Strength of Materials
Two non-collinear parallel equal forces acting in opposite direction

constitute a moment of couple
balance each other
constitute a couple
constitute a moment

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Strength of Materials
According to principle of moments

if a system of coplanar forces is in equilibrium, then their algebraic sum is zero
if a system of coplanar forces is in equilibrium, then the algebraic sum of their moments about any point in their plane is zero
positive and negative couples can be balanced
the algebraic sum of the moments of any two forces about any point is equal to moment of theiwesultant about the same point

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