If a force is applied along a line that passes through the joint axis, what is the moment arm?

If a force is applied along a line that passes through the joint axis, what is the moment arm?

• A. Zero
• B. Equal to distance to origin
• C. Equal to lever length
• D. Equal to force magnitude

Answer: (A)

The moment arm is the perpendicular distance from the joint axis to the line of action of the force. If the force is applied along a line that passes through the joint axis, that distance is zero, so the moment arm is zero. Since torque equals force times this moment arm, the rotational effect about the joint is zero. The force can still act along the axis (compressing or pulling), but it won’t cause rotation about that axis.

The moment arm is the perpendicular distance from the joint axis to the line of action of the force. If the force is applied along a line that passes through the joint axis, that distance is zero, so the moment arm is zero. Since torque equals force times this moment arm, the rotational effect about the joint is zero. The force can still act along the axis (compressing or pulling), but it won’t cause rotation about that axis.