Hi. I need to find the mass (relativistic mass?) Of an object in rotation. Say I have a string with a small rock tied to the end. The relevant variables are: A) The circumference of the weights path. B) The weight of the rock itself. C) The revolutions per minute that B travels around A. Now, what is the equation needed to put these together to tell me how much 'weight' in kg the spinning rock will exert or pull on the string? Or in other words, how do we know the amount of centripetal force (In kg) the string requires to hold to the spinning rock?