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Homework Statement
A puck of mass m is attached to a cord that passes through a hole in a horizontal frictionless surface. The puck is originally moving in a circle of radius r and speed v. The cord is then slowly pulled from below, decreasing to a circle with radius r1. What is the work done by the person pulling the string?
Homework Equations
W=Fd
Iw=Iw conservation of rotational momentum
F=ma
The Attempt at a Solution
I need to show that W is equal to the change in rotational kinetic energy. So I did W=FD.
F=m(v^2)(r^2)(1/R^3) where R is an arbitrary radius from r to r1. Thus I integrated what I set F equal too and multiplied by D=(r-r1). My answer is off by a factor of 1/4 b/c I calculated the change in rotational kinetic energy. Any tips?