# A question about work/rotational Ke

Here is the question thats been giving me trouble:

A electric motor can accelerate a ferris wheel of moment of inertia I = 25300 kgm^2 from rest to 11.9 rev/min in 11.5 s. when the motor is turned off, friction causes the wheel to slow down from 11.9 rev/min to 6.33 rev/min in 7.53 s.

Determine the torque generated by the motor to bring the wheel to 11.9 rev/min.

Determine the power needed to maintain a rotational speed of 11.9 rev/min.

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The 2nd part seems fairly straight forward. When i find the torque i use
Power = (torque)(rotational speed).

The rotational speed = 11.9 rev/min = .198333 rev/sec = 1.24617 rad/sec.

So the power should be easy to find.

I assume i can find the change in kinetic energy (and therefore the work) by using the following equation:

The final rotating speed = 6.33 rev/min = .662876 rad/sec

change in K = W = .5(25300)(.662876^2) - .5(25300)(1.24617^2)
= -14086.2 J

Im not sure, however, how to find the torque after this?

Any help would be greatly appreciated!

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I was thinking about using Work = (torque)(rotational speed) to find the torque required for the first part of the problem.

I guess i didnt calculate the work/change in k correctly above, as i tried using this approach but got the problem incorrect.'

Still puzzled as to what im doing wrong...

arildno