Pressure

The force acting on per unit area of a surface is called pressure. We can say force per unit area is called pressure

                                        Pressure = Force / Area

From the above formula, we can say that with the same applied force, Pressure is indirectly proportional to the area, thus pressure decreases with an increase in area and increases with a decrease in area.

Example:

(i) When we cut an apple, we need to use the sharp edge of the knife instead of a blunt knife because the sharp edge of knife has the small surface area and we need to exert less force with high pressure to cut the apple.

(ii) When we put a nail into a wooden board, the pointed end of the nail is kept at the front. The pointed end of the nail has a very small surface area and this enables us to apply greater pressure with the applied force.

Pressure exerted by Liquid and Gas

(i) Liquid and gas can also exert pressure on the inner walls of the container in which they are kept.

(ii) When air is filled, a rubber balloon gets inflated from all sides. This happens due to air or gas exerts pressure over the inner walls of the balloon.

(iii) Water starts leaking if there is a pore in the bottle. This happens due to water exerts pressure over the walls of the bottle.

Atmospheric Pressure

(i) Our atmosphere is made of different types of gases. Since gases exert pressure, thus Pressure exerted by air (present in atmosphere) is called atmospheric pressure.

(ii) The weight of air in a column of the height of the atmosphere and area 10 cm x 10 cm is as large as 1000 kg. The reason we are not crushed under this weight is that the pressure inside our bodies is also equal to the atmospheric pressure and cancels the pressure from outside.

(iii) A pressed rubber sucker on a plane surface does not come off because of atmospheric pressure.