Pressure

Pressure: Pressure is defined as force acting normally on unit area of the surface.

Pressure (P) = F/ A = Normal force on the surface    /   Area of the surface

SI unit of pressure is N/ m² also called Pascal (Pa). Pressure is a scalar quantity.

Atmospheric Pressure: Atmospheric pressure is that pressure which is exerted by a mercury column of 76 cm length at 0˚ C at 45˚ latitude at the sea-level. It is equal to weight of 76 cm column of mercury of crose sectional area 1 cm². Generally it is measured in bar. 1 bar = 10⁵ N/ m²

Atmospheric pressure 1 atm = 1.01 bar = 1.01 x 10⁵ N/ m² = 760 torr

One torr is the pressure exerted by a mercury column of 1 mm length.

Atmospheric pressure decreases with altitude (height form earth‘s surface). This is why (i) It is difficult to cook on the mountain (ii) The fountain pen of a passenger leaks in aeroplane at height.

Atmospheric pressure is measured by barometer. With the help of barometer, weather forecast can be made.

Sudden fall in barometric reading is the indication of storm.

Slow fall in barometric reading is the indication of rain.

Slow rise in the barometric reading is the indication of clear weather, 

Pressure in Liquid:  Forest excreted on unit area of wall or base of the container by the molecules of liquid is the pressure of liquid.

The pressure exerted by liquid at depth h below the surface of liquid is given as p= hdg where d is the density of liquid.