Gravitation Class 9 Physics

Hello students, in this lesson we are going to learn most important discovery made by Sir Isaac Newton. Before proceeding with the article, just try to find out the answers for the following questions.

Why there is high tide and low tides takes place on the ocean coast? Though nobody applies force on ripe fruits they fall on earth, which force is behind this? Why the planets are bounded to rotate around the sun? Answer of these entire question lies in the same concept i.e. Gravity. As you may have heard about the story of Newton’s that, he was sitting below an apple tree and suddenly apple falls on his head. Instead of finishing the juicy fruit he wasted the time in thinking that how it came down? Why it does not flies off? And the solution was lying in the mystery of gravity which was unfolded by Newton.

He observed that the apple fall on earth in straight line. He studied the reason behind this and had some questions to himself

  1. Why the apple travelled toward earth and in straight line only?
  2. Why does an apple can’t pull the earth towards it?

It was observed that Earth exerts large force on object which pulls every object towards its centre, this force is known as gravitational force. Every particle on earth surface is directed ( pulled) towards the centre of the earth so that gravitational force obeys the inertia of unidirection and always acts along the straight line.

Gravitational force is centre seeking force which helps planets to rotate around the sun in circular orbit. The centre seeking force is also known as centripetal force, which can understand by a simple example, illustrated below.

If a small stone is tied to end of string and rotating in circular orbit with one end in hand, we can see that the stone performs the circular motion. The force for the circular motion of stone is provided from the centre of circle, this force is known as centripetal force. This force is observed in planets also due to their accelerated motion. Due to this centripetal force, the gravitational force exerted by sun on the planets can be balanced so as to maintain the fix orbit of planet.

The magnitude of centripetal force is given by

Where, m is mass of object, v is velocity of object and r is orbital radius.

Characteristics of gravitational force:

1) It is attractive and long range force.

2) It is centre seeking force.

3) It is conservative force.

4) It forms action-reaction pair.


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