Undergrad Why if net force=zero, moment at any point is same?

[gen x]

Why as long net force= zero, moment at any point is the same but if net force= non zero, that is not case anymore?

Here is simple example:

Net force=zero case: F1=10, F2=40, N=50

Moment at A= +20Nm

Moment at B=+20Nm

Any point you choose you will get same moment..

Net force = non zero: F1=10, F2=40, N=30

Moment at A=+20Nm

Moment at B=+100Nm

Now, moment is not the same in every point.

 

[Baluncore]

Please show the equations that you use to compute the moments, for the cases you present.