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2-1 (d) Finding one force from the resultant

2-1 (d) Finding one force from the resultant

Point 4 of 12 — Parallel Coplanar Forces: Resultant and EquilibriumQuestion bank: 0 questions (0 with pictures)

04 2-1 (d) Finding one force from the resultant

Method

  • Take the unit vector along the resultant and use R=F1+F2 with signs.
  • Take moments about the point where the resultant acts; the sum of moments of the forces about that point is zero.
✍️ Worked examples

Example 4:

  • One of two parallel forces is 20 N.
  • The resultant is 35 N, and its line of action is 15 cm from the 20 N force.
  • Find the other force and its position in each case: (a) the 20 N force and the resultant have the same direction, (b) opposite directions.

Solution:

  • (a) 35=20+F2, so F2=15 N, in the same direction.
  • Moments about C (point of the resultant): 20×15=15×BC, so BC=20 cm. The 15 N force is on the other side of C, so AB=15+20=35 cm.
  • (b) The resultant (35 N) is opposite to the 20 N force, so 35=F2−20 and F2=55 N in the direction of the resultant.
  • Moments about C: 20×15=55×BC, so BC=6011 cm.
  • Both C and the 55 N force are on the same side of the 20 N force, so AB=15−6011=10511 cm.

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