PURPOSE: To demonstrate levitation of a ball by an air stream.

DESCRIPTION: A slow cart moves either horizontally or at an angle while creating an air stream which supports a styrofoam ball.

The ball levitates in the upward air stream due to the Coanda effect, causing the ball to follow along with the car. Click your mouse on the photograph above to see an mpeg video of the action.

This demonstration is often explained incorrectly using the Bernoulli effect. According to the INCORRECT explanation, the ball (or balloon or beachball, etc.) positions itself at the edge of the moving air, with the inside part in the rapidly moving air stream and the outside in the quiescent adjacent air. The pressure is lower in the moving air jet, so the differential pressure keeps the ball levitated in the air stream.

The correct explanation involves the Coanda effect. When the air stream flows past the ball, some of the air follows the contour of the ball and only leaves after it moves a significant distance along the surface of the ball, as illustrated in the drawing below. In effect, the ball is "pulling" the air around its surface. There must always be some reaction force on the ball, which points in the direction of the air stream and upward, holding the ball in the air. Click your mouse on the drawing to see the web page from which it was taken.

The Coanda effect is also responsible for such phenomena as the pulling of the convex side of a spoon into the water stream from a faucet, as seen in Question of the Week #228.

SUGGESTIONS:

REFERENCES: (PIRA 2C20.30)

EQUIPMENT: Cart on track with styrofoam ball, block for tilting track on request.

SETUP TIME: 5 min.