Finding the torque needed to accelerate a wheel

[Pablo]

Homework Statement

Object of fifty kg and a r= 0.025m. What's the torque to accelerate from 0 to 0.4 in 20s.

Homework Equations

Torque = Moment of inertia * angular acceleration
T = I * α

The Attempt at a Solution

[/B]
m = 50kg
r = 0.25m
α = (0.4) / 20 = 0.02

I = mr^2 = 50 * 0.25^2 = 3.125
T = I * α = 3.125 * 0.02 = 0.0625 Nm

I got 0.0625 Nm for my torque, but this does not match any of the choices. Am I doing something wrong?

 

[berkeman]

Welcome to the PF.

α = (0.4) / 20 = 0.02

The units for angular acceleration should be in radians/s^2. How many radians are there in one revolution?

 

[berkeman]

BTW, it's a good idea to carry units along in your calculations. That helps you to see errors like that early and correct them along the way.

 

[Pablo]

Welcome to the PF.

The units for angular acceleration should be in radians/s^2. How many radians are there in one revolution?

Wow, I can't forgot to multiply by 2 pi, thanks! I got 0.39Nm :)