Angular Acceleration of toy train

[carleon]

Hi, thanks for taking a look!

A toy train rolls around a horizontal 1.4-m-diameter track. The coefficient of rolling friction is 0.12.
(a) What is the magnitude of the train's angular acceleration after it is released?
(b) How long does it take the train to stop if it's released with an angular speed of 16 rpm?

I think I know which equations to use: α = ω^2*r and a = rα, but there is no mention of velocity in the problem so I am stuck.

 

[tiny-tim]

welcome to pf!

hi carleon! welcome to pf!

(a) What is the magnitude of the train's angular acceleration after it is released?

find a (from F = ma), then divide by r to get α

 

[carleon]

Thanks, but since mass isn't given, how would I find F=ma?

 

[tiny-tim]

call the mass "m" … it'll cancel

 

[carleon]

I got it! α=1.68rad/s^2

Thanks a bunch.