Chapter-10. Gravitation Science class 9 in english Medium CBSE Notes

10. Gravitation

Thrust And Pressure:

Weight:  Weight is the force acting vertically downwards.

Thrust: The force acting on an object perpendicular to the surface is called thrust.

Pressure: The force (thrust) exerted by an object per unit area is called pressure. SI unit of pressure N/m² or N m^-2 This unit is known as Pascal, which is denoted by Pa. 

Pressure = Thrust/area

Understanding this by a simple example:

lets force (thrust) = 10 N

      And Area = 2 m²,

then, 

     Pressure = 10 / 2 N/m² 

                     = 5 N/m² 

    if area increases by 5 m² on same force.  

    Pressure = 10 / 5 N/m² 

                    = 2 N/m² 

We get conclusion that if increases the area on same force    

Important points:

• The effect of thrust depends on the area on which it acts.
• Area is inverse proportionally to the pressure, It means the same force acting on a smaller area exerts a larger pressure, and a larger area exerts a smaller pressure. 
• if larger thrust (force) acting on same area pressure will be larger and smaller thrust acting on same area pressure will be smaller, so relation between thrust and pressure is direct proportionally. 
•  

Up thrust or buoyant force depends on: 

(i) The size or volume of the body immersed in a liquid.

(ii) The density of the liquid in which the body is immersed.