Calculating Angular Acceleration and Velocity for a Rotating Disk

[DrummingAtom]

Homework Statement

a) A disk is turned on starting from rest and has constant angular acceleration. If it takes .750s for the drive to make it's second revolution, how long did it take to make its first complete revolution?

b) What is it's angular acceleration?

Homework Equations

theta = 1/2αt² + wt
w = w_o + αt

The Attempt at a Solution

I tried solving b) first and set theta = 2(2pi), so I have:
4pi = 1/2αt² and after solving for α it's saying the answer is wrong..

This isn't making sense to me.. Thanks for any help.

 

[zhermes]

That really should be right (unless I'm totally missing something...). It sounds like you're using an online-homework thing---perhaps its a sig.fig. issue or something like that. Try redoing your calculation, what do you get for the angular acceleration?

 

[DrummingAtom]

44.7m/s²... They want it with 3 sig figs.. This is so annoying, it's still wrong. Haha

 

[zhermes]

Try 44.680? (Note that its not m/s^2, its radians per second squared where radians are unitless).

 

[DrummingAtom]

It will reject it with more than 3 sig figs. Regardless of any amount of sig figs it wants 3 in the answer. My bad on the m/s², I have a feeling most of the problems on this homework were punched in wrong. There were many complaints from the people that tried this stuff early.

Thanks for your reassurance regardless.