When a number of like or unlike parallel forces act on a rigid body and the body is in equilibrium, then the algebraic sum of the moments in the clockwise direction is equal to the algebraic sum of the moments in the anticlockwise direction. In other words, at equilibrium, the algebraic sum of the moments of all the individual forces about any point is equal to zero.
In the illustration given in figure, the force F1 produces an anticlockwise rotation at a distance d1 from the point of pivot P (called fulcrum) and the force F2 produces a clockwise rotation at a distance d2 from the point of pivot P. The principle of moments can be written as follows: Moment of clockwise direction = Moment of anticlockwise direction
F1 × d1 = F2 × d2
