Finding the angular velocity of a pendulum

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Homework Help Overview

The discussion revolves around calculating the angular velocity of a pendulum, focusing on the moment of inertia for different components, specifically a sphere and a rod. Participants are examining their calculations and the application of the parallel axis theorem.

Discussion Character

  • Mathematical reasoning, Assumption checking

Approaches and Questions Raised

  • Participants are questioning the calculations of the moment of inertia for the rod and sphere, particularly the application of the parallel axis theorem. There are inquiries about the correct formulas and the distances involved in the calculations.

Discussion Status

Some participants are exploring different interpretations of the moment of inertia calculations, with one individual expressing uncertainty about their initial results. Guidance has been offered regarding the correct application of the parallel axis theorem and the moment of inertia for the rod.

Contextual Notes

There is a focus on ensuring that calculations include appropriate units, and participants are encouraged to clarify the definitions and assumptions related to the center of mass and axis of rotation.

I_Try_Math
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Homework Statement
A pendulum consists of a rod of length 2 m and mass 3 kg with a solid sphere of mass 1 kg and radius 0.3 m attached at one end. The axis of rotation is as shown below. What is the angular velocity of the pendulum at its lowest point if it is released from rest at an angle of 30 degrees?
Relevant Equations
## I_{sphere} = \frac{2}{5}MR^2 + Md^2 ##
## I_{rod} = \frac{1}{3}ML^2 + Md^2 ##
7-27-q.png


My answer disagrees with the textbook and I have a feeling it may be due to how I calculated the moment of inertia. Is there anything obviously wrong with my calculation? Any help is appreciated.

## I_{sphere} = \frac{2}{5}MR^2 + Md^2 ##
## I_{rod} = \frac{1}{3}ML^2 + Md^2 ##

## I_{sphere} = \frac{2}{5}(1)(0.3)^2 + (1)(0.3)^2 = 0.126 ##
## I_{rod} = \frac{1}{3}(3)(2)^2 + 3(0.6)^2 = 5.08 ##

## I = I_{sphere} + I_{rod} = 5.21 ##
 
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I_Try_Math said:
## I_{rod} = \frac{1}{3}ML^2 + Md^2 ##
## I_{rod} = \frac{1}{3}(3)(2)^2 + 3(0.6)^2 = 5.08 ##
Why?
What is the equation for the parallel axis theorem?
What is ##I_{\text{cm}}## for the rod?
What is the distance of the CM of the rod from the axis of rotation?
Also, if you put numbers down remember to include the appropriate units.
 
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kuruman said:
Why?
What is the equation for the parallel axis theorem?
What is ##I_{\text{cm}}## for the rod?
What is the distance of the CM of the rod from the axis of rotation?
Also, if you put numbers down remember to include the appropriate units.
Ah I think I see where I was going wrong. I believe this should be the correct moment of inertia for the rod.
## I_{rod} = \frac{1}{12}ML^2 + Md^2 ##
## I_{rod} = \frac{1}{12}(3 kg)(2 m)^2 + (3 kg)(1.6 m)^2 ##
 
That looks fine.
 
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