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

[theBEAST]

Homework Statement

Assume the system is massless.

The Attempt at a Solution

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

What I am not sure about is the direction of the force at F. Should the force be A or B?

 

[cepheid]

Can't you just assume a direction, and if the force comes out negative, it's in the opposite direction from what you assumed?

 

[theBEAST]

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?

 

[cepheid]

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

 

[cepheid]

You didn't actually write down a well-defined problem statement. What exactly is it that you're being asked to solve for?

 

[cmmcnamara]

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.