I've come across this question, and I'm at at a loss.
Two masses, m and M, are in an isolated system. The gravitational forces, F (by M on m) and F (by m on M), are equal and opposite. Why do they not add to a zero net force? Explain your answer.
I approached the question by simply adding vectors. In the question F(1) and F(2) are defined as equal and opposite. Mathematically that is F(1) = -F(2). Hence, is stands to reason that F(1) + F(2) = 0.
If it's an isolated system there there should be no other forces present. Also, even if the forces are moving towards each other, then they could be at a constant velocity (zero acceleration).
Is this flawed reasoning, or is there another explanation?