class 9 gravitation notes | Universal law of Gravitation | gravitation|| whats gravity | gravitation class 9 notes

                                   Gravitation Class 9 Notes

Gravitational Force :    Every object in this universe attract every other object with a certain force .
The force with which two object attract each other is called gravitational force .

Gravitational force is a non contact force .

> The strength of the gravitational attraction between two object     depends on  two factors 

• How big the object are
• How far apart they are 

                      Universal law of Gravitation 

Given by newton also known as newton's law of gravitation ,  Everybody in the universe attract every other body with a force which is directly proportional to the product of their masses and inversely proportional to the squire of the distance between them 

 .

                     GM₁M₂

_{F =                 __________}

                                R²

where G is the universal gravitation constant.

Value of G = 6.673*10-11Nm2Kg-2 

Gravitational force

                  Importance of Universal law of Gravitation 

• It bind us to the earth 
• It is responsible for the motion of the moon around the earth 
• It is responsible for the motion of planets around the sun 
• Gravitational force of moon causes tides in the sea  on earth .

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  class 9 gravitation notes 

  
  

Free Fall:     The falling of a body or object from height towards the earth under  gravitational force of earth ( with no other force acting on it ) and such a body is called freely falling body.

   Acceleration due to Gravity (g) :

The uniform acceleration produced in a freely falling body due to gravitational force of the earth is known as acceleration due to gravity.

Value of acceleration due to gravity is 9.8 m/s²

> g does not depends on the mass of the falling object of smaller mass . 

Calculation of acceleration due to gravity ( g) - 

 

F= mass * acceleration 

F= ma = GM₁M₂

_{_____________________________}

                R²

 ^{acceleration came due to the gravity of the earth} 

                 g     =       GM

                          _________

                                   R²

 

   G   g = 6.67x10^-11 x  6 x 10²⁴

_______________________                      = 9.8 m/s²

                (6.4 x 10⁶)²

The three equation of motion

Equation of motion	Motion due to gravity
·         v= u +at    ·          S = ut + ½ at2    ·         v2- u2 = 2as	·         v= u +gt   ·          S = ut + ½ gt2 ·         v2- u2 = 2gs

>  When a body is falling vertically downwards its velocity is increasing. So the acceleration          due to gravity is taken as positive    

.                     g  =  + 9.8 m/s²

 

^>  When a body is thrown vertically upwards  its velocity is decreasing . So the acceleration            due to gravity is taken as negative    

                     g  =  - 9.8 m/s²

 

> If an object is just drop or fall  from a height then  u = 0

Difference between Mass and Weight 

Mass	Weight
·         Actual content present in a body ·         Mass is always constant ·         Mass is never zero or negative ·         SI unit  = Kg	·         It is the force exert by the body on earth. ·         Weight is variable  ( change from place to place ) ·         Weight = mass * acceleration ·         Weight can be zero. ·         SI unit = N ( newton)

Gravity on moon is 1/6 less than the earth 

                                                         gravity on earth

      Gravity on moon   =              __________________

                                                                    6

Frictional Force : The force which always opposes the motion of one body over another body is called frictional force .