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# Explain inverse square rule and its derivation for class 9.

According to the inverse-square law of gravitaion, the force of gravitation on a planet should decrease as the square of the distance of the planet from the sun. Newton further showed that the gravitational force is proportional to the product of the masses of the two objects.

It is said that Newton guessed this inverse square law from Kepler's 3rd law of planetary motion which states that the cube of the mean distance of a planet from the Sun is proportional to the square of its orbital period T.

Any point source which spreads its influence equally in all directions without a limit to its range will obey the inverse square law. This comes from strictly geometrical considerations. The intensity of the influence at any given radius r is the source strength divided by the area of the sphere.