**UNIVERSAL LAW OF GRAVITATION**

Every object in the universe attracts every other object with a force . Attraction force is directly proportional to the product of their masses.

** F ∝ m _{1} × m_{2}** Equation 1

The force between two objects is inversely proportional to the square of the distance between them, that is,

** F ∝ 1/ r ^{2}** Equation 2

Combining Eqs. (1) and (2), we get

F ∝ m_{1} m_{2 }/ r^{2} Equation 3

F = G m_{1} m_{2 }/ r^{2} Equation 4

where G is the constant of proportionality and is called the** universal gravitation constant.**

m_{1} = Mass of one body

m_{2} = Mass of other body

r = Distance between two body

By multiplying crosswise, Eq. (4) gives

F × r^{ 2} = G m_{1 }× m_{2}

G = F × r^{ 2} / m_{1 }× m_{2} Equation 5

The** SI unit of G** can be obtained by substituting the units of force, distance and mass in Eq. (5) as** N m ^{2} kg^{–2} = kg^{-1} m^{3} s^{-2}**

The** C G S unit of G** = **Dyn cm ^{2} g^{–2} = g^{-1} cm^{3} s^{-2}**

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The value of G was found out by Henry Cavendish (1731 – 1810) by using a sensitive balance. The accepted value of G is

**6.673 × 10**

^{–11}N m^{2}kg^{–2}, C G S value of G =6.673 × 10^{–8}dyn cm^{2}g^{–2}.