Statics- equilibrium of rigid bodies

[Litcyb]

Homework Statement

An opening in a floor is covered by a 1×1.2-m sheet of plywood of mass 18 kg. The sheet is hinged at A and B and is maintained in a position slightly above the floor by a small block C. Determine the vertical component of the reaction (a) at A, (b) at B, (c) at C.

Homework Equations

The Attempt at a Solution

I know how to approach this problem, however, I am just a bit confused on how to set up my F.B.D. i attached the correct f.b.d for this problem . My question is, how come if we take the moment about point A, we only have a reaction force in the j direction at support B?

 

[SteamKing]

The plywood is hinged at A and B and the only loads imposed are the weight and the reaction due to the block at C. All of these forces are in the direction of the y-axis. It doesn't matter about which point you take moments, it's just the nature of the forces for this particular problem.

 

[pongo38]

You take moments about an axis, not a point. That is the key to solving 3-dimensional problems like this.