Rotational Kinematics Help (constant angular acceleration)

[sirfederation]

The Tub of a washer goes into a spin cycle, starting from rest and gaining angular speed steadily for 8 s, when it is turning at 5 rev/s. At this point the person doing the laundry opens the lid, and a safety switch turns off the washer. The tub smoothly slows to rest in 12s. Through how many revolutions does the Tub turn while it is in motion?

Ok, I am going to do this problem in 2 stages: One for Wi[/size] =0 and Wf[/size]=5 rev/s and the second for Wi[/size]=5rev/s and W[size=.5]f[/size]=0

First stage:

Known
Wi[/size]=0
Wf[/size]=5 rev/s
T=0
T=8s
Oi[/size]=0

Unknown
A=?
Of[/size]=?
Wf[/size]= (5 rev/s)(2(pi)rad/rev)
Wf[/size]= 10(pi) rad or 31.4 rad

A = (Wf[/size]-Wi[/size])/T
A = 31.4/8
A = 3.9 rad/(s^2)

Of[/size]=Oi[/size]+(Wi[/size](T)+.5(A)(T)^2
Of[/size]= 0 + (10)(pi)(8) + .5(3.9)(8)^2
Of[/size]= 80(pi) + 40(pi)
Of[/size]= 377 rad (I am going to leave it in this form because I am going to have to plug it into stage two.

Stage two:

Known
Wf[/size]= 0
Wi[/size]= 5 rev/s or 10(pi) rad/s
Oi[/size]= 377 rad (found in stage one)
T=12 s

Unkown
A=?
Of[/size]=?

A = (Wf[/size]-Wi[/size])/T
A = (0-31.4)/12
A = -2.6 rad/s^2

Of[/size]=Oi[/size]+(Wi[/size])(T)+.5(A)(T)^2
Of[/size]= 377 + 31.4(12) + .5(-2.6)(12)^2
Of[/size]= 377 + 376.8 - 187.2
Of[/size]=566.8 rad

Now we need to convert to revolutions:
566.8 rad(57.3/rad)= 32477.64 degrees

32477.64/(360/rev) = 90. 2 rev

Ok here is what I do not get.

My revolutions for the washer when its acceleration is increasing in stage one is slower than the revolutions when the acceleration is decreasing in stage two. I think I am correct but it doesn't make sense.

 

[sirfederation]

Ok, I redid this problem a couple of times and came up with a different answer. I forgot to set wi to zero in the equation Of[/size] = 0i[/size] + Wi[/size](T)+.5(A)(T^2) in stage one. So Of[/size]=125.66 rad which changes the final answer to 50.2 revolutions