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Rotating Ball

  • Thread starter Hollywood
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  • #1
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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?

230rev/min= 24.1 rad/s

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?
 

Answers and Replies

  • #2
28,562
4,867
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?

230rev/min= 24.1 rad/s

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!
 
  • #3
5
0
63 cm

But doesn't 63cm need to be converted to meters making it .63 meters?
 
  • #4
28,562
4,867
It needs to be converted to a radius too!
 
  • #5
5
0
Thanks

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

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