Swimming

Going swimming

When was the last time you went for a swim? It is a common experience that when you duck your head under the surface of the water you feel that characteristic effect on your ears. We say (quite correctly) that this is due to the pressure of the water.

If you hold your head straight up under the water you feel the effects of the pressure; if you turn it sideways, it's just the same effect. Furthermore, if you go deeper in the water the effect is stronger.

The first-hand experience gained in the sea or a swimming pool is absolutely in line with what physics has to say: The pressure in a fluid, such as water, results in the same force acting on a surface arranged in any direction.

No matter how you hold your head, at a given depth in the pool you feel the same effect on your ears.

In this respect, pressure is a different kind of quantity to force:

• A force is exerted by an environment in a specific direction, which we can show with an arrow, and is called a vector quantity (it has magnitude and direction).
• The pressure in a fluid just is and is called a scalar quantity (it has magnitude only).

For example, it makes no sense to say that the pressure at the bottom of the swimming pool is very large, straight downwards. The pressure is just very large.

Here's a summary:

Force is a vector, so can be represented by an arrow. It's useful to say that a force acts on or is exerted by an object.

Pressure is a scalar, so cannot be represented by an arrow. It's useful to talk about the pressure in a fluid.