# Homework Help: Free Body Diagram Help

1. May 22, 2012

### USN2ENG

1. The problem statement, all variables and given/known data

I feel that I am a bit fuzzy whether things are reaction forces. In the diagram I see there is a beam holding point B, and there are 3 forces: Cable BC, 500N, and 780N.

My professor wrote:
Ask yourself: is the beam holding the point from moving up or down, right or left? If so you will need to include the appropriate reaction forces in your FBD.

To me, it is, as the beam is not hinged. Unfortunately I am not quite sure how to handle how to label the reaction force. Any advice would be great!

2. Relevant equations

3. The attempt at a solution

My FBD is attached.

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• ###### FBD.jpg
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2. May 22, 2012

### Endorser

At A, the reaction force is Upwards

3. May 22, 2012

### PhanthomJay

When you draw a FBD of joint B, you must include ALL forces, both external and internal, acting on the joint. You neglected to include the force of the beam acting on the joint. You should also break down the applied external forces at B into their x and y components.

It is often best to find the reaction forces at A and C first, by using the 3 equilibrium equations and noting that the reaction force at C must be directed along the cable direction BC.

4. May 23, 2012

### PhanthomJay

I don't see any reference to the weight of the beam, so I assume it is negligible, in which case there cannot be any upward force at A, since this is a pure truss with axial tensile or compressive forces only, in the cable and beam.