1. The problem statement, all variables and given/known data we know that, every force systems can be generally replaced by a resultant force(R) and a couple(M) at a point O and the position of point O is optional. but magnitude and direction of M is dependent to this point while magnitude and direction of R is independent. In static equilibrium R and M are zero at an optional point O. now this is the question: While M is zero at an optional point O, why should we conclude that M would be zero at every point chosen( infinite in number of points)...please pay attention that " magnitude and direction of M is dependent to the point chosen".... I mean we don't know the object is in static equilibrium or not and we want to determine it... why do we consider that if M is zero about a point, it means that it is zero about any point? is there a theorem about this? is it provable? 2. Relevant equations Static equilibrium conditions. 3. The attempt at a solution In fact I have no answer to the question...it's not a numerical problem.