What is the significance of F being a roller joint in this system?

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Homework Help Overview

The discussion revolves around the significance of a roller joint in a massless system, particularly focusing on the force at point F and its implications for system constraints.

Discussion Character

  • Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants explore the direction of the force at F, questioning whether it should be assumed as A or B. Some suggest that assuming a direction is valid, as a negative result indicates the opposite direction. Others discuss the implications of F being a roller joint and its lack of resistance in the x-direction, raising questions about over-constraining the system.

Discussion Status

The discussion is ongoing, with participants seeking clarification on the concept of roller joints and their role in preventing over-constraining the system. Some guidance has been provided regarding the nature of forces at roller joints, but multiple interpretations and questions remain.

Contextual Notes

There is a mention of a lack of a well-defined problem statement, which may affect the clarity of the discussion. The original poster's assumptions and the professor's comments on the nature of roller joints and system constraints are central to the conversation.

theBEAST
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Homework Statement


tUv0v.png

Assume the system is massless.

The Attempt at a Solution


Here is my quick attempt at drawing a free body diagram for this system:
LSe0p.jpg


What I am not sure about is the direction of the force at F. Should the force be A or B?
 
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Can't you just assume a direction, and if the force comes out negative, it's in the opposite direction from what you assumed?
 
cepheid said:
Can't you just assume a direction, and if the force comes out negative, it's in the opposite direction from what you assumed?

The prof posted answers online and it says that F is a roller joint and there is no force in the x direction. The reason why it is a roller joint is because if it wasn't it would be over constrained. Does anyone understand what is meant by this?
 
theBEAST said:
The prof posted answers online and it says that F is a roller joint and there is no force in the x direction. The reason why it is a roller joint is because if it wasn't it would be over constrained. Does anyone understand what is meant by this?

Hmm...try looking at this:

http://en.wikipedia.org/wiki/Overdetermined_system
 
You didn't actually write down a well-defined problem statement. What exactly is it that you're being asked to solve for?
 
theBEAST said:
The prof posted answers online and it says that F is a roller joint and there is no force in the x direction. The reason why it is a roller joint is because if it wasn't it would be over constrained. Does anyone understand what is meant by this?

The reason there is no force in the x-direction at F is because the joint is a roller, it resists no forces in the x-direction. A force in the x-direction on a roller would cause it to move.

The idea your professor is mentioning is that if the joint at F was not a roller and rather a pin or some other type of constraint which imposed more than one resistance on the system, the system would be over constrained or statically indeterminate. These types of problems are solvable only through some sort of continuity condition such as material property relations that go beyond just performing force analysis.
 

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