Dabour Hub EGEgyptian Curriculum 2026/2027
3-1 (b) The moment of a couple

3-1 (b) The moment of a couple

Point 2 of 8 — Couples and the Resultant CoupleQuestion bank: 0 questions (0 with pictures)

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: M=F×L where F is the magnitude of each force and L 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 F⃗ and −F⃗ act at A and B, and let O be any point.
  • M⃗=OA→×F⃗+OB→×(−F⃗)=(OA→−OB→)×F⃗=BA→×F⃗.
  • The point O 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: 40×4=160 N·cm.
  • Second couple: 20×8=160 N·cm.
  • The moments are equal in magnitude: halving the force while doubling the arm leaves the turning effect unchanged.

📝 Question bank for this point

0 questions linked to this point only, 0 of them with pictures.