Ever wondered how
a submarine manages to sink to the bottom of the ocean and then rise again upon
command? What scientific principle is involved that allows an object to both sink
and float? Read on...
- Fill the plastic bottle with water.
- Attach a piece of clay to the arm of a plastic pen cap.
- Put the cap in the bottle so that it floats.
- Put the lid on the bottle and tighten so that it does not leak any air.
- Squeeze the sides of the bottle.
What do you think causes the pen cap to sink when you squeeze the sides of the bottle?
By squeezing the bottle, you increase the pressure inside, thus forcing more water
up into the pen cap. The added water in the cap increases its weight and causes
the cap to sink.
A submarine works along these same principles. If the average density of the submarine
is less than that of the water then the submarine will float. If the average density
of the submarine is more that that of the water then the sub will sink (or dive
in submarine terminology). While the submarine contains a lot of air (which would
make it float since the density is less than that of the water) it also contains
a lot of steel (which has a high density). So you see, it's the average density
that makes the submarine act the way it does. How can you change the average density
of the sub 'on the fly' to make it float or sink on command? Just like with the
pen cap, water is pumped in and out of ballast tanks by the submarine crew.
Parent's Note. If a fluid is at rest, pressure is transmitted equally
to all its parts and, at any one point, is the same in all directions. The fluid
acts this way because the molecules in it move freely. The molecules are far apart
in a gas and comparatively close together in a liquid.
The French scientist Blaise Pascal discovered the fact that pressure in a fluid
is transmitted equally to all distances and in all directions. He formulated Pascal's
law to describe the effects of pressure within a liquid.