"A mass of 3.900 kg is suspended from a 1.450 m long string. It revolves in a horizontal circle. The tangential speed of the mass is 3.247 m/s. Calculate the angle between the string and the vertical (in degrees)." Here is a diagram, labelling angle theta I'm supposed to solve for: http://capaserv.physics.mun.ca/msuphysicslib/Graphics/Gtype11/prob03_pendulum.gif I've deduced so far that: sin(theta) = opp/hyp In this case, opp = radius and hyp = length of string (L) or (1.450 m) So sin(theta) = r/L In turn, r can be solved by means of Fc = mv^2/r, as r = mv^2/Fc so sin(theta) = mv^2/FcL M is given, V is given, and L is given. Fc I'm sort of puzzled on, since I can't use the MV^2/R equation again. Any clues?