Work and rotational Kinetic Energy

[frig0018]

Homework Statement

A 32.0 kg wheel, essentially a thin hoop with r=1.20 m is rotating at 280 rev/min. It must be brought to stop in 15.0 s. How much work must be done to stop it?

Homework Equations

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The Attempt at a Solution

I=mr^2=46.08 kg*m^2
W=(1/2)I\omegaf^2 - (1/2)I\omegai^2
W=\tau(\Delta\theta)
\tau=I\alpha
Rev in 15 sec = 70 rev => 140\Pi radians in 15 sec.

I'm not sure which equation(s) to use. I've tried plugging numbers into all of them and getting stuck or wrong answers. Thanks!

 

[Nabeshin]

Remember, work is a change in kinetic energy. Do you know the formula for kinetic energy of a rotating object?

 

[frig0018]

i thought it was
W = delta K = 1/2(I)(omega final)^2 - 1/2(I)(omega initial)^2...?
or do i use the formula
W= (integral from theta initial to theta final) torque d-theta?

 

[tiny-tim]

… one step at a time …

A 32.0 kg wheel, essentially a thin hoop with r=1.20 m is rotating at 280 rev/min. It must be brought to stop in 15.0 s. How much work must be done to stop it?

Hi frig0018!

First step: what angular acceleration is needed to stop it in 15 s?