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The force acting normally on (at right angles to) a unit area of surface, or the ratio of force to area. The SI unit of pressure is the pascal (Pa = newton/meter2) but several other pressure units, including the atmosphere (101.325 kPa), the bar (100 kPa), and the millimeter of mercury (mmHg = 133.322 Pa), are in common use.

If a weight of 20 newtons (N) acts at right angles to on an area of 2 m2, the pressure it exerts is calculated as follows:

pressure = force / area = 20 / 2 = 10 Pa.

thumb tack
A good example of how the same force applied to different areas can have dramatically different effects is provided by the thumb tack (or drawing pin in the UK). Press down firmly on the large round head and the pin will sink easily into wood. Make the mistake of pressing down with the same force on the pointed end and the pin will sink easily into your finger! Similarly, flat shoes won't damage a wood floor whereas stilettos will.

According to the kinetic theory of gases, the pressure in a closed container of gas arises from the bombardment of the container walls by the molecules: it is proportional to the temperature and inversely proportional to the volume of the gas.

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