What is the equation for the force of gravitation between two particles?
A force is acting along the line joining two particles 1 and 2. If particle 1 attracts particle 2 with a force F12, then particle 2 pulls particle 1 with a force F21 of equal magnitude. Both the forces are along the line joining the particles. 
According to Newton's third law, the force F12 is the action and the force F21 is the reaction. These forces form a pair of forces, which are equal in magnitude and opposite in direction. Hence the magnitude of the force of gravitation between these two particles of masses m1 and m2 separated by a distance r is
F ∝
or F = G
where G is a constant of proportionality. It is the universal constant of gravitation.
According to Newton's third law, the force F12 is the action and the force F21 is the reaction. These forces form a pair of forces, which are equal in magnitude and opposite in direction. Hence the magnitude of the force of gravitation between these two particles of masses m1 and m2 separated by a distance r is
F ∝
or F = G
where G is a constant of proportionality. It is the universal constant of gravitation.
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