Derive an expression for atmospheric pressure.

Explanation

1. All objects on the surface of the earh are subject to constant atmoshpheric pressure.
2. We know that the pressure is thrust (force) per unit area.
3. Atmospheric pressure `=("Force of the atmosphere")/("Surface are of the earth")`
`=("Weight of the atmosphere")/("Surface area of the earth")`
`=("Mass of atmosphere"xxg)/("Surface area of the earth")[Weight W=mg]`
`=("Density of atmosphere"xx"Volume of atmosphere"xxg)/("Surface of the earth")[because"Mass"="density"xx"volume"]`
`=(rhoxx"surface are of the"xx"height of the atmosphere"xxg)/("surface are of the earth")[because"Volume"="area"xx"height"]`
`=rhoxx"height of the atmospher"xxg`
`therefore"Atmospheric pressure"P_(@)=rhoxxhxxgrArrP_(@)=rhohg`