The moment arm is the distance from the fulcrum to the line of action of the force, and the mathematical model of the moment arm is the distance from the point to the straight line. Where the "point" is the fulcrum of the lever; "Line" is the line of action of force, that is, a straight line drawn along the direction of force through the point of action of force. It should be noted that the moment arm can never be understood as "the length from the fulcrum to the stress point".

If the directions of several forces acting on the same point are different, the arm of force is generally different. If the line of action of the force passes through the fulcrum, then the arm of force must be zero.

Extended data:

First, on the lever diagram, determine the fulcrum. Draw the line of action of strength and the line of action of resistance, and extend the line of action of strength appropriately with dotted lines when drawing.

Draw a vertical line from the fulcrum to the line of action of the force and a right angle at the vertical foot. The distance from the fulcrum to the vertical foot is the arm of force. Use one right-angle side of the triangle to coincide with the line of action of the force, let the other right-angle side cross the intersection point, draw a vertical line from the fulcrum to the line of action of the force, make a force arm and a resistance arm, circle the distance from the fulcrum to the vertical foot with braces or tick off the line segment with arrows.