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A clothes dryer is spinning at 75 rpm. A student opens its door and it comes to rest after rotating through four revolutions. Assume constant deceleration. What was the magnitude of the dryer's angular deceleration?

physics 
Damon,
Vi = 75 * 2 pi radians/min *1 min/60 sec
= 5 pi/2 radians/second
v = Vi  a t
0 = 5 pi/2  a t
so t = 5 pi/2a
4*2pi = Vi t + (1/2) a t^2
8 pi = ( 5 pi/2)(5 pi/2a) + (1/2)a (5 pi/2a)^2
etc 
sign repair 
Damon,
Vi = 75 * 2 pi radians/min *1 min/60 sec
= 5 pi/2 radians/second
v = Vi  a t
0 = 5 pi/2  a t
so t = 5 pi/2a
+++++++++ a is assumed negative +++
4*2pi = Vi t  (1/2) a t^2
8 pi = ( 5 pi/2)(5 pi/2a)  (1/2)a (5 pi/2a)^2
8 pi a = (5 pi/2)^2 = (1/2)(5 pi/2)^2
8 pi a = (1/2)(5 pi/2)^2
16 pi a = 25 pi^2/4
a = (25/64) pi
remember it is negative (deacceleration)