Posted by Bernadette on .
A 1050kg car has four 15kg wheels. When the car is moving, what fraction of the total kinetic energy of the car is due to the rotation of the wheels about their axles? Assume that the wheels have the same rotational inertia as uniform disks of the same mass and size. Also, why do you not need the radius of the wheels?

AP Physics 
bobpursley,
I will be happy to critique your thinking on this.
You do not need the radius, because you are dealing with fractions of energy. When you work out the KE, radius will be there, and the same radius term will be in the rotational energy part, and the radius terms divide out. 
AP Physics 
Bernadette,
The equation I came up with to use was KE(rotational)/KE(total)=4(.5mr^2w^2)/(4(.5mr^2w^2)+(.5mr^2w^2)) and got an answer of 5.4%. My physics teacher gives us answers to problems in our book that are almost the same as our homework problems, but when I used this equation with the corresponding question, I did not get the right answer. Where am I going wrong?

AP Physics 
bobpursley,
I agree with your formula, except how you used it. The masses of the wheels are not the same as the mass of the car, and the portion of KE translational has to use the mass of the cars.

AP Physics 
Buddy,
The formula for the moment of inertia is actually incorrect for you.
I = (1/2)mr^2
so
K = (1/2)((1/2)mr^2)(w^2)