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3.1 Pressure In Solid

Definition and the S.I unit of Pressure (P)

Pressure is defined as the force acting normally on a unit of surface area.


@ Pressure   =  Normal force


@          P     =  F 


P = Pressure    F = Force   A = Surface area


The S.I. of pressure is    Nm-2  or pascal(Pa)


Example 1


On a table is a full box of groceries which has a weight of 160 N. The area of the base of the box is 0.2 m2 . What is the pressure between the box and the table top?










Example 2


A metal block with the dimensions of  0.05 m x 0.10 m x 0.20 m is placed on a floor. If the mass of the metal block is 0.5 kg , what is 

(a) the maximum pressure and

(b) the minimum pressure

      that can exerted by the metal block on the floor






Application of pressure

(1)     The surface area of the head of thumbtack is large but the cross-sectional area of tip of the thumbtack is small. The high pressure of the tip causes the thumbtack to go into the wood easily.

(2)     A lady’s high heel shoe with a small base  exerts a higher pressure than a man shoe with a flat base.

(3)     A smaller area of the edge of knife causes the knife can cut and penetrates  hard materials.

(4)     Walking over deep snow is difficult and tiring because your feet sink at every steps. Skies or snowshoes make it much easier because they stop you from sinking in. The skies causes the pressure becomes smaller.

(5)     The sole of an ice is fixed a narrow metal bar. The high pressure on surface of the ice so that the ice melts and allowing the ice skater to glide smoothly .

(6)     Racing bicycles need very high air pressure inside the tyres,because the narrow tyres have a very small contact area with the road. The hard road surface can support the high pressure under the wheels.

(7)     Heavy vehicles which travel over soft ground need  very wide tyres. This provides a large contact  area and a lower pressure against the ground and prevents them sinking in.

(8)     A high bulding possesses a huge surface so that the lower pressure acting on it prevents the building from sinking into the ground.

(9)     Studs on the soles of a pair of football give  the player a better grip of the ground as he dribbles the   or running  on the field .

(10) An elephant is able to move on soft ground or mud because the large area of its feet reduces the pressure on the ground.


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