Pascal's Principle

The Earths atmosphere is about 500 km high. All the air above you – 500 km of it – is pushing down on you with a force of over 100,000 N on each square metre. If your body presents a horizontal cross section of 1000 cm 2 or 10 -3 m 2 the force bearing down on you due to the atmosphere – using- is 100 N, equivalent to a mass of about 10 kg. You do not however feel the heavy weight of the atmosphere pushing down on you. This is because of Pascal's Principle. It states:

Pressure in a fluid acts uniformly in all directions and at each point.

This means that pressure act in at the sides, back and front, and even upwards – apart form on the soles of his feet when he is standing on the ground. The man does not feel the atmosphere pushing down on him because it is also pushing up on him, and the force with which it is pushing down is very nearly equal to the force with which it is pushing up.

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