Circular Motion of a 8.0cm CD: Finding Speed and Revolution Using Kinematics

[reverse]

A magnetic computer disk 8.0 cm in diameter is initially at rest. A small dot is painted on the edge of the disk. The disk accelerates at 600 rad/ s^2 for 1/2 s, then coasts at a steady angular velocity for another 1/2 s . a) What is the speed of the dot at t = 1.0 s?
b)Through how many revolutions has it turned?


for a) i kept getting the answer wrong:
i did :
d=8.0cm
so r=4.0 cm
ti=0s
tf=0.5s
a=600 rad/s^2

vi=(2)(pi)(4.0cm) / 1.0s

so; vi=25m/s


and for part b)
i think you have to use the kinematic equation after you get vi.. I'm not sure tho.. am i right?? please help. thanks in advance!

 

[Philosophaie]

For part a) What is the velocity after .5s - v=a*t
then coasts at a steady angular velocity for another .5 s - same Angular velocity as accelerated to