# Atmospheric pressure

What is the significance of atmospheric pressure being referenced as 100 kPa ? Does that really mean 10000 kg/m$^2$ acting upon us ?

If we are measuring only collision force on the surface and not the weight of the entire column of air above us, then why figure 100 kPa (100000 N force/m$^2$) which is huge and significantly more than what is required to raise mercury level by just 76 cm.

Let's work out the force exerted per square metre of $0.76m$ of mercury. The density of mercury at room temperature is 13591 kg/m$^3$, so the mass of a block of mercury 1 square metre by $0.76$ metres high is $10329 kg$, and multiplying by the acceleration due to gravity $9.81 m/s^2$ gives the weight as $101325N$. The pressure is therefore $101325Pa$, which is one atmosphere.
So one atmosphere is exactly the pressure exerted by $0.76m$ of mercury.
A pressure of 100 kPa exerted on a surface means that on 1 cm$^2$ of that surface there is a total force of 10 N normal to the surface of that square centimeter. One must remember, however, that there may be a pressure applied to the other side of that surface resulting in a force in the opposite direction.