Find an Expression for the Frequency - Pendulum

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SUMMARY

The discussion focuses on deriving the frequency of small angle oscillations for a solid sphere suspended from a thin rod. The correct formula for frequency is identified as f = 1/2(pi) sqrt(5g/7R), where R represents the distance from the pivot point to the center of mass, not the radius of the sphere. The moment of inertia I used in the frequency equation must be the correct value for the system's configuration, which is not simply 2/5MR². Clarification on the definitions of R and I is crucial for accurate calculations.

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  • Understanding of pendulum dynamics and oscillatory motion
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  • Basic grasp of gravitational effects on oscillating systems
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AbigailG
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Homework Statement


[/B]
A solid sphere of mass M and radius R is suspended from a thin rod. The sphere can swing back and forth at the bottom of the rod. Find an expression for the frequency of small angle oscillations.

Homework Equations



f = 1/2(pi) sqrt(MgR/I)

I for a solid sphere 2/5MR^2

The Attempt at a Solution


[/B]
I simply plugged the moment of inertia into the equation for the frequency and that yielded:

1/2(pi) sqrt(5g/2R)

but the answer is:

1/2(pi) sqrt(5g/7R)

Does anyone know what my missing is?
 
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Review the meaning of R and I in the equation f = 1/2(pi) sqrt(MgR/I). R is not the radius of the sphere and I is not the moment of inertia of the sphere about an axis through the center of the sphere.

EDIT: It could be that R does in fact equal the radius of the sphere depending on the interpretation of the problem. It's not clear how the rod is arranged in the problem.
 
TSny said:
Review the meaning of R and I in the equation f = 1/2(pi) sqrt(MgR/I). R is not the radius of the sphere and I is not the moment of inertia of the sphere about an axis through the center of the sphere.

Generically, R = l which is the distance from the pivot point to the center of mass. I thought the moment of inertia would be that of a sphere about an axis through the center. ..am I just not visualizing this correctly?
 
Is the setup like either of the pictures below?

upload_2017-12-11_14-49-22.png
 

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AbigailG said:
Generically, R = l which is the distance from the pivot point to the center of mass.
Yes
I thought the moment of inertia would be that of a sphere about an axis through the center.
No, check out this page and pay particular attention to the meaning of ##I_{\rm support}##.

http://hyperphysics.phy-astr.gsu.edu/hbase/pendp.html
 

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