A machinist turns the power on to a grinding wheel, at rest, at time t = 0 s. Th

Question

A machinist turns the power on to a grinding wheel, at rest, at time t = 0 s. The wheel accelerates uniformly for 10 s and reaches the operating angular velocity of 29 rad/s. The wheel is run at that angular velocity for 27 s and then power is shut off. The wheel slows down uniformly at 2.7 rad/s2 until the wheel stops. In this situation, the average angular velocity in the time interval from t = 0 s to t = 25 s is closest to:

A) 17 rad/s

B) 15 rad/s

C) 13 rad/s

D) 11 rad/s

E) 8.7 rad/s

The answer is A... What I dont understand is how to get there. Please work out the problem for points.

Explanation / Answer

A machinist turns the power on to a grinding wheel, at rest, at time t = 0 s. The wheel accelerates uniformly for 10 s and reaches the operating angular velocity of 29 rad/s. The wheel is run at that angular velocity for 27 s and then power is shut off. The wheel slows down uniformly at 2.7 rad/s2 until the wheel stops. In this situation, the average angular velocity in the time interval from t = 0 s to t = 25 s is closest to:

A) 17 rad/s

B) 15 rad/s

C) 13 rad/s

D) 11 rad/s

E) 8.7 rad/s

Answer: A

constant angular acceleration..use

?=?0t+1/2?t2

and

?=?0+?t

2.there is no acceleration..use

?=?t

3.there is constant angular retardation..use

?2=?20?2??

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