Moments, couples and centre of gravity

Repair the common break

Common misconception: Moment uses any distance from the pivot.

Correct model: Use the perpendicular distance to the force’s line of action; a couple has zero resultant force but a non-zero torque.

Worked transfer

A 12 N force whose line of action is 0.25 m from a pivot produces 3.0 N m. Moving the point of application along the same line of action does not change that moment.

Guided practice

Explain why “Moment uses any distance from the pivot.” fails. Then construct one example that correctly applies moments, couples and centre of gravity.

Independent practice

Two equal 18 N forces form a couple with perpendicular separation 0.40 m. Find the torque and explain why the resultant force is zero.

Return to the recorded repair plan when the explanation and application are secure enough to re-test.