Two Can Two-Can

Physics:	Air Pressure
Grade Range:	Elementary School
Format:	Hands-on

This is one of our most commonly performed demonstrations. It is easy to perform, easy to understand, and the topic matter is easily adjusted for all grades and age levels.

Materials

• Soda Cans (two per pair)
• String (two 6in pieces per pair)
• Popsicle Sticks (one per pair)
• Straws (one per pair)

Safety Precautions

Please read the General Safety Precautions section of the Demonstration Safety page before performing this demonstration.

Demonstration

1. Put the students into pairs, and give the demo supplies to each pair.
2. Have each pair of students tie their soda cans to the popsicle stick. Have one student in each pair hold up their finished model, and the other student adjust the can strings as needed until the cans hang very close to each other (1/2 in apart).
3. Have the students take turns holding up their models, and their partner using the straw to blow air between the hanging cans. They should see the cans move towards each other!
4. Now have them take turns holding up their models while their partner blows air between the hanging cans without the straw. Make sure they stand the same distance from the cans as they did when they used the straw. Can they still make the cans move towards each other?

Why This Works

There is air all around us, and that air pushes on us all the time. This is called Air Pressure, and we are using air pressure to make the cans move in this demonstration. When the cans are hanging in the still air, they experience air pressure equally on all sides, so they don't experience a push in any given direction. However, when air is blown between the cans with the straw, this moving air is no longer applying air pressure equally in all directions. Instead, the moving air is pushing mostly in a single direction (forward) and does not push as much on the cans (sideways). The still air on the other side of each can is pushing more on them than the moving air between them, so the cans get pushed to the center and towards each other!

When you blow at the cans without the straw, there are a few things that change. First, your breath is moving slower than when you used the straw, so the change in air pressure is smaller. Second, your breath doesn't blow directly in-between the cans, but rather is spread out more and hits the cans from the front. These two changes can affect the demonstration a lot, causing the cans to just bounce around from you blowing at them, instead of being pulled together.

Additional Information

• This demonstration benefits from the Bernoullis Principle demonstration, as they show similar concepts. However, be careful to explain that there is a difference in these demonstrations; Although they both talk about air pressure, this demonstration is not using the Bernoulli's principle.
• This demonstration pairs well with the STAHP demonstration.