Help needed for transformation of stresses in beer and johnston book

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Discussion Overview

The discussion revolves around the transformation of stresses as presented in the "Mechanics of Materials" by Beer and Johnston. Participants are examining the resolution of force components in the x' and y' directions, specifically addressing discrepancies between their calculations and those in the textbook. The focus is on understanding the mathematical approach to resolving these forces.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Homework-related

Main Points Raised

  • One participant expresses confusion about how the force components in the x direction are resolved in the textbook, noting a different outcome in their calculations.
  • Another participant requests to see the calculations that led to the differing results, suggesting that the problem is straightforward.
  • A participant mentions that their resolution of the horizontal force in the x' direction yields a different expression than that in the book, specifically questioning the use of cosine in the calculations.
  • There is a request for clarification on the mathematical representation of the forces involved, indicating a potential misunderstanding of the geometric relationships in the problem.
  • One participant points out that the expression σΔAcosθ is the hypotenuse of a triangle, suggesting that the other participant may be misinterpreting the components involved.
  • A later reply indicates that the participant has recognized their mistake regarding the reference taken for the resolution of forces.

Areas of Agreement / Disagreement

Participants do not reach a consensus, as there are differing interpretations of the mathematical resolution of forces. Some participants express confusion and seek clarification, while others attempt to guide them towards understanding the geometric relationships involved.

Contextual Notes

There are unresolved assumptions regarding the geometric interpretation of the forces and the application of trigonometric functions in the context of stress transformation. The discussion reflects a reliance on visual representations that may not be fully articulated in the text.

chiraganand
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Transformation of stresses in beer and johnston mechanics of materials. While reading the section on trsnformation of stresses they have solved by using the force components in x' and y' directions. I have attached a screenshot of the relevant page and the figure. I have few doubt as to how the force components in the x direction are resolved as shown in the book.

When i tried resolving the forces in x' and y' direction i am not getting the solution as in the book but a totally different solution. using the pythagoras theorem to determine the resolved forces.

Could someone please help me out as to how the answer in the book is done?
 

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Can you show us what you got when you tried? It's a pretty straightforward problem of resolving vectors into their components.
 
whenever is try to resolve the horizontal force in the x' direction i am getting the force component as sigma x into area.. whereas in the book he's getting it as sigma x into (cos theeta) ^2 ... don't know where i am gong wrong..
 
Well, I can't make sense of what you're trying to say. Could you write it out in math?
 
vela said:
Well, I can't make sense of what you're trying to say. Could you write it out in math?

When i resolve the component of the horizontal force σ ΔA cos θ in the x' direction i am getting = σ ΔA
whereas in the book it is σ (ΔA cos θ)cosθ
 
Why are you dividing by the cosine?
 
coz when i a m resolving it i am getting cos θ = σΔAcosθ/(component in x' direction)
 
You do realize that σΔAcosθ is the hypotenuse, right? It's not one of the legs go the triangle as you appear to be assuming.
 
yep now i got it.. taking the reference wrong..
 

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