GRAVITATION

We know that the moon goes around the
earth. An object when thrown upwards,
reaches a certain height and then falls
downwards. It is said that when Newton was              
                                                                                 
sitting under a tree, an apple fell on him. The
fall of the apple made Newton start thinking.
He thought that: if the earth can attract an
apple, can it not attract the moon? Is the force
the same in both cases? He conjectured that
the same type of force is responsible in both
the cases. He argued that at each point of its
orbit, the moon falls towards the earth,
instead of going off in a straight line. So, it
must be attracted by the earth. But we do
not really see the moon falling towards the
earth.
The motion of the moon around the earth
is due to the centripetal force. The centripetal
force is provided by the force of attraction of
the earth. If there were no such force, the
moon would pursue a uniform straight line

motion.
It is seen that a falling apple is attracted
towards the earth. Does the apple attract the
earth? If so, we do not see the earth moving
towards an apple. Why?
According to the third law of motion, the
apple does attract the earth. But according
to the second law of motion, for a given force,
acceleration is inversely proportional to the
mass of an object. The mass of an
apple is negligibly small compared to that of
the earth. So, we do not see the earth moving
towards the apple. Extend the same argument
for why the earth does not move towards the
moon.
In our solar system, all the planets go
around the Sun. By arguing the same way,
we can say that there exists a force between
the Sun and the planets. From the above facts
Newton concluded that not only does the
earth attract an apple and the moon, but all
objects in the universe attract each other. This
force of attraction between objects is called the
Gravitational force.