Teaching Physics with the Physics Suite

Edward F. Redish

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At the pool

If an inflated beachball is placed beneath the surface of a pool and released, it shoots upward, out of the water. Explain why.

Note to the instructor: This is a touchstone example for the idea of buoyant force. Many students have the impression that the "weight of the air" is what is responsible for gravity. Many texts encourage this by a poor choice of diagrams and examples. This problem can usefully be related to the idea that immersion in air produces an upward force on everything -- as is demonstrated by its effect on a helium balloon.

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Page last modified October 11, 2002: M09