- 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/Teacher/Advanced Notes [click to expand]
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.
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