Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top


A capstan is a rotating drum or cylinder over which a rope or cord slides to provide a great amplification of the rope's tension while keeping both ends free. Since the added tension in the rope is due to friction, the capstan generates thermal energy.

If the difference in tension ($T_0-T'$) between the two ends of the rope is $520N$ and the capstan has a diameter of $10.0 cm$ and turns once in $0.90s$, find the rate $P_{thermal}$ at which thermal energy is being generated.

My Work:

$F = 520N$

$d = 0.1m$

$\Delta T = 0.90s$

$\Delta U = Q - W \implies Q = \Delta U + W$

$W = \tau \Delta \theta = Fd(2\pi) \implies W = 326.7J$

$P = \frac{\Delta W}{\Delta T} = 363W$

However, this is the incorrect answer. (I don't know the correct one.) Can anybody give me any guidance here?

share|cite|improve this question
up vote 2 down vote accepted

Maybe you should use radius $r = d/2$ instead of diameter $d$?

share|cite|improve this answer
Yes, thank you for that. I don't know what I'm doing today... – Brandon Amos Apr 24 '12 at 20:00
Oh no problem. It happens. I just have an eye for such things, I've corrected thousands of tests ;) – Pygmalion Apr 24 '12 at 20:15

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.