02 3-1 (b) The moment of a couple
Definition
- The moment of a couple is the sum of the moments of its two forces about any point in space.
- Its magnitude is the product of one force and the arm: where is the magnitude of each force and is the arm.
- Its direction is perpendicular to the plane of the couple, given by the right-hand rule. In a plane we use a sign: anticlockwise positive, clockwise negative.
- Units: N·m, N·cm, kg·wt·cm, dyne·cm.
The moment is a constant vector
- Let the forces and act at and , and let be any point.
- .
- The point has vanished from the result: the moment is the same about every point.
Observations
- If the arm becomes larger, the same moment needs a smaller force.
- Only the moment (size and sense) matters; the couple can be moved anywhere in its plane or into a parallel plane without changing its effect.
- A couple can be rotated in its own plane, and its forces and arm can be changed, provided the moment stays the same.
✍️ Worked examples
Example 1:
- Forces of 40 N form a couple with arm 4 cm in one case, and forces of 20 N form a couple with arm 8 cm in another.
- Compare the moments.
Solution:
- First couple: N·cm.
- Second couple: N·cm.
- The moments are equal in magnitude: halving the force while doubling the arm leaves the turning effect unchanged.
Example 2:
- Two forces of 20 N act in opposite directions at the ends of a rod 50 cm long, each making an angle of with the rod.
- Find the moment of the couple they form.
Solution:
- The arm is the perpendicular distance between the two lines of action: cm.
- N·cm.
Example 3: The forces at and at form a couple. Find , and the moment.
Solution:
- For a couple : and , so .
- Moment about : with .
- , so : a clockwise couple of magnitude units.
Example 4: Forces and N act at and m. Show that they form a couple and find its moment.
Solution:
- The forces are equal and opposite, and the points differ so the lines of action are not the same: it is a couple.
- ; moment .
- N·m (clockwise, magnitude 10 N·m).