Phantastic Physics Toys: The Blow Pipe
The Blow Pipe is an inexpensive toy that has been around for a long time. The basic design is a small plastic pipe and a little tiny ball that sits in the pipe. As a person blows air through the pipe the ball hovers above the pipe. The pipe can actually be tipped slightly from side to side and the ball will continue to hover above the pipe. The animation below shows a person using the pipe on the left and a representation of the air flow and the forces on the ball at the right.
While examining this animation you should notice the following things:
- The fastest moving air is in the center of the air stream. The air gets slower as you get farther out on the sides of the air stream.
- As the ball moves to the left side of the air stream, there is now faster moving air on the right so the force on the right side of the ball drops which allows the ball to be pushed back toward the center by the force from the slower moving air on the left side.
- The force of Drag is created any place the air strikes the surface of the ball head on. (the bottom of the ball)
- When the force of Drag is greater than the weight then the ball rises. When the Drag is equal but opposite the weight then the ball hovers (or rises with a constant velocity).
- Fnet = ma tells us that the ball will accelerate whenever there is a net force acting on the ball.
A large version of this pipe can be seen in appliance stores in early summer. Appliance stores trying to demonstrate how strong their fans are will often point one toward the ceiling and put a beach ball hovering above it.