Strength of Materials
According to principle of moments

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

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Strength of Materials
Limiting force of friction is the

Ratio of limiting friction and normal reaction
Tangent of angle between normal reaction and the resultant of normal reaction and limiting friction
The friction force acting when the body is just about to move
the friction force acting when the body is in motion

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