Free Body Diagram Explained: Why is Right FBD Upward?

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Discussion Overview

The discussion revolves around the interpretation of free body diagrams (FBDs) in the context of mechanics, specifically addressing the forces acting on members in a system. Participants explore the reasoning behind the direction of forces in the diagrams and the number of unknown forces present.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant questions the upward component in the FBD of a member, seeking clarification on its necessity.
  • Another participant cites Newton's 3rd law to explain that for every action, there is an equal and opposite reaction, which applies to both horizontal and vertical forces.
  • A participant expresses confusion regarding the number of unknown forces in the FBD, noting a discrepancy between their observation of four unknowns and the book's claim of three.
  • In response, another participant suggests that the inclusion of coefficients of friction provides an additional equation that can reduce the number of unknowns.
  • A follow-up question seeks to clarify whether the three unknown forces refer specifically to the components bx, by, and the normal force Na.
  • A participant confirms that they expect the three unknown forces to be those components.

Areas of Agreement / Disagreement

Participants express differing views on the number of unknown forces and the interpretation of the FBDs, indicating that the discussion remains unresolved regarding the exact nature of the forces involved.

Contextual Notes

The discussion highlights potential limitations in understanding the assumptions underlying the FBDs and the definitions of the forces involved, as well as the implications of including friction in the analysis.

Drain Brain
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Can you explain why the free body diagram of each members are as shown? I kind of understand why the FBD of the member on the left is drawn as shown. But the FBD on the right is confusing, Why does it have a component in Y (upward) direction?
thanks!
 

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Drain Brain said:
Can you explain why the free body diagram of each members are as shown? I kind of understand why the FBD of the member on the left is drawn as shown. But the FBD on the right is confusing, Why does it have a component in Y (upward) direction?
thanks!

Hi Drain Brain!

It's Newton's 3rd law: whenever a body pushes against another body, that body pushes back with an equal and opposite force.
For the horizontal forces that should be intuitive.
For the vertical forces we pick one down, meaning the other must be up.
 
I like Serena said:
Hi Drain Brain!

It's Newton's 3rd law: whenever a body pushes against another body, that body pushes back with an equal and opposite force.
For the horizontal forces that should be intuitive.
For the vertical forces we pick one down, meaning the other must be up.

Hi I Like Serena! Now I understand!

There's another thing I want to ask, my book says that there are 3 unknown forces on each member, but I see 4 unknowns. Can you tell me why that is?
 
Drain Brain said:
Hi I Like Serena! Now I understand!

Good!

There's another thing I want to ask, my book says that there are 3 unknown forces on each member, but I see 4 unknowns. Can you tell me why that is?

They're probably jumping a bit ahead.
Since the coefficients of friction are given, that gives us the extra equation $F_A=\mu_A N_A$, effectively eliminating $F_A$ as an unknown.
 
I like Serena said:
Good!
They're probably jumping a bit ahead.
Since the coefficients of friction are given, that gives us the extra equation $F_A=\mu_A N_A$, effectively eliminating $F_A$ as an unknown.

Do you mean to say that the 3 unknown forces my book is referring to are the components bx, by and the normal force Na?
 
Drain Brain said:
Do you mean to say that the 3 unknown forces my book is referring to are the components bx, by and the normal force Na?

That's what I expect yes.
 

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