To Do and Notice
Start with a tank full of water big enough to plunge two hands into at the same time. A fish tank for example.
Cut three squares of aluminum foil about 10 cm on a side.
Crumple one piece of aluminum foil into a ball.
Hold it next to a flat piece of aluminum foil in the water half way between the top and bottom of the water. Hold the crumpled ball next to it. Ask the observers what will happen when you release both pieces at once.
Release the flat piece and the ball.
Notice that the crumpled ball rises to the surface and the flat piece sinks slowly.
Next take the last piece of aluminum foil and hold it under water, crumple it into a ball. Ask people what will happen when you release it, then release it. Notice that this ball sinks.
What's Going On?
Aluminum is denser than water, even when it is aluminum foil.
When you crumple the foil in the air, air gets trapped in the ball making the ball less dense than water, so that it floats. When the ball is crumpled underwater no air is trapped so the aluminum remains denser than water.
The flat piece of aluminum foil experiences a large friction force as it sinks through the water, it sinks slowly.
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Scientific Explorations with Paul Doherty
1 Feb 2000