# Homework Help: Kinetic energy of compound rotational objects

1. May 11, 2008

### Pantherezz

I have built a ferris wheel out of three parts, a tambourine ring, the cover that came with the tambourine, and a wooden shaft on the other side. I know that to find the kinetic energy would be Iw^2 but I don't really understand how to input the data into the equation.

I know I need to find each individual pieces' moment of inertia, using (0.5)mr^2, (0.5)m(r1^2 +r2^2). I don't know how to find the shaft's moment of inertia though...

And for the angular velocity, do you need to find each pieces separate angular velocity and add them together or do you just need it once because they would all be the same...?

See, I tried finding the answer using just the moment of inertia of the ring and the thin disk (cover) but I got a very very small number....

2. May 11, 2008

### Nick89

Find the moment of inertia for each part seperately, through the same axis (!) and add them up.
The angular velocity is the same everywhere on the object (as long as everything is fixed and cannot move relative to eachother).

Also, the rotational energy is $$\frac{1}{2}I \omega^2$$.

3. May 11, 2008

### Pantherezz

thank you!!!

Last question... I think.
How do you find the mechanical energy efficiency of the whole thing? I'm a little confused as to how to convert the kinetic energy the wheel has into watts. I don't know what the time would be, because essentially the energy is constant.
..
I know the end result is that there is no mechanical energy efficiency but I dont know how to find the time so as to prove that there is no energy efficiency...