# Two-force member significance

1. Oct 23, 2014

### scottalot

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

2. Relevant equations
None given

3. The attempt at a solution
What I tried to do was figure out the force equations and moment equations, but maybe assuming that there is no moment nor net force is incorrect... I know I'm missing something here.

I did an equation equating the positive y-axis forces (from the roller's normal force and the tension on ED) to the force pulling down on G (201 lb), another equating the tensions of DE and BC with A's normal (now that I type that out, I'm thinking maybe BC is a compression?), and another equating the net moment to zero by taking the vertical forces of A's normal and DE's tension and multiplying them by 40in. and 15in. respectively.

I really think I'm missing something because the hint provided mentions this:
"Before taking any other action, draw an FBD of the steel plate, cutting through supports DE and BC as well as underneath the roller support at A. Member E and hydraulic cylinder BC are examples of "Two-force members' , supports whose reaction forces are aligned with their geometry (along pins DE and BC. The roller support prevents any motion normal to the incline."

I'm not understanding the significance of any of that.

I can provide my equations if requested, but I think my issue is more a misunderstanding of the concepts rather than incorrect work.

EDIT: I apologize if this shouldn't be in advanced... I saw a couple titles and thought it'd fit, but now I'm seeing some crazy advanced ones.

Last edited: Oct 23, 2014
2. Oct 24, 2014

### SteamKing

Staff Emeritus
The roller at A can only develop a force which is normal to the surface on which it rests.

The linkages at BC and DE can support forces which are applied axially along their lengths or perpendicular to their lengths. The pin connections at either end of each link, however, mean that the links can rotate if a moment is applied to them.

The type of attachment, with either two-force members or rollers, helps to determine what sort of unknown reactions for which you are trying to solve.