Newton inverse square law: Newton considered the orbits of the planets as circular. For circular orbit of radius r, the centripetal acceleration towards the centre is
a = \(-\frac{v^2}{r}\) .... (1)
The velocity in terms of known quantities r and T, is
\(v= \frac{2πr}{T}\) .... (2)
Here T is the time period of revolution of the planet. Substituting this value of v in equation (1) we get,
Substituting the value ‘a’ from (3) in Newton’s second law, F = ma, where ‘m’ is the mass of the planet.
By substituting equation (6) in the force expression, we can arrive at the law of gravitation.
Here negative sign implies that the force is attractive and it acts towards the center. But Newton strongly felt that according to his third law, if Earth is attracted by the Sun, then the Sim must also be attracted by the Earth with the same magnitude of force. So he felt that the Sun’s mass (M) should also occur explicitly in the expression for force (7). From this insight, he equated the constant 4π2k to GM which turned out to be the law of gravitation.
Again the negative sign in the above equation implies that the gravitational force is attractive.
