Statics: Reactions at rocker and pin on an A-shaped frame

[JoeS4]

Homework Statement

Determine the reactions at rocker support A and pin support B.

Homework Equations

ƩF = 0
ƩM = 0

The Attempt at a Solution

ƩFx = 0: -Asin(25) - (800lb) + Bx
ƩFy = 0: Acos(25) - (500lb) + By

I'm getting thrown off because of the rocker on an incline.

 

[pongo38]

You have made an assumption without declaring it about the direction of the reaction at B, and it contains an inconsistency. Before you apply equilibrium equations blindly, look at the problem geometrically so that you know the rough directions of the answers you seek. In this case, if you draw a line with a gradient 5 to 8 through E in the approximately south west direction, where does this line of the resultant applied force meet the direction of the reaction at A? Why is it that the reaction at B must also pass through this same point? What can you now conclude about the directions of the two reaction forces... and hence their components? Can you now sketch a triangle of forces and obtain an answer to the question within 20% of the algebraically "correct" answer. It is wise to do the rough thought process I have described, before doing the algebra. I concede that very systematic people can solve mechanics problems with no mental picture of it, but it is rare to do so without one or more errors. Looking at the problem geometrically is a form of error trapping.