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Water Level Lowers
Puzzler 4

You think that the pool's water level will lower? That's absolutely right!

The reason is that, when the rock is in the boat, it displaces its total weight. If it weighs ten pounds, for example, then it's making the boat ten pounds heavier.

When the rock is sitting at the bottom of the pool, on the other hand, it displaces its volume. As explained in Buoyancy Basics and in the answer to the Buoyancy Question, an object sinks when it weighs more than the water it displaces. To follow the example given above, the ten-pound rock may only displace three pounds of water. (A given volume of rock is typically three to five times heavier than the same volume of water.)

At any rate, the rock displaces more water when it's in the boat than when it's in the water, and so the pool's water level is lower when the rock is in the water.

By the way, this is the question that the physicists J. Robert Oppenheimer and George Gamow answered incorrectly.

On to Puzzler #3...

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