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Angular Momentum Question?

  • Thread starter math_head7
  • Start date
  • #1
Angular Momentum Question???

Homework Statement


Figure 11-45 (attached) shows a rigid structure consisting of a circular hoop of radius R and mass m, and a square made of four thin bars, each of length R and mass m. The rigid structure rotates at a constant speed about a vertical axis, with a period of rotation of 2.5s. Assuming R=0.50m and m=
2.0kg, calculate (a) the structure's rotational inertia about the axis of rotation and (b) its angular momentum about that axis.

Homework Equations


I=[tex]\int[/tex]r2dm
L=I[tex]\omega[/tex]

The Attempt at a Solution


(a) I did mr2 = (10)(.5)2 = 2.5 (the answer is supposed to be 1.6
Then I tried msrs2 + mcrc2 = (8)(.25)2 + (2)(.5)2 = 1.5 (still incorrect)

(b) Due to not being able to solve "a" I could not began my attempt on "b"
 

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Answers and Replies

  • #2
Doc Al
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(a) I did mr2 = (10)(.5)2 = 2.5 (the answer is supposed to be 1.6
Then I tried msrs2 + mcrc2 = (8)(.25)2 + (2)(.5)2 = 1.5 (still incorrect)
Not sure what you're doing here. The thin rods and the hoop are standard shapes--you can look up their rotational inertia. (You'll need to use the parallel axis theorem.)
 
  • #3
1,137
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first find I of hoop along any diameter using perpendicular axis theorem and then use parallel axis theorem to find I along axis of rotation
do same for square

then just simply add their I
 
  • #4


Thanks. I got it.
 

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