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2-2 (c) Tilting and limiting positions

2-2 (c) Tilting and limiting positions

Point 10 of 12 — Parallel Coplanar Forces: Resultant and EquilibriumQuestion bank: 0 questions (0 with pictures)

10 2-2 (c) Tilting and limiting positions

Idea

  • A beam on two supports is about to tilt about one support when the reaction at the other support becomes zero.
  • Take moments about the support that remains in contact.
✍️ Worked examples

Example 14:

  • A uniform beam AB of length 6 m and weight 40 kg·wt rests on supports at C (1 m from A) and D (1 m from B).
  • A man of weight 100 kg·wt walks from the middle towards B.
  • How far from A is he when the beam is about to tilt?

Solution:

  • At the limit the reaction at C is zero, so take moments about D (5 m from A).
  • The beam's weight is 2 m to the left of D: anticlockwise moment 40×2=80.
  • The man at distance x from A is (x−5) to the right of D: clockwise moment 100(x−5).
  • 100(x−5)=80, so x=5.8 m from A (0.2 m from the end B).

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