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Problem Solving Classroom

Tips from a Master Teacher (This is a work in progress.)

Be a Rock Star

Be a Rock Star

Suppose you put one of these rowboats in your pool at home. You're quite happy for a while just floating in the sun and sipping on iced tea. Then you become curious about the following:

If you took a big rock from your parents' garden, would it raise the water level of the pool more if your put it in the boat or if you dropped it directly in the water?

Difficulty: 3/5

Keywords: science, physics, density

Solution

Although it might be tempting to say that it doesn't matter—that either one will raise the water level in the pool the same, this isn't true.

If you drop the rock in the pool, it will displace an amount of water equal to its volume. If you put the rock in the boat, you will displace an amount of water equal to its mass. Since the rock is more dense than the water, the volume of the water displaced will be more than the volume of the rock. That means the rock in the boat will displace more water than the rock in the pool.

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