# Gravity

## Gravity

Dr. G.Singh

We all are familiar with the word ‘Gravity’, it is not just a word but it is part of our every day’s life.

Even if someone is not familiar with Gravity he know about a story of Newton & Apple. I won’t go into detail of the story. In brief we can say :

Isaac Newton was in his garden and suddenly an apple fell on his head. He though why the apple come down toward Earth and an excellent theory was developed, we know it as the theory of gravitation.

It looks simple yet complicated. We saw things fell down toward Earth as soon as we drop them. Did we even think about the reason behind this behavior of objects. Fortunately we don’t need to discover the reason Isaac Newton already has done this for us. But we need to understand what Newton told us.

Yes friends theory of gravitation is about the behaviors of objects under the influence of mass. I used the word mass not the Earth. Because it is not about Earth, we can observe the similar behavior on Moon also. Even on any planet we will observe the same behavior. Question is if the behavior is same is the magnitude also same ?

This is the point where things become complicated. I can say behavior of objects will be same on all planets but the speed of falling will be different on different planet. Why ? This understanding comes from theory of gravitation.

The theory of gravitation tell us two bodies attract each other. Here apple was a body and Earth was another body. Does that mean Earth and Apple attract each other ? Answer is Yes. Does that means when Apple was falling down toward Earth, Earth was also ? Theoretically yes. But when we compare the masses of Earth & Apple we can safely conclude that because of Earth’s mass it was not affected (atleast not visibly) by the attraction of  Apple.

What will happen if another body is not apple but it is very huge body like Earth, for example Earth & Moon ? In the case of Earth & Moon because both bodies have huge masses both will attract each other. The force of attraction between two bodies is known as Gravitational Force and it is long range force, that means does not matter how far bodies are the force will be affective. Of course the magnitude of force will come down as bodies goes away, but yes the force will be there. Also the force depends on masses of both the bodies. In simple words we can say bigger mass attract each other by higher force. Also interesting fact is the force is always attractive.

Newton’s Law of Universal Gravitation can be describes in the following words :

“Two particles attract each other with a central force proportional to the product of their masses and inversely proportional to the square of the distance between them”

The proportionality constant here is Newton’s constant G = 6.67 x 10-8 dyn cm2 g-2, also called the gravitational constant.

The theory explain the falling of objects toward Earth. The same theory is capable of explaining the attraction between planets. The interesting fact we can observe from the theory is force depends on mass not on shape. What does that mean ?

Does that mean force between two ball will remain same even if one ball we replace with some other material ball but with same mass ? Yes it is true force between two iron ball of 1 gms will be same as force between one iron and one nikle ball of 1 gms. Even if we change the shape, one ball we replace with a cube of same mass, still force will be same.

In short we can conclude if I drop a iron ball of 1 gms and a plastic ball of 1 gms, both will feel the same force and finally will fell down with the same speed and hit the Earth on same time. Yes two different object touch the earth together.

If we do the experiment at home we can understand this fact easily. But we also need to remember we are staying in an environment where Gravitational Force is just one force, there are many more forces we need to consider. What I wanted to say is if you don’t see a paper sheet & a paper ball coming down and hitting surface together don’t say Theory of Gravitation is failed. Yes dear the theory is not failed but something happened which is not part of this article, we will discuss that something some other time.