Solving Simple Forces Problem | FE Exam Prep | Mental Block

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Discussion Overview

The discussion revolves around a problem related to forces acting on a circular rod, specifically focusing on determining the displacement relative to a point C. The context is preparation for the FE exam, and the problem involves concepts of equilibrium and force analysis.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • A participant describes a problem involving a circular rod with specified forces and dimensions, seeking to calculate displacement using Young's modulus.
  • Another participant notes the absence of an attachment that would clarify the problem setup, suggesting a description of the diagram instead.
  • A detailed description of the rod's orientation and forces is provided, including forces acting at points A, B, and C.
  • One participant questions the force at point C, suggesting it should be 2 kip to maintain equilibrium, indicating a potential error in the problem statement.
  • Another participant suggests using a free-body diagram to resolve forces after addressing the equilibrium issue.
  • There is a claim of an error in the problem, with a participant asserting that the forces should all be 2 kip, particularly at point C.
  • A question is raised about the presence of supports on the rod, indicating that the absence of supports would imply acceleration and the potential use of d'Alembert's principle.

Areas of Agreement / Disagreement

Participants express disagreement regarding the correct values of the forces acting on the rod, particularly at point C. There is no consensus on the correct interpretation of the problem or the forces involved.

Contextual Notes

Limitations include the lack of a visual diagram, which is critical for understanding the problem setup. There are also unresolved questions about the forces and supports acting on the rod, which affect the analysis.

nikkelm
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I am studying for the FE exam and came to this problem:

A circular rod of 1in. cross-section area is subjected to several forces. Determine the displacement relative to C (in the attachment). Assume E =29x10^6 psi.

my solution is:

del=1/29x10^6(Fa-b(10in)/1in+Fc-b(8in)/1in)

yet I don't know how to get the right forces Fa-b and Fc-b

signed,

Mental Block
 
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Welcome to PF!

nikkelm said:
A circular rod of 1in. cross-section area is subjected to several forces. Determine the displacement relative to C (in the attachment) …

Hi nikkelm! Welcome to PF! :smile:

erm … no attachment … ! :cry:

Actually, it takes ages to get an attachment approved.

It'll be much quicker if you just describe the picture to us in words! :smile:
 
Oh... I guess that's for copy right infringement stuff... anyways, the cylinder axis is oriented horizontally and it is 18 in long from A to C and 10 inches long from A to B. A is at the left C at the far right with B in the middle, of course. The force at A is 2kip to the left. The force at B is 4kip to the right. The force at C is 4kip to the left again.
 
Before you go too far, I think you mean the force at C is 2K to the left, or else you're not in equilibrium.
 
nikkelm: Once you resolve the equilibrium issue, you could perform a section cut anywhere you wish, then solve for the unknown force acting on your free-body diagram, to obtain the force in each segment.
 
That explains things.. there is an error in the problem because it definitely shows a 4kip force
 
Is there any support on the rod? We would need to know about any support(s) on the rod, since we can't see your diagram. If it has no supports, then the rod is accelerating to the left. If accelerating, you might be able to use d'Alembert's principle to obtain the force on any section cut.
 
no, it isn't a kinematics question I am pretty sure it is a mistake. it is a multiple choice and the forces are supposed to be 2kip ea. which makes sense if they meant for it to be 2kip to the left at C.
 

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