An object m of mass m=1kg is attached to a string that passes through a hole in the center of a table. The object m moves in a circle of radius r=1.3m. An object M of mass M=2.1kg is tied to the other end of the string, and hangs vertically beneath the table. Assuming the tabletop is frictionless, what speed must the object m have if the mass M is to remain at rest?

Answer 1

m g = M V^2/r

Solve for V. You know M, m, g and r.