Strength of Materials
The necessary condition for forces to be in equilibrium is that these should be

Meet at one point
All be equal
Coplanar
Coplanar & Meet at one point

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Strength of Materials
According to principle of moments

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
if a system of coplanar forces is in equilibrium, then the algebraic sum of their moments about any point in their plane is zero
if a system of coplanar forces is in equilibrium, then their algebraic sum is zero

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