Pressure is the amount of force pushing over a certain area. The size of the pressure depends on the size of the force and the area it is applied on. This can be shown using the following formula:
High pressure can break things more easily than low pressure. Keeping the size of the force the same will lower the pressure on a large area, whereas for a small area the pressure will increase. Force is measured in Newtons (N) and the area in square metres. The unit for pressure is Pascal (Pa). 1 Pascal is equal to 1 Newton per square metre.
Tiny particles make up matter around us. The particles move and collide with each other; this means they bump onto each other and any surfaces they come into contact with when they move. Pressure is caused by the force of the particles hitting things and it comes from all directions in liquids and gases.
When we pump up a bicycle tyre, we squash the gas inside it; this gas is air. We add more air to it and the pressure is increased. The gas is compressed.
Pressure in water increases as we go deeper, because the weight (gravitational force) of the water increases, too.
If we go up a high mountain, the weight of the air above us decreases. This causes the pressure to decrease, too. The graph below shows how pressure alters with height and depth.