Physics · Laws Of Motion · NEET
No. Equilibrium means net force = 0, so acceleration = 0. By Newton's first law, the particle either stays at rest OR moves with constant velocity in a straight line. Both are equilibrium. A skydiver falling at terminal velocity is in equilibrium even though moving fast, because weight balances air drag.
Concurrent forces are forces whose lines of action all pass through one common point (the same particle). Because they act at one point, we only need to make the vector sum zero for equilibrium; we do NOT worry about torque or rotation (that comes with extended bodies later).
Draw the free body diagram, choose x and y axes, and resolve every force into components. The particle is in equilibrium only if sum of x-components = 0 AND sum of y-components = 0 at the same time. If even one direction has a leftover force, there is acceleration and it is not equilibrium.
If three concurrent forces add to zero, placing them head-to-tail as vectors brings you back to the start point — a closed triangle. NCERT states this directly: three forces in equilibrium can be represented by the sides of a triangle with arrows taken in the same sense. For n forces it becomes a closed polygon.
The resultant is the single vector that replaces all the forces (their vector sum). The equilibrant is the single force equal in magnitude but opposite in direction to the resultant; adding it makes the total zero. In equilibrium the resultant is already zero, so no equilibrant is needed.
A particle moving with velocity V is acted upon by three forces represented by the sides of a triangle PQR (taken in order). The velocity of the particle will:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The vector sum of all forces acting on the particle must be zero. In two dimensions this means sum of Fx = 0 and sum of Fy = 0 together, giving zero acceleration.
Yes. Constant velocity means zero acceleration, which means net force = 0. That is exactly the equilibrium condition, so uniform motion in a straight line is an equilibrium state.
If three concurrent forces keep a particle in equilibrium, they can be drawn as the three sides of a triangle taken in the same sense (head-to-tail), because their vector sum is zero and the figure closes.
Two independent scalar equations: sum of horizontal components = 0 and sum of vertical components = 0. These let you solve for two unknown forces or angles.
Many Laws of Motion problems — strings, hanging masses, blocks on inclines, and connected bodies at rest — are solved by setting net force to zero. It is a fast, high-yield tool that avoids long calculations.
