Solving a Pin Joint System with Vertical Load

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Discussion Overview

The discussion revolves around a pin joint system subjected to a vertical load, focusing on the mechanics of determining reaction forces at the joints. Participants explore the application of equilibrium equations and the role of moments in analyzing the system, with an emphasis on understanding the basic principles of mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • The thread starter questions why the vertical components of the reactions at points A and C are not included in the moment equations, suggesting a possible misunderstanding of the mechanics involved.
  • One participant asks how a vertical force at point A would create a moment about point C, prompting a discussion on the definition of moment and the relevance of distances in this context.
  • Another participant inquires about the interaction between points A and C when a load is applied at point B, specifically whether the reaction forces can prevent movement and how the components of these forces contribute to the system's stability.
  • A clarification is provided regarding the meaning of "pinned" points in the context of the problem, indicating that these points do not move and will generate reactionary forces in response to applied loads.
  • One participant explains that moments are calculated as force times the perpendicular distance, noting that the reaction forces at A and C do not create moments due to the lack of a perpendicular distance.

Areas of Agreement / Disagreement

Participants express varying levels of understanding regarding the mechanics of moments and reaction forces. There is no consensus on the initial confusion about the exclusion of certain components in the moment equations, and the discussion remains unresolved on this point.

Contextual Notes

Some participants highlight the importance of understanding the definitions and roles of forces and moments in mechanics, indicating that assumptions about the system's behavior may not be fully articulated.

Who May Find This Useful

This discussion may be useful for students beginning to learn about mechanics, particularly those interested in understanding the fundamentals of pin joint systems and the application of equilibrium equations.

sriram123
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Hi all,I just started to learn mechanics so please forgive me if i ask something elementary.I have an example from Egor P Popov's Engineering mechanics of solids.(Please refer attachment and sorry about quality of drawing).

This is a problem of a pin joint system subjected to vertical load that acts at point B.Since the pin joint system cannot allow x or y movement there should be reactions at A and C which are resolved into their horizontal and vertical components.For determining their value the moment about the point C and A are taken to zero ,This is where I got stuck,

I cite the step in the book

ƩMc=0 => FAx*(a+b)-P*c
ƩMa=0 => P*c +FCx*(a+b)

What happened to the y components of the reactions at A and C.Why FCy and FAy are not taken,Is that because the force applied is vertical ?.I am self studying so I cannot clarify this with someone.I know this is elemantary but pls help me..
 

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Take the equation you've got there for the moment about (c). How would a vertical force at (a) apply a moment about (c)? To find a moment you take a force and multiply it by a distance, what is that distance in the case of Fay about point (c)?
 
What I'm about ask is very basic.But I don't understand one thing.In the image the thread starter has shown,the points A and C are connected through B so if the load applied at B tries to push it down will the point C remain stationary because of the reaction offered by X component of A ?.If the points A and C are not collinear,will the Y component also offer resistance ? .Please explain me how with an example ?

And again please remember I'm learning the basics and I just wanted to learn this properly.Sorry if I'm asking something very basic

And thanks in advance.
 
Those circles at the A,B, and C locations generally denote "pinned" or "fixed" points. In problems like this, this means that any forces on the system will create reactionary forces, but the location of the pins will not move.
 
Moment is a force times the perpendicular distance from the force to the point of rotation. There's no perpendicular distance for those two reactant forces, therefore no moment is created by them.
 

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