# A student holds a bike wheel and starts it spinning with an initial angular speed of 9.0...

## Question:

A student holds a bike wheel and starts it spinning with an initial angular speed of 9.0 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 77.5 complete rotations.

Assuming the frictional torque remains constant, how much more time {eq}\displaystyle \Delta t_s {/eq} will it take the bike wheel to come to a complete stop?

The bike wheel has a mass of 0.725 kg and a radius of 0.315 m. If all the mass of the wheel is assumed to be located on the rim, find the magnitude of the frictional torque {eq}\displaystyle \tau_1 {/eq} that was acting on the spinning wheel.

## Rotational Motion

A torque is necessary for a body to impart or stop rotational motion.

Physical quantities associated with rotational motion are :

angular displacement {eq}\theta {/eq} - is the angle covered by the rotating body

Angular velocity {eq}w {/eq} -is the rate of change in angular quantites

Angular acceleration {eq}\alpha {/eq}- is the rate of change in angular velocity

All the above quantities are vectors directed along the axis of rotation.

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The relation between angular displacement, angular velocity and time is

{eq}\theta = w_{0} t +\frac{1}{2} \alpha t^{2} {/eq}

{eq}\alpha =...

Practice Applying Rotational Motion Formulas

from

Chapter 17 / Lesson 15
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Physics students should be comfortable applying rotational motion formulas. Review the definition of rotational motion and practice using the relevant formulas with the provided examples.