This demonstration fits in well with the Science theme I am featuring this month.
Bernoulli’s Principle Demonstration
- Two ping pong balls
- table or other flat surface
- one straw for each scout
- Cut two pieces of string, each about six inches long
- Using the tape, attach one end a piece of string to a ping pong ball. Repeat with the second piece of string and the second ping pong ball.
- Tape the loose end of each string to the table top so that the ping pong balls are hanging next to each other. The gap between the balls should be about one centimeter (half inch).
- Read Bernoulli’s principal: “The pressure of a moving gas decreases as its speed increases.”
- State the problem being explored. “What will happen if you blow between the ping pong balls?”
- Have the scouts make a hypothesis for the problem. Will the balls move further apart, closer together, or not move at all?
- Using their straws, have them blow a stream of air between the balls one at a time. The balls should move closer together and bounce off of each other.
- Discuss the results and draw a conclusion. Was their hypothesis correct or incorrect?
The air being blown between the ping pong balls is moving faster than the air on the opposite sides of the balls. So the air pressure between the balls is less that the air pressure on the outside. This difference in pressure causes a force on the ping pong balls which causes them to move toward each other.
This is the same principal which causes lift in an airplane wing. See the Webelos handbook for a detailed explanation.