When a force is applied on a given area,Pressure is exerted on the surface.

Pressure is defined as the,force acting normally(perpendicularly)per unit area.

Units of pressure

From the definition,

Pressure P=Force (F)/Area (A). Therefore, the SI Unit of pressure =SI Unit of force/SI unit of area. =N/m^2.

The SI unit of pressure is thus newton per square metre, which is also called the pascal(Pa).

Other units include the mmHg,the cmHg and an atmosphere (atm).

Example 1

A man of mass 84kg stands upright on a floor. If the area of contact of his shoes and floor is 420cm^2, determine the average pressure he exerts on the floor.(Take g=10N/kg).

Solution

Pressure = force/area

Force=mg =84 x 10N =840N.

Area=420 cm^2 =0.042m^2

Thus pressure = 840/0.042 =20000N/M^2.

Example 2

A metallic block of mass 40kg exerts a pressure of 20N/m^2 on a flat surface.Determine the area of contact between the block and the surface.(Take g=10N/kg).

                 solution

Force = weight of the block

40 x 10N = 400N

Since P = F/A  A=F/P = 400/20 = 20m^2

Factors affecting pressure in solids

1. Force applied.

Pressure increases with the increase in force applied if area is kept constant.

2. Area incontact with the ground.

Pressure increases with decreasing in area and decreases with increasing area,if the force is kept constant.