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 to. This can be shown using the following formula:
This can be rearranged to:
Force = pressure x area
Area = force/pressure
High pressure can break things more easily than low pressure. If you stand on a piece of cardboard wearing high heeled shoes, you're very likely to make a hole in it because the pressure will be high from that one narrow heel. If an elephant stood on that same piece of cardboard, the pressure would be less because of the larger surface area of its foot and so there would be less chance of making a hole.
Having feet with a large surface area really helps camels to spread their weight when walking in a sandy desert - it reduces the pressure of their feet on the ground and makes it easier for them to walk without sinking into the sand.
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. A large surface area can also mean that more force needs to be applied. For example, if you try to cut an apple with the flat side of a knife, you're going to have a bit of trouble - because the area of the knife is large, more force will be needed to exert the pressure necessary to cut the apple. Turn the knife to its thin, sharp edge, and the area is reduced, meaning that less force is needed to chop that apple. This is also the reason why nails have sharp points, so that less force needs to be applied to hammer it into a piece of wood.
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 into 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.