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2-1 (b) The resultant of two like parallel forces

2-1 (b) The resultant of two like parallel forces

Point 2 of 12 — Parallel Coplanar Forces: Resultant and EquilibriumQuestion bank: 0 questions (0 with pictures)

02 2-1 (b) The resultant of two like parallel forces

Rule

  • Let F1 and F2 be like forces at points A and B of a rigid body.
  • Their resultant R is parallel to them, in the same direction, with magnitude R=F1+F2
  • Its line of action cuts the line AB at an inner point C which divides AB internally in the inverse ratio of the forces: F1⋅AC=F2⋅BCorACBC=F2F1
  • So C is nearer to the larger force.

Where the rule comes from

  • Add two equal and opposite forces F along AB at A and at B; they do not change the effect.
  • Combine F1 with the force at A, and F2 with the force at B, by the parallelogram law. The two resultants meet at a point O.
  • Resolve them again at O: the added horizontal forces cancel and F1, F2 act together along the line through O.
  • Similar triangles then give ACOC=FF1 and BCOC=FF2, so F1⋅AC=F2⋅BC.
✍️ Worked examples

Example 1: Two like forces of 5 N and 7 N act at A and B with AB=36 cm. Find their resultant.

Solution:

  • R=5+7=12 N, in the direction of the two forces.
  • ACCB=75 with AC+CB=36, so AC=712×36=21 cm.
  • The resultant acts 21 cm from A (the end of the 5 N force) and 15 cm from B.

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