# Calculating Power of a rotating shaft

1. Feb 21, 2008

Hi, it's been a number of years since I've even thought about physics, so this is a very simple question. I have a rig set up where I have a power source that is rotating a shaft. Attached to this shaft is a wheel with a string, I have this string going through a pulley in the ceiling, and pulling a hanging weight. What I need to do is calculate the power that I'm getting out of this device, I remember how to calculate work and power, but can't for the life of me figure out where the radius of the wheel comes into play, and the torque. Any help would be appreciated

Cheers,
Corey

2. Feb 21, 2008

### FredGarvin

If you have the power, you can calculate the torque (neglecting friction) from:

$$P = T \dot{\theta}$$

where
$$P$$ = Power
$$T$$ = Torque
$$\dot{\theta}$$ = Angular speed

Just make sure your units are consistent!