Compression of an object under it's own weight.

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Discussion Overview

The discussion revolves around the compression of a cube under its own weight, focusing on the calculation of the decrease in height of the center of mass due to deformation. Participants explore theoretical approaches and mathematical formulations related to this problem.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested, Mathematical reasoning

Main Points Raised

  • One participant proposes starting with a cube of side length a, mass m, and Young's modulus E to calculate the decrease in height of the center of mass.
  • Another participant expresses uncertainty about the approach and seeks thoughts on how to tackle the problem.
  • A different participant shares a solution assuming the cube deforms before the normal force balances with gravity, resulting in a calculated decrease of 2.27 μm, but questions the validity of this approach due to the lack of consideration for the normal force.
  • One participant presents a differential force balance equation involving compressive stress, density, and gravitational force, inviting others to discuss the derivation of this result.
  • Another participant acknowledges a misunderstanding in their earlier argument, indicating a shift in their perspective.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus, as there are multiple competing views on how to approach the problem and the role of the normal force in the deformation process.

Contextual Notes

Some limitations include assumptions about the timing of deformation relative to the normal force and the need for clarity on the definitions of stress and density in the context of the problem.

LordGfcd
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Let's first consider a cube side length a, mass m, Young's modulus of the block is E. How do we calculate the decrease of the height of the center of mass of that cube ?
 
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What are your thoughts so far on how to approach this problem?
 
Actually I solved this problem assuming the cube is deform before the normal force make balance with gravity. I eventually find an acceptable result (a=10cm,m=1kg,E=10^7 Pa) : 2,27 μm. But I still think my approaching is wrong because I didn't consider the normal force (action equal minus reaction ofcourse). So I must consider the normal force too, but I don't know how . And, if the normal force balance with gravity, isn't the cube will stop deforming ?
 
The differential force balance on the section of the cube between z and z + ##\Delta z## (z is measured downward from the top) is $$a^2\frac{d\sigma}{dz}=\rho g a^2$$ where A is the cross sectional area, ##\sigma## is the compressive stress, and ##\rho## is the density of the material ##m/(a^2L)##. Do you see how this result is derived?
 
Last edited:
Thank you very much, I was completely wrong with my argument.
 

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