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3-1 (d) Equivalent couples

3-1 (d) Equivalent couples

Point 4 of 8 — Couples and the Resultant CoupleQuestion bank: 0 questions (0 with pictures)

04 3-1 (d) Equivalent couples

Definition

  • Two couples in the same plane are equivalent if their moments are equal (in size and in sense).
  • Equivalent couples have the same effect on a rigid body even if their forces and arms are different.
✍️ Worked examples

Example 8:

  • ABCD is a rectangle with AB=12 cm and BC=16 cm.
  • Equal forces of 96 N act along AB and CD (opposite directions).
  • Find the equal forces acting at A and C parallel to BD whose couple is equivalent to this one.

Solution:

  • First couple: arm =BC=AD=16 cm, moment 96×16=1536 N·cm (clockwise).
  • BD=122+162=20 cm.
  • The distance of A from BD is 12×1620=9.6 cm, and the same for C on the other side, so the arm of the second couple is 2×9.6=19.2 cm.
  • Equal moments: F×19.2=1536, so F=80 N, acting clockwise.

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