describe equilibrium of particle?

A particle is in equilibrium if the vector sum of the external forces acting on it is zero. Hence a particle is in equilibrium if:

1. It is at rest and remains at rest − Static Equilibrium
2. It moves with constant velocity − Dynamic Equilibrium

If there are only two forces acting on a particle that is in equilibrium, then the two forces must be equal (in magnitude) and opposite in direction to each other. If three forces act on a particle that is in equilibrium, then when the three forces are placed end to end they must form a triangle.