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3-2 (c) The three-point test

3-2 (c) The three-point test

Point 7 of 8 — Couples and the Resultant CoupleQuestion bank: 0 questions (0 with pictures)

07 3-2 (c) The three-point test

Rule

  • If the sum of the moments of a system of coplanar forces about three non-collinear points in its plane has the same non-zero value k, the system is equivalent to a couple of moment k.

Why

  • A coplanar system reduces to a single force, a single couple, or equilibrium.
  • It is not in equilibrium (the moments are non-zero).
  • If it reduced to a single force F at distances L1,L2,L3 from the three points, then FL1=FL2=FL3, so the three points would lie on a line parallel to the force; they do not. So it must be a couple.
✍️ Worked examples

Example 16: A square ABCD of side 10 cm is acted on by the forces in Example 14. Use the test to confirm that it is a couple.

Solution:

  • Moments about A: +200 (computed above).
  • Moments about B: the forces along AB and BC pass through B; CD has arm 10 and DA has arm 10, both in the same sense: +200.
  • Moments about C: similarly +200.
  • Equal non-zero values at three non-collinear points, so the system is a couple of moment 200 N·cm.

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