physics - drwls?

A disk rotates about its central axis starting from rest and accelerates with constant angular acceleration. At one time it is rotating at 7 rev/s. 55 revolutions later, its angular speed is 21 rev/s.
(a) Calculate the angular acceleration.
(b) Calculate the time required to complete the 55 revolutions mentioned.
(c) Calculate the time required to attain the 7 rev/s angular speed.
(d) Calculate the number of revolutions from rest until the time the disk attained the 7 rev/s angular speed.

(a) Divide the change in angular rotation rate by the time required for the acceleration to occur.
alpha = (14 rev/s)*(2 pi rad/rev)/Time
Since 55 revolutions were required and the average rotation rate was 14 rev/s, the time required was 55/14 = 3.93 s.
(c) It will take 1/2 as long to go from 0 to 7 rev/s as it takes to go from 7 ro 21 rev/s
(angle) = (1/2) alpha t^2, where t is the time you get in part (c). Divide by 2 pi for he number of revolutions

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