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2-2 (d) A rod on a rough surface and a smooth wedge

2-2 (d) A rod on a rough surface and a smooth wedge

Point 11 of 12 — Parallel Coplanar Forces: Resultant and EquilibriumQuestion bank: 0 questions (0 with pictures)

11 2-2 (d) A rod on a rough surface and a smooth wedge

Method

  • Forces on the rod: weight W, reaction of the wedge S (perpendicular to the rod, since the wedge is smooth), the ground reaction R and friction F at the lower end.
  • Use X=0, Y=0 and moments about the foot of the rod.
  • At the point of slipping, F=μR.
✍️ Worked examples

Example 16:

  • A uniform bar AB of weight 5 kg·wt and length 30 cm rests with A on rough ground and touches a smooth wedge at C, 12.5 cm above the ground, at 30∘ to the horizontal.
  • It is about to slip.
  • Find the reaction of the wedge and the coefficient of friction.

Solution:

  • AC=12.5sin⁡30∘=25 cm. The weight acts 15 cm from A.
  • Moments about A: 5×15cos⁡30∘=S×25, so S=3cos⁡30∘=332≈2.60 kg·wt.
  • The wedge reaction is perpendicular to the bar, so it makes 30∘ with the vertical. Horizontal: F=Ssin⁡30∘=334.
  • Vertical: R+Scos⁡30∘=5, so R=5−332⋅32=5−94=114.
  • μ=FR=33/411/4=3311≈0.47.

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