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January 28, 2015

January 28, 2015

Posted by **Disy** on Saturday, January 5, 2008 at 7:57pm.

I need some hints to do this problem. Please help. Thanks.

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**Damon**, Saturday, January 5, 2008 at 8:26pmThe kinetic energy stored in a rotating object is

(1/2) I w^2

I is the moment of inertia of the disk about an axis perpendicular to the disk at the center, which you can look up (If they want you to derive it reply to me that you need a derivation)

I = (1/2) M R^2 where R is the radius of the flywheel and M is the mass

w is the angular velocity in radians per second

w = 2 pi (revolutions per second)

so set your Joules equal to (1/2) I w^2 and solve for w

then change w from radians per second to revs per minute

revs/min = w in rad/sec * 1 rev/2pi radians * 60 seconds/1 minute

- physics. -
**Disy**, Saturday, January 5, 2008 at 8:35pmI did the same thing, but I can't get the right answer.

My answer is 4.3*10^5 rev/min

- physics. -
- physics. -
**Damon**, Saturday, January 5, 2008 at 8:48pmI agree with your answer

- physics. -
**Disy**, Saturday, January 5, 2008 at 8:51pmBut the book says the right answer should be 6.0*10^5rev/min

- physics. -
- physics. -
**Damon**, Saturday, January 5, 2008 at 8:54pmThat is pretty mysterious.

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