At point A and point B two forces act which rotate the bar in anticlockwise direction.
The perpendicular distance between the two forces called AB is the couple arm.
Moment of force F at A = F × OA (anticlockwise)
Moment of force F at B = F × OB (anticlockwise)
Total moment of couple = F x OA + F × OB
F x (OA + OB) = F × AB
= F × d
Hence proved : Moment of force = Force × Couple arm
