Force & Pressure Notes

• Pressure is defined as the force per unit area. The SI unit of pressure is pascal (Pa).

• Some very important and useful devices such as syringes, dropper, drinking straw, etc., work on the principles of pressure.
• Force exerted per unit area is called pressure.
• Liquid and gases also exert pressure in all directions.
• Pressure exerted by air is called atmospheric pressure.

Variation of Pressure with Area:

• Liquids and gases are together called fluids.
• Fluids exert pressure on all bodies immersed in them and on the walls of the container that holds them.
• E.g. A balloon expands when we blow air into it. The air inside the balloon exerts a pressure on the inner wall of the balloon. If we blow in too much air, and the material of the balloon is not capable of expanding further, increasing the pressure inside can cause the wall of the balloon to break at one or more points. This is why a balloon bursts when too much air is blown into it.

• Atmospheric pressure is defined as the pressure exerted on an object by the weight of the air above it. The Atmospheric pressure on the Earth’s surface at sea level is about one hundred thousand pascal, i.e., 100 kPa.
• Atmospheric pressure is measured using an instrument called barometer.
• We know that there is air all around us. This envelop of air is called as the atmosphere.

• 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.