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## Homework Statement

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?

## Homework Equations

Static equilibrium conditions.

## The Attempt at a Solution

In fact I have no answer to the question...it's not a numerical problem.