A student holds a bike wheel and starts it spinning with an initial angular spee

Question

A student holds a bike wheel and starts it spinning with an initial angular speed of 7.0 rotations per second. The wheel is subject to some friction, so it gradually slows down. In the 10-s period following the initial spin the bike wheel undergoes 57.5 complete rotations. Assuming the frictional torque remains constant, how much more time delta t_s will it take the bike wheel to come to a complete stop? Delta t_s = The bike wheel has a mass of 0.825 kg and a radius of 0.385 m. If all the mass of the wheel is assumed to be located on the rim, find the magnitude of the frictional torque tau_f =

Explanation / Answer

wi = 7 rev/s = 14pi rad/s

theta = wi t + alpha t^2 / 2

57.5 x 2pi = 14pi x 10 + ( alpha x 10^2 / 2)

alpha = - 1.57 rad/s^2

now applying wf = wi + alphat = 0

0 = 14pi - 1.57t

t = 28 sec

More time taken = 28 - 10 = 18 sec .........Ans

torque = I alpha

I = m r^2

torque = (0.825) (0.385^2) (1.57)

= 0.19 N m

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