(adsbygoogle = window.adsbygoogle || []).push({}); 1. The problem statement, all variables and given/known data

The image in the attachment shows two uniform ladders (red and blue) in equilibrium resting on grass(green). The distance between A and B is "L" units.

The two ladders are freely hinged at C.

Because the ladders are uniform their Centers of mass lie at their mid points.

As the ladders are on rough grass we can conclude that friction forces act at A and B. (not shown in the diagram due to lack of space).

the friction at A, [itex]F_{A}[/itex], is directed towards B

the friction at B, [itex]F_{B}[/itex], is directed towards A.

The normal reactions at A and B are shown in the diagram.

2. Relevant equations

these are the rules i learned in my lesson on Equilibrium of forces.

Moment aboutanypoint is zero

Vector sum of the forces is zero

3. The attempt at a solution

I dont really think that the first rule is correct because clearly a force will act at the point C (although i dont know the direction or magnitude). According to my intuition a force will definitely act at C where the two ladders are hinged.

And if that is true (which it should be) then there is a net moment about A and about B.

Another thing which confuses me is :

"the line of action of all forces must pass through a common point for the system to be in equilibrium"

I don't understand this point either.

Why is this necessarily true and is it true in this particular case ?

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# Homework Help: Moments and equilibrium

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