# Rotational motion -- Energy stored in a flywheel

## Homework Statement

Cylindrical shape pulley ( m = 6 kg, R = 0.18 m) is rotating at a frequency f = 10 s-1. Due to constant torque it stops. Calculate the work done by the breaking force.

w = 2πf

## The Attempt at a Solution

[/B]
I can calculate the angular velocity:
w = 2πf = 62.8 rad/s

I don't know what to do now.

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berkeman
Mentor

How much energy is stored in that spinning flywheel? What is its moment of inertia?

I don't know, it is not given. I think I'm suppose to find some kind of ratio.

berkeman
Mentor
Calculate it. Look up the formula for the moment of inertia for a cylinder in your textbook or use Google or Wikipedia...

Here is moment of inertia for a cylinder:

I = 1/2 MR2 = 1/2 * 6 kg * 0.18 m = 0.54 kg/m

berkeman
Mentor

## Homework Statement

Cylindrical shape pulley ( m = 6 kg, R = 0.18 m) is rotating at a frequency f = 10 s-1. Due to constant torque it stops. Calculate the work done by the breaking force.

w = 2πf

## The Attempt at a Solution

[/B]
I can calculate the angular velocity:
w = 2πf = 62.8 rad/s

I don't know what to do now.
I don't know, it is not given. I think I'm suppose to find some kind of ratio.
Here is moment of inertia for a cylinder:

I = 1/2 MR2 = 1/2 * 6 kg * 0.18 m = 0.54 kg/m
And what is the energy stored in that rotating cylinder? Once you have the energy stored in the rotating cylinder, how much work would it take to slow it down and stop it?

Energy stored in a rotating cylinder:

Erotational = ½Iω2 = 1065 J

So in order to stop this cylinder one must do 1065 J work?