Ok here it goes: A mass m1 moves without any friction in a circle with radius r on a table. On to this mass a string is attached that goes through a hole in the table and is attached to another mass m2 under the table. http://img358.imageshack.us/img358/5636/circularmotion0ny.gif [Broken] There is no friction between the rope and the table. Derive a formula for the radius r in terms of m1, m2 and the time T for one complete revolution. What I did is the following: F= mtot * a, with a= ac= v^2/r Therefore F= mtot * (v^2/r) v= 2pi r/T. Substitution gives: F= mtot * ((2pi r/T)^2)/ r = (mtot 4pi^2 r^2)/ (r T^2) = (mtot 4pi^2 r)/ T^2 so this gives r= (FT^2)/ (mtot 4 pi^2)... But how will I get rid of the F term in the formula? Or is there some other way to solve this problem?