Angular acceleration in revs/s^2

[vorcil]

http://img15.imageshack.us/img15/858/masteringf.jpg

my attempt,

$$\alpha$$= dw/dt

550-150 = 400 rev/min change

400/3.5 = 114 revs

that wasn't right so i tried
114*60, which gives -6857 revs^-2

can someone help please? I'm not sure if I've got this whole revolution thing correct in my head lol

 

[rl.bhat]

550-150 = 400 rev/min change
Convert rev/min to rev/s

 

[vorcil]

550-150 = 400 rev/min change
Convert rev/min to rev/s

why does angular acceleration come out right whether you use Angular velocity or revolutions?

$$\alpha$$ = w/t

$$\alpha$$ = revolutions/t

 

[rl.bhat]

You can express angular velocity either in rev/min or rad/s.
In the problem α should be rev/s^2. So you have to convert rev/min to rev/s.