Pressure is the force acting on a unit area on an object.

Pressure = Force/Area on which it acts

In this equation, force is the numerator and area is the denominator. This means that pressure is directly proportional to the applied force but inversely proportional to the area on which it is acting.

Following examples illustrate this effect:

  • It is easier to push a nail into a wooden door through its pointed end than through its blunt end because a small area of the pointed end helps in creating more pressure and it becomes easier to insert the nail.
  • Shoulder straps of your school bag are wide. A larger area helps in reducing the pressure on the shoulder and makes it less painful to carry the bag.
  • Camel’s feet are wide; making for the larger area. Due to this, a camel can easily walk on sand.
  • Eskimos wear ski-like footwear. The wider area of the footwear reduces pressure and thus prevents the Eskimo’s feet from sinking in ice.

Pressure Exerted by Liquids and Gases

Liquids and gases exert pressure as follows:

  • Pressure on the bottom of the container depends on the height of the column of gas or liquid. Due to this, divers have to withstand a large pressure at the bottom of the sea.
  • A fluid exerts pressure on walls of the container. Containers for gases and liquids are usually cylindrical ion shape to equally distribute the pressure on all portions of the wall.
  • A fluid exerts equal pressure at same depth.

Atmospheric Pressure:

The pressure exerted by atmospheric air is called atmospheric pressure. The weight of air in a column of the height of atmosphere and area 10 x 10 cm is 1000 kg. This is roughly same as the area of our head. We have around 1000 kg of air on our head.

The pressure of all the air above our heads is the force that pushes air into our lungs and squeezes oxygen out of it and into our bloodstream.  As soon as that pressure diminishes (such as when we ascend a high mountain) less air is pushed into the lungs, hence less oxygen reaches our bloodstream and hypoxiation results; again, not due to a lessening of the amount of available oxygen, but to the lessening of atmospheric pressure.