We had drenching rains today, and
a comment about needing a boat to get to my shows tomorrow helped me
decide on this week's experiment. Think about the boats that you have
seen. What were they made of? Some may have been made of wood, but most
boats and ships today are made of metal, like steel. Does steel float?
Tonight, help with washing the dishes and see if the forks and spoons
float in the water. If steel sinks, how can a steel boat float? To
discover that, you will need:
* a large
container of water. A sink or bathtub works very well * a metal can
Fill the container with water. So
you won't waste water, wait until you are going to use the water
anyway, for washing dishes or taking a bath. 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. 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.
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. Why does it float now? The can is 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.
Why did we leave some water in
the can? Try floating the empty can and you will see. 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!