A Moment of Inertia/physical pendulum Problem

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Homework Help Overview

The problem involves a physical pendulum, specifically a wrench swinging from a hook with a given period and spring constant. Participants are tasked with determining the moment of inertia of the wrench about the hook based on the provided parameters.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the calculation of mass from the spring's stretch and question the interpretation of the wrench's center of mass relative to its length. There are attempts to apply the moment of inertia formula, but confusion arises regarding the parameters used.

Discussion Status

Some participants have provided insights into the need for a visual reference to clarify the problem setup. Others have suggested using the period of the pendulum to derive further relationships, indicating a potential path forward without reaching a consensus on the interpretation of the wrench's dimensions.

Contextual Notes

There is a noted absence of the figure referenced in the problem, which is critical for understanding the configuration of the wrench and its center of mass. Participants are also grappling with the implications of the wrench's length and its measurement from the pivot point.

dominatesean
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Homework Statement


The 14-cm-long wrench in the figure swings on its hook with a period of 0.90 s. When the wrench hangs from a spring of spring constant 380 , it stretches the spring 2.8 cm
What is the wrench's moment of inertia about the hook?

cm of wrench is also 14 cm according to pic

Homework Equations


I = m*g*L*T^2/2pi

(How I found the mass)
Fsp = -ks
Fg = mg

The Attempt at a Solution


I=1.08571428*9.8*.14*.028^2/2 pi
Not the right answer not sure what to do or where to go
 
Last edited:
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It would really help if we had the figure; that has a lot of the information in it.

From the second sentence you should be able to compute the mass of the wrench.

It is pretty hard to understand how the CM can be 14 cm from the hook when the wrench is 14 cm long. That is why we really need to see the figure.
 
According to the figure you posted, that 14 cm is the distance from the pivot to the CM, so this is a very, very strange "14 cm long wrench."

Now write the equation of motion, and use the definition of the radius of gyration in order to express the mass moment of inertia. From the period information you should be able to find the radius of gyration. From there, you can finally go back to the MMOI itself.
 

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