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Treebeard's Stumper Answer
7 September 2001

Thinking About Sinking

We're studying water in my Dunn Middle School Science class, beginning with this classic stumper. Your boat is floating in a small pool with a load of heavy rocks. What will happen to the water level of the pool if you throw the rocks over the side? Will the water level rise, fall, or stay exactly the same? Suppose the boat develops a leak and starts to slowly sink. How will the water level change as your boat takes on water and finally sinks to the bottom? Be careful, these are stumpers! Think first, but with summer still upon us, a wet experiment is called for!
Here are a few bonus stumpers:


It seems surprising, but the water level drops when you throw rocks from your boat. A volume of rock is heavier than the same amount of water, so it has more effect displacing its weight in the boat than displacing its volume in the lake. As your sinking boat takes on water, there's less water in the lake, but the heavier boat sinks that much deeper. The water level will stay the same until your boat finally goes down, and then it drops suddenly, like tossing another rock over the side. Mass, volume, density, and buoyancy are the concepts involved, and these are the new topics in my science class.

Notes:

It's easy to do this experiment with a graduated cylinder or measuring cup for a lake, a test tube or plastic cup for a boat, and some real rocks or lead fishing weights. Poke a small hole in a plastic cup with a nail to make a sinking boat. It's fun and surprising, and a great way to start a science class. There's a rumor that several fine physicists, including Einstein, George Gamov, Felix Bloch, and Robert Oppenheimer, answered this question wrong. That must be an urban legend. I doubt they all missed that same Physics lecture on Archimedes' Principle.

Graybear had the privilege of sending his answer with a clear mind on Monday before the September 11 attack. He's safe. I'll let it stand.

I figure it's always proper to answer a question with another question, and Graybear has a good one:

What if you had an empty barrel (which normally would float high) but had enough weight attached to it so that it would be held underwater. It you threw the contraption overboard, would the water level rise, fall, or remain the same?

Here are some links for your own research on thinking about sinking:

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Copyright © 2001 by Marc Kummel / mkummel@rain.org