State and explain the principle of moments.

Explanation

(i) Sum of the clockwise moments is equal to sum of the anticlockwise moments when a body is in rotational equilibrium or algebraic sum of moments at any point is zero.
(ii) Consider a ligth rod of negligible mass which is pivoted at a point along its length. Let two parallel forces `F_(1) and F_(2)` act at the two ends at distances `d_(1) and d_(2)` from the point of pivot and the normal reaction force N at the point of pivot as shown in Figure.
(iii) If the rod has to remain stationary in horizontal position, it should be in translational and rotational equilibrium. Then, both the net force and net torque must be zero.

For net force to be zero, `- F_(1) + N - F_(2) = 0`
`N = F_(1) + F_(2)`
For net torque to be zero, `d_(1) F_(1) - d_(2) F_(2) = 0`
`d_(1)F_(1) = d_(2)F_(2)`
The above equation represents the principle of moments.