Question

what is valency in flotation chapter class 9th

Answer 1

The S.I. unit of thrust is Newton (N).
 e.g. For fixing a poster on a bulletin board one has to press drawing pins with the thumb.


When pressing a drawing pin, force is applied on the surface area of its head. 


The force is directed perpendicular to the surface of the board. This force is called thrust.

Pressure – The thrust per unit area is called pressure.

Pressure = Thrust/ Area

The S.I. unit of pressure is Newton per square metre (N/m2) which is also called pascal (Pa). 


Many times a bigger unit of pressure called kilopascal (kPa) is used.

The pressure depends on two factors:
(a). Force applied              (b). Area over which force acts.


The same force can produce different pressures depending on the area over which it acts 

e.g. when a force acts over a large area of an object, it produces a small pressure. 


But if the same force acts over a small area of the object, it produces a large pressure.

Let we take  two similar bricks lying on the ground, one in the lying position and another in the standing position. The two bricks exert the same force on the ground because they have the same weight. 


But the two bricks exert different pressures on the ground because their areas in contact with the ground are different. 


The brick in the lying position has a large area in contact with the ground. So, the force of the weight of the brick falls on a large area of the ground and the ‘force per unit area ‘ or pressure on the ground is less. 


The brick in the standing position has a small area in contact with the ground. So, the force of the weight of the brick falls on a smaller area of the ground and the pressure on the ground is more.

A school bag has wide straps made of thick cloth 


=  The weight of bag may fall over a large area of the shoulder of the child producing less pressure on the shoulder. And due to less pressure, it is more comfortable to carry the heavy school bag. 


=  On the other hand, if the school bag has a strap made of thin string, then the weight of school bag will fall over a small area of the shoulder. This will produce a large pressure on the shoulder of the child and it will become very painful to carry the heavy school bag.