It Stays Up!


Reverse the hose on a vacuum cleaner so that it blows. Point the hose nozzle up, and, while the machine is operating, place a Ping-Pong ball in the airstream. If the ball doesn’t remain in the stream of air, try different positions until it does.

What Happens and Why

When placed at the correct height, the ball will vibrate and rapidly bounce about in the airstream without falling.

Note that the ball is attracted to the center of the moving air, since the air is moving faster there and the greater air pressure on the outside of the ball pushes it toward the center of the airstream. The ball is held up because of the push of the airstream beneath it.

Now inflate a round balloon until it is about six inches in diameter, then weight the balloon with a piece of soft wire wrapped around the knot of the balloon, and place it above the Ping-Pong ball in the airstream. Will the balloon behave the same way the Ping-Pong ball does?

How far can you change direction of the vacuum cleaner nozzle before the ball and balloon no longer remain in the airstream?

[illustration] Illustrated by Shauna Mooney