Here the angular acceleration is constant, so we have,
θ=ωt+21αt2.
When flywheel is rotated from rest in the first second, then θ1=21α(1)2=5rad
⇒α=10rads−2
The angle turned by flywheel in the next second is θ2=21α(2)2=21(10)(4)=20rads−2
Thus, the angle turned during next second is θ=20−5=15rad