wheel starts from rest and rotates with constant angular acceleration about its

Question

wheel starts from rest and rotates with constant angular acceleration about its axis it completes its first revolution 6s after it started how long after it started will the wheel complete second revolution?
i kno the answer but iam asking why when i answer by different way gives me another result thats wrong my answer is:
w'=w+alpha*t and w=0 and w'=2pi*f
alpha=w/t=2pi*f/t=pi/18( which is wrong angular accelration)
theta=wt+1/2(alpha)t t=144s which is totally wrong
i see him answer by theta=1/2(alpha)t and he get right angular acc.

then he got right time

Explanation / Answer

w= 2f

can be used if it is moving with constant angular velocity,

but here it has an angular acceleration,so you cant use w= 2f, as the final angular velocity

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