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If a particle is subjected to a system of coplanar and concurrent forces in X-Y plane, the vector equation must satisfy :

  • A.\(\Sigma f_{x} = 0\) and \(\Sigma f_{y} \neq 0\)
  • B.\(\Sigma f_{x} = 0\)
  • C.\(\Sigma f_{y} = 0\)
  • D.Both \(\Sigma f_{x} = 0\) and \(\Sigma f_{y} = 0\)

Answer: D

The particle must satisfy both \(\Sigma f_{x} = 0\) and \(\Sigma f_{y} = 0\).

For a particle in equilibrium under coplanar concurrent forces, the vector sum of the forces must be zero, \(\Sigma \vec{F} = 0\). In the X-Y plane that one vector equation splits into two scalar equations, one along each axis.

If only \(\Sigma f_{x} = 0\) held, a resultant along Y would remain, and the particle would accelerate in that direction; the same is true the other way round. The first option, with \(\Sigma f_{y} \neq 0\), describes a particle that is not in equilibrium at all. No moment equation is needed, because concurrent forces all pass through one point.

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