Force acting normally on a surface is called the thrust i.e. the force that acts on an object perpendicular to its surface is the thrust, measured in newton in the SI system.
When you stand on loose sand, the force, that is, the weight of your body is acting on an area equal to area of your feet. When you lie down, the same force acts on an area equal to the contact area of your whole body, which is larger than the area of your feet. Thus, the effects of forces of the same magnitude on different areas are different. In the above cases, thrust is the same. But effects are different. Therefore the effect of thrust depends on the area on which it acts. The effect of thrust on sand is larger while standing than while lying.
The thrust acting on unit area of the surface is called the pressure.
If a thrust F acts on an area A, then
Thus, the same force acting on a smaller area exerts a larger pressure and a smaller pressure on a larger area. This is the reason why a nail has a pointed tip, knives have sharp edges and buildings have wide foundations.
The SI unit of pressure is called pascal(Pa).
1 Pa = 1 N/m2
One pascal is defined as the pressure exerted on a surface of area of 1 m2 by a thrust of 1 N acting normally on it.
Other units of pressure are bar and millibar(m bar)
where 1 bar = 105 N/m2 and 1 millibar = 102 N/m2
or 1 bar = 105 Pa and 1 m bar = 102 Pa
1 atmospheric pressure (1 atm) = 101.3 k Pa = 1.013 bar = 1013 m bar
