Phantastic Physics Demos: Blow 2 Cans Together
A great proof of Bernoulli's principle is to set two empty soda cans on a table. Place them parallel to each other and a few centimeters apart. When they are just sitting there, the air has the same slow speed on all sides of the cans. This means the force from the slow air is equal in all directions. Now put your face down near the table and blow across the table between the two cans. By blowing between the cans, you are making the air between the cans move faster than the air on the other side of the cans. The movie below shows this happening.
While examining this animation you should notice the following things:
- The reduced force from the faster moving air allows the force from the slower moving air to force the two cans together.
- At first part of the air stream strikes the pink can almost head on, so the can actually gets pushed away at first. An instant later the air stream smoothes out and the force between the cans decreases.
- We are tempted to describe this as though the cans got pulled or "sucked" together. But notice the forces, they actually got pushed together by the force from the slower moving air.
This can be a concern for captains of large ships as two ships pass next to each other in a harbor. If they are parallel to each other and the wind blows, they have the potential of behaving like the cans.