Every object in the Universe attracts other object with a force F which is (i) directly proportional to the product of their masses, and (ii) inversely proportional to the square of the distance between them. If the masses of the two objects be M and m respectively and they are separated by a distance r, then the gravitational force, F between them is, F = (GMm/r^2) in magnitude and is directed towards the attracting mass. This is called the Newton's Universal Law of Gravitation. The quantity G is called the Universal Gravitational constant and G = 6.67 * 10^-11 Nm^2/kg^2.
The value of G was experimentally determined by the British Physicist Henry Cavendish about a century later, by doing a simple experiment with a torsion pendulum as described in here.
Test Yourself
(1) Calculate the magnitude of the force between two 1 kg point masses separated by a distance of 1m.
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