# Rotating Ball

A uniform, solid sphere of mass 3.2 kg and diameter 63 cm is rotating about its diameter at 230 rev/min. What is the kinetic energy of rotation?

I know that KE=1/2Iw^2 and interia of a solid sphere is I=2/5MR^2

I=(2/5)(3.2)(.63)^2
=.508032

KE=(1/2)(.508032)(24.1)^2
=147.5 J

For some reason my answer is wrong. Am I missing something? What am I doing wrong?

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Dale
Mentor
A uniform, solid sphere of mass 3.2 kg and diameter 63 cm is rotating about its diameter at 230 rev/min. What is the kinetic energy of rotation?

I know that KE=1/2Iw^2 and interia of a solid sphere is I=2/5MR^2

I=(2/5)(3.2)(.63)^2
=.508032

KE=(1/2)(.508032)(24.1)^2
=147.5 J

For some reason my answer is wrong. Am I missing something? What am I doing wrong?
Yup, just a little mistake.

PS Welcome to the PF!

63 cm

But doesn't 63cm need to be converted to meters making it .63 meters?

Dale
Mentor
It needs to be converted to a radius too!

Thanks

I see now! LOL! My bad. Very dumb mistake! Thanks!