The correct option is B 6.35rads−2
Given,
v=72kmph=72∗518ms−1
=20ms−1.
Now , angular velocity of wheel intially=vr
where , r is radius of wheel.
r=50cm {given}
=0.5m
angular velocity intially=40rads−1
and also given, That the wheel is rotated by 20revolutions.
angle of rotation say ( θ ) =20×2πrads−1
θ=40πrads−1
As we know that,
ω2final−ω2intial=2×α×θ {let α be angular accerlation}
By substituting the known values,
⇒02−402=2×α×40π
⇒α=−6.3661rads−1{- sign indicates retardation}
Therefore answer is Option C.