The gravitational force between two objects that can be regarded as particles can be calculated by the following formula: f = g m 1 m 2/r 2, that is, the gravitational force is equal to the product of the gravitational constant multiplied by the mass of two objects divided by the square of their distance. Where g represents the gravitational constant, and its value is about 6.67× 10? -1 1 unit n m? /kg？ . It was measured by British scientist cavendish through the torsion balance experiment.

Deduction of gravity: If the orbit of the planet is approximately regarded as a circle, the angular velocity of planetary motion can be obtained from Kepler's second law, namely:

Ω = 2π/t (period)

If the mass of the planet is m, the distance from the sun is r and the period is t, then from the equation of motion, the force acting on the planet is

mrω？ =mr(4π？ )/T？

In addition, it can be obtained from Kepler's third law

t？ /r cubic; = constant k'

So the force in the direction of the sun is

mr(4π？ )/T & amp； ? ; = MK’(4π？ ; )/r？ ;