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Consider a heap of loose sand. When you lie down on the loose sand, your body does not sink much into the sand. But if you stand on loose sand, your feet sink more deeply into the sand. In both the cases, the force exerted on the sand is the same and it is equal to weight of your body.When you stand, the force is exerted on a smaller area and when you lie down, the same force acts on a larger area of the sand surface. So we define a physical quantity called pressure as follows:

The force per unit area is called pressure. Thus,
Pressure = Description: 58816.png

If a force F acts on an area A, then the pressure Description: 58826.png.

Pressure depends on the following factors.
  1. The area of the surface on which the force acts. The smaller the area, the larger is the pressure exerted by the force.
  2. The amount of the force acting on the surface. The greater the force on a surface, the more is the pressure.
The total force acting on a given surface is known as thrust.

Units of Pressure

In SI units, force is measured in newton (N) and area in m2. Therefore, the unit of pressure is Nm−2. Nm−2 is also called pascal (Pa) in the honour of French physicist Blaise Pascal.

One pascal is defined as the pressure exerted on a surface of area of 1 m2 by a force of one newton (N).

In CGS system, unit of pressure is dyne cm–2.

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