You may get told off if you swing around on one leg of a chair instead of sitting properly. Apart from the risk that you will damage the chair or hurt yourself, the chair leg can damage the floor. This is because it puts too much pressure on the floor.

Working out pressure

To work out pressure, we need to know two things:

1. the force or weight applied

2. the area over which the force or weight works.

Pascals

Notice that the unit of pressure here is N/m² (newtons per square metre). Sometimes you will see another unit being used. This is called the pascal, Pa.

1 Pascal = 1 N/m², so in the example above the pressure is 10 Pa.

Using pressure

Drawing pins

Drawing pins have a large round end for you to push. The round end has a large area, so it applies a low pressure to your thumb. The sharp end has a very small area. The same pushing force produces a high pressure there, so it pushes into the notice board.

Chair legs

If you swing round on one leg of a chair, you put four times as much pressure on one point of the floor as you do if you sit properly. This is because four chair legs spread the pressure over four times more area than one chair leg can.