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2-2 (a) Conditions of equilibrium

2-2 (a) Conditions of equilibrium

Point 8 of 12 — Parallel Coplanar Forces: Resultant and EquilibriumQuestion bank: 0 questions (0 with pictures)

08 2-2 (a) Conditions of equilibrium

The two conditions

  • A rigid body acted on by coplanar forces is in equilibrium if and only if:
  • the resultant vector is zero: R⃗=0⃗;
  • the sum of moments about any point is zero: ∑M=0.

In components

  • Take i^ and j^ in the plane and k^ perpendicular to it.
  • R⃗=Xi^+Yj^ and the moment is along k^, so the conditions become X=0,Y=0,∑M=0
  • For parallel forces (say vertical), the first reduces to one equation: the forces up equal the forces down.
  • The moment equation can be taken about any point; the best choice is a point where an unknown force acts, to eliminate it.

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