Which option best defines equilibrium in a physical system?

• A. The condition of a body when it is at rest or moving with constant velocity
• B. A body in accelerated motion
• C. A system with increasing energy
• D. Motion in a circular path only

Answer: (A)

Equilibrium means all forces on a system balance so there is no net force acting. When the net force is zero, acceleration is zero, so the object's motion doesn’t change: it can be at rest or move with constant velocity. That’s why the best description is a body that is at rest or moving with constant velocity. If motion is accelerated, there’s a net force at work, so the system isn’t in equilibrium. If energy is increasing, that usually comes from work done on the system, which also indicates an unbalanced influence. Even circular motion at constant speed isn’t equilibrium because maintaining it requires a continuous inward force, i.e., a net force. In everyday terms, a book resting on a table is in equilibrium, and a car cruising steadily on a straight road is also in equilibrium, with forward and resistive forces balancing out.