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Pressure 
The idea of pressure is useful in situations in which a force is distributed over a surface.
For example, the wind can provide a force to move a boat but the force provided by the wind is spread over the whole of the surface of the sails of the boat.
 
Pressure is defined as force per unit area.
 
Therefore, the relation between force and pressure can be expressed as
where F represents the magnitude of the force and A the area over which it is spread.
 
Many units are used for pressure depending on the situation but the fundamental (S.I.) units for pressure are Nm-2.
1Nm-2 is called 1Pascal after Blaise Pascal, French mathematician and physicist.
 
Pressure Exerted by a Fluid
"Fluid" means liquid or gas but in the example below we will consider a liquid of density, ρ.
We can consider the pressure acting on the arbitrarily chosen area, A, to be due to the weight of the liquid vertically above it (represented by the darker area).
 
 
The mass of the liquid vertically above the area A is given by
 
and the weight of this liquid is
 
Therefore, the pressure at depth h below the surface of a liquid of density, ρ is given by
 
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