The distance from a datum to the point where a force is applied is called what?

• A. Arm length
• B. Moment arm
• C. Datum distance
• D. CG distance

Answer: (B)

A moment arm is the perpendicular distance from the axis of rotation (the datum) to the line of action of the force. This distance is what determines how effectively the force can produce rotation: torque equals the force times the moment arm (τ = F × d). If the force isn’t perpendicular to the lever, only the perpendicular component of the force contributes, so torque can be thought of as F sin(theta) times the distance, or, in vector terms, the cross product r × F.

Other terms don’t fit as precisely: arm length is a generic distance and doesn’t specify the critical perpendicular distance to the force’s line of action; datum distance and CG distance refer to different concepts (reference points or center of gravity) and don’t describe rotational effect about an axis.