(a) According to universal law of gravitation: Every body in the universe attracts every other body with a force (F) which is directly proportional to the product of their masses (m and M) and inversely proportional to the square of the distance (d) between them.

F = \(G \times \frac{m \times M}{d^2}\)

Sir Isaac Newton gave this law.

(b) The gravitational constant G is numerically equal to the force of gravitation which exits between two bodies of unit masses kept at a unit distance from each other.

G = \(F \times \frac{d^2}{m \times M}\)

Unit of gravitational constant = Nm²/kg²