# Forces and Moments

## Homework Statement

A triangular plate is attached to a wall by two pins A and B. It is loaded at point C by A force F

View attachment diagram.bmp

## Homework Equations

Sum of Moments = 0
Sum of Forces = 0

## The Attempt at a Solution

Moments act around bottom of plate
F*L1=L2*Fa+L3*Fb

Vertical Forces
F=Fa+Fb
Fb=F-Fa
Horizontal Forces
F=0

So vertical forces into moments
F*L1=L2*Fa+L3*(F-Fa)

Fa= F (L1-L3)
-------------
L2 - L3

It doesn't feel right can someone tell me if I've done it right cheers

Fa = (F (L1 - L3))/(L2-L3)

PhanthomJay
Homework Helper
Gold Member

## Homework Statement

A triangular plate is attached to a wall by two pins A and B. It is loaded at point C by A force F

View attachment 32117

## Homework Equations

Sum of Moments = 0
Sum of Forces = 0

## The Attempt at a Solution

Moments act around bottom of plate
Moments act about any point you choose, not just the bottom of the plate. For objects in equilibrium, the sum of moments about any point is zero.
F*L1=L2*Fa+L3*Fb
This moment equation about the bottom of the plate is not correct. You first need to use F_ax and F_bx, not Fa and Fb, And your moment arm from F_ax to the bottom of the plate is wrong. It is easier to sum moments about B to solve for F_ax.
Vertical Forces
F=Fa+Fb
Fb=F-Fa
Again, here it should be F_by = F - F_ay, and you can assume that F_ay and F_by are equal.
Horizontal Forces
F=0
You mean F_netx = 0?
So vertical forces into moments
what do you mean by this?
F*L1=L2*Fa+L3*(F-Fa)

Fa= F (L1-L3)
-------------
L2 - L3

It doesn't feel right can someone tell me if I've done it right cheers