# Engineering Statics – Moments problem

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

I have attached the diagram. The question says "Compute the Moment Mo of the 1.2kN force about the axis O-O.

**2. Relevant equations**

M = r x F

AB = OB – OA

Resolving forces, Fx = F cos Θ.. Fy = F sin Θ etc.

**3. The attempt at a solution**

Here are the axes I’ve made, using the Right Hand Rule:

OA = (-300, 0, 105)

OB = (0, 0, 200)

So AB = (300, 0, 95) .. this is the r vector.

For the F vector, Fz = 1.2 sin 60 = 1.04 kN

Fxy = 1.2 cos 60 = 0.6 kN

Fx = -0.6 cos 50 = -0.39 kN

Fy = 0.6 sin 50 = 0.46 kN

M = r x F (since we need the moment about the O-O axis, I’ll just get the j component).

| 300 0 95 |

| -0.39 0.46 1.04 |

Solving gives M @ o-o as -349.05 Nm. However the answer is +327 Nm. What have I done wrong?

http://ift.tt/1jF6rBT

## Leave a comment