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# Physics/Converting rotational force into watts of energy

Question
Hello,

I'm trying to understand how to convert rotational force into watts of energy and I'm getting confused with exactly what a Newton meter is and how it applies. This may sound like a homework question, but I assure you, I'm not a student of any kind. I have to ask it like a homework question so that I can learn how to solve it. Say I have a disk of 0.25 meters in diameter, rotating at say10 rpm. If I know it's rotating with 10 kg of force, how would I convert this energy into watts?

OK, the basic question doesn't make any sense.  Force is measured in Newtons, not kilograms.  Kilograms is a unit of mass, the amount of matter an object contains.  It's commonly confused with weight (the force which gravity pulls on that mass with), but they're not the same.  Then, you're asking about the energy of a disk when we don't know it's mass.  If you just apply a simple rotational energy term to the object, you can get the energy.  But that's in Joules, not Watts.  Watts is a unit of power, energy per unit time.  If I use one Joule of energy to do something in a millionth of a second, I have a million Watts of power.  If I do it in a whole second, I have one Watt of power.  The general term for the energy of a rotating, uniform disk is 1//2MR^2*w^2, where M is the mass in kg, R is the radius in meters, and w is the angular velocity (the rotations per second * 2pi).

Perhaps you can clarify what this is for and I can be more direct and not just plunk down an answer which is so lengthy, but I hope this clears up the basics.

Physics

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#### Dr. Stephen O. Nelson

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