# Stumped - work done by rotating mass

1. Mar 14, 2005

### UrbanXrisis

stumped -- work done by rotating mass

I got the answers to questions (A) and (B) but I cant get (C)
My work is shown http://home.earthlink.net/~urban-xrisis/clip001.jpg [Broken]

the books gives me an answer of $$W=0.5mv_i^2(r_i^2/r^2-1)$$

any help on where I went wrong?

Last edited by a moderator: May 1, 2017
2. Mar 15, 2005

### Staff: Mentor

You went wrong when you tried to find the work by multiplying the tension by $r_i - r$, which treats the tension as though it were constant. (It's not.) Instead, you must integrate to find the work done, since the tension is a function of r.