Consider two particles 1 and 2 that can interact with each other but are isolated from their surroundings. If an internal force from particle 1, for example, a gravitational force, acts on particle 2, then there must be a second internal force—equal in magnitude but opposite in direction—that particle 2 exerts on particle 1. But if the particle 2 exerts a gravitational force too? Will the net force exerted on particle 1 be the sum of the gravitational forces 1 to 2 and 2 to 1?
the force is given by [tex]F=G \frac{m_1 m_2}{r^2}[/tex] the force on 1 = - force on 2 so F_{1}= -F_{2}