Rotary Kinetic Energy

[Marky]

I have a problem with this in working out getting it to watts/. I think I am doing it wrong.

I can either work it out two ways. For an example, a circular solid disk with a diameter of 2 mtr and a weight of 7 kg or 70 newton

this is rotating at 57rpm and I have this as 104 watts of power.

 

Or I can do it with mass rotating around which would be .5 *7Kg = 3.5 * half the diameter V2 * Mtr/s at half radius =3 V2 and I come to an answer of 7.9 watts. Where am I going wrong !!!!.

Hope that some pne can help me get some sleep.

 

Many thanks

[swansont]

I have a problem with this in working out getting it to watts/. I think I am doing it wrong.

I can either work it out two ways. For an example, a circular solid disk with a diameter of 2 mtr and a weight of 7 kg or 70 newton

this is rotating at 57rpm and I have this as 104 watts of power.

 

Or I can do it with mass rotating around which would be .5 *7Kg = 3.5 * half the diameter V2 * Mtr/s at half radius =3 V2 and I come to an answer of 7.9 watts. Where am I going wrong !!!!.

Hope that some pne can help me get some sleep.

 

Many thanks

I don't see how you get 104 Watts, nor is it clear what your equation is that gives you 7.9 Watts.

For a rotating system the KE is [math]\frac{1}{2}I\omega^2[/math]

where I is the moment of inertia and omega is the angular frequency

[InigoMontoya]

Another point: Watts is a measure of power, not energy. So if you're looking for energy (and it sounds like you are) and your units come out watts, then you've definitely screwed something up.

[Marky]

Many thanks for the replies. Yes I am going to re-write the question as it is related to a turning turbine. I will need to understand the Latex code prior to placing.

For your interest the turbine has a diameter of 2 mtr, and has a force of 4.611 kg or 45.18 n pushing it around at that speed 115rpm this is the force measured at the edge or the tip of the blades of the turbine at 115 rpm . I have calculated the power to be 544 watts. something is telling me this is in correct.

 

Many thanks

[InigoMontoya]

If I remember correctly....

 

Power = Torque (N*m) * Angular Velocity (rad/s)

 

So....

 

Power = (45.18 * 1.0) * (115/60 * 2 * pi) = 544 watts.

 

I concur.

 

 

edit: Note, however, that just because 544 watts is the power required to keep the turbine spinning, that does NOT mean that it has any relation to the energy stored in the rotational motion. That's a totally different entity (and has already been identified by Swansont).

Edited October 2, 2012 by InigoMontoya