**Gravitation** is the name given to the force of attraction acting between any two bodies of the universe.

Newton’s reasoning was that the moon revolving in an orbit of radius R_{m} was subject to a centripetal acceleration due to earth’s gravity of magnitude

where V is the speed of the moon related to the time period T by the relation V = 2π R_{m }/T . The time period T is about 27.3 days and R_{m} was already known then to be about 30.84 × 108m.

If we substitute these numbers in above equation, we get a value of a_{m} much smaller than the value of acceleration due to gravity g on the surface of the earth, arising also due to earth’s gravitational attraction.

## Universal Law of Gravitation:

Everybody in the universe attracts every other body with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.

where G is the universal gravitational constant.

### Gravitational constant (G):

It is equal to the force of attraction acting between two bodies each of unit mass, whose centers are placed unit distance apart. Value of G is constant throughout the universe. It is a scalar quantity. The dimensional formula G = [M^{-1} L^{3} T^{-2}]. In SI unit, the value of

**G =6.67X10 ^{-11} Nm^{2} kg^{-2}**.

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