Demos: 2C-06 Ball in Air Stream

A small glass tube, tapered at the end, is attached to the lecture desk air supply. The rubber tubing connected to the air supply is clamped just below the glass tube so that the latter is held in a vertical position but is free to be pivoted. After the air is turned on, a ping pong ball is placed in the air stream and released. The ball remains “floating” in the air stream and will remain there even if the glass tube is tipped several degrees away from the vertical.

Two effects are at work here: The ball stays in the stream because the pressure in the stationary air is larger than that in the moving air. The ball stays supported in the stream because of direct impact of the air (sometimes referred to as “ram pressure.”)

Directions: Turn on the air supply about half way at first, to test the strength of the stream. Place the ping pong ball in the stream and let go. This might take a try or two before you get it to stay in the stream. Adjust the air flow to get the height you want. After the ball has stabilized, SLOWLY tip the glass tube back and forth and the ball will stay in the stream. (If it doesn’t, do it again. Sometimes instabilities kick the ball out, but usually it works.) Other balls in the supply box can be tried also.

Suggestions for Presentation: Start the air flow and ask what would happen if you placed a ball on top of the stream. Do it and allow the ball to be moved around. Why does it stay in the stream?
An even more dramatic version of this demo is to use a leaf blower and a large ball.

Applications: Many department stores attract attention to product areas by having a beach ball suspended in the air stream from a fan.

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