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 
