1. The problem statement, all variables and given/known data If two particles are orbiting each other in a circular orbit with a period of tau, show that if you stop them both in an instant, they will fall into each other after a time tau/ 4 sqrt2 . 2. Relevant equations 3. The attempt at a solution So, we know that they must be along a diameter and we can use f = m v^2/ r_0 to express f in terms of tau and then V in terms of tau. I get: [tex] f = m \tau^2 4 \pi^2 r_0 [/tex] V = -f * r But don't we know that V = -GM1M2/r so why is my expression for V different than that?