Theory:

We see many objects around us. Some of them are moving, and some of them are at rest.
 
What causes the objects to move?
  
Before entering your house on your way from school, you should open the closed door by pushing it; while you get up from the school desk, you push yourself against the desk. So, to move an object which is actually at rest, we need force.
 
Observe the below images
  
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A ball dropped from a certain height
  
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The Motion of planets around the sun
  
From the above two images, it is very clear that a force acting on the objects makes them move.
 
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Sir Isaac newton
 
Isaac Newton found this phenomenon when he sat under the apple tree. He called the force which is responsible for the motion of planets is the gravitational force. In this part, we shall learn about the universal law of gravitation and objects' movement under the influence of gravitational pull on the earth. We will analyse how the weight of a body differs from one place to another and the conditions for objects to float in liquids.