Thinking It Through (IMAGE)
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Q: If a stone aboard a boat is dropped in the water, does the level of the water rise, sink, or stay the same? An innovative physics course at Washington University coaxes students to reason their way through problems like this one instead of memorizing the answers.
A: When the stone is in the boat it is displacing an equivalent mass of water. When the stone is thrown over the side it is displacing it's own volume in water. So which of these is greater? The stone must be more dense than water because it sinks. So the volume of water equivalent to the mass of the stone is greater than the volume of the stone. Less water is displaced after the stone is thrown overboard, and the water level goes down.
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Monica Duwel
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