Science Projects
Why Do Boats Float?

Items needed:
	A large container of water. A sink or bathtub works very well
	A metal can

Fill the container with water. Put the can in the water, letting it fill up with the liquid. Once the can is full, release it and watch what happens. It sinks.
Now, pick up the can and pour half of the water out of it. With the can half full, carefully place it back into the water, trying not to let any more water get inside. This time, it floats, just like a boat. If it sinks, pour out a little more water and try again.
Try floating the empty can and you will see.

Why? The steel can is denser than the water. By denser, I mean that a cubic inch of steel weighs more than a cubic inch of water. If something weighs less than the same amount (volume) of water it will float. If something weighs more than the same amount of water, it will sink.
When the can is partly filled with water it is also partly full of air. The combination of the steel can, the water inside and the air inside gives us something that is less dense than the water around it, so it floats.
Without some weight in the bottom, the can tends to float so high that it tips over, letting water flow in and sink the can. In a ship, this weight at the bottom is called ballast, and it serves the same function. It helps keep the ship from tipping over and filling with water.
To investigate further, you might try making boats from other materials that are denser than water. Try using aluminum foil, modeling clay, a potato, etc. As long as the total is less dense than the surrounding water, it will float. During World War II, they actually made some ships out of concrete!

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