects. Air flows more quickly over the top surface of a wing than over its lower surface. An upward force is produced by the slower moving air beneath.

Aircraft wings come in a variety of designs. In all designs air is made to flow faster over the wing's top surface than under the lower surface. This is mainly accomplished by an upward tilt in the wing, called angle of attack.

Lift is greater when there is a large wing area and when the plane is travelling fast. Ailerons, or the moving parts of the wings, can move up and down to change the angle of attack, therefore tilting the plane upward or downward.

### Objectives

• Explain how differences in air pressure create lift.

### Materials

• Per Student Pair:
2 pieces of string (approx. 30 cm)
2 balloons (large size works best)
water (optional – adding a very small amount of water to the balloon will stabilize it)

### Key Questions

• What happens when you blow between the balloons?
• Where is the air moving fast? Where is it moving slowly?
• How can we relate this phenomenon to the wing on an aircraft?

### What To Do

Preparation:

1. Cut string into lengths of approximately 30 cm (12 in).

Instructions:

1. The students blow up the balloons and tie them off.
2. Tie a piece of string to the neck of each balloon. One partner holds the balloons so that they are hanging loosely about 5 – 10 cm apart.
3. The other partner blows hard between the two balloons.
4. Reverse roles and try it again.

​Teacher Tips:

• Use a small amount of water in the balloons – otherwise they’ll be too heavy.
• Larger-inflated balloons are better, as they will hold more air.
• You may have to experiment beforehand to find the optimal distance at which to hold the balloons apart.

### Extensions

• Draw the balloon setup in your notebook and indicate the relative air pressure between the balloons and around the balloons. What causes them to come together?
• This experiment can also be conducted using 2 empty pop bottles or 2 empty pop cans, side by side on a table so that they roll toward each other when you blow between them.
• Next time you take a shower make some observations about the shower curtain. Before the shower head is turned on the curtain is hanging vertically. When the fast moving water is moving past the inside surface of the curtain what does it do? How does Bernoulli’s Principle explain the curtain’s behaviour?

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Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

Egg BB

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

Comet Crisp

Artist: Jeff Kulak

Jeff is a senior graphic designer at Science World. His illustration work has been published in the Walrus, The National Post, Reader’s Digest and Chickadee Magazine. He loves to make music, ride bikes, and spend time in the forest.

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Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

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Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

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Artist: Michelle Yong

Michelle is a designer with a focus on creating joyful digital experiences! She enjoys exploring the potential forms that an idea can express itself in and helping then take shape.

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Artist: Ty Dale

From Canada, Ty was born in Vancouver, British Columbia in 1993. From his chaotic workspace he draws in several different illustrative styles with thick outlines, bold colours and quirky-child like drawings. Ty distils the world around him into its basic geometry, prompting us to look at the mundane in a different way.

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Artist: Ty Dale

From Canada, Ty was born in Vancouver, British Columbia in 1993. From his chaotic workspace he draws in several different illustrative styles with thick outlines, bold colours and quirky-child like drawings. Ty distils the world around him into its basic geometry, prompting us to look at the mundane in a different way.