Finding the resultant moment for two directions is simple enough. However when there are forces in four directions (at right angles) how do you add the moments for the two axises?
For instance in the following example there is a moment of 5Nm downwards (or anticlockwise) and of 15Nm to the left.
I thought as both these moments were anticlockwise you could add them but the correct answer is 120Nm (a far cry from my 20Nm).
So how do you add moments in more than two directions?
Best Answer
You should add all the moments separately.
You are summing all the forces together when you shouldn't (based on the little amount of info you provided on how you answered this). If you do sum them, you have to account for the couple moments (the forces may be equal and opposite, but the moments add up to a bigger one, not cancel since the are on different sides of the origin). Try finding the moment for each force on it's own then adding them after.
When forces aren't applied on the centre of a body, you can cancel the forces, but it can still result in a moment about the centre.