State the universal law of gravitation and derive its expression.

The universal law of gravitation states that the force of attraction between two masses m₁ and m₂ separated by a distance r is directly proportional to the product of the masses and inversely proportional to the square of the distance between the masses.

That is,

F α m₁m₂

F α 1/r²

Or, F = Gm₁m₂/r²

Where, G is a constant called the universal gravitational constant and is equal to 6.67 × 10^-11 Nm²/kg²