Equilibrium — Definition

Definition

Imagine an object that is perfectly balanced. It's not speeding up, slowing down, or changing its direction of motion. It's also not spinning faster, slower, or changing its axis of rotation. This state of perfect balance, both in terms of its linear motion and its rotational motion, is what we call 'equilibrium' in physics.

\n\nTo understand equilibrium, we first need to recall Newton's First Law of Motion, which states that an object at rest stays at rest, and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.

Equilibrium is essentially the condition where this 'unbalanced force' (or net force) is precisely zero. If the net force on an object is zero, its acceleration is zero. This means its velocity is constant.

If the constant velocity is zero, the object is at rest, and we call this 'static equilibrium'. Think of a book lying motionless on a table – it's in static equilibrium because the gravitational force pulling it down is perfectly balanced by the normal force from the table pushing it up.

The net force is zero.\n\nHowever, equilibrium isn't just about being at rest. An object can also be in equilibrium if it's moving at a constant velocity. This is called 'dynamic equilibrium'. Imagine a car cruising on a straight highway at a steady 60 km/h.

The forward thrust from its engine is balanced by air resistance and friction, so the net force is zero, and its velocity remains constant. It's not accelerating. Even though it's moving, it's in equilibrium.

\n\nBeyond linear motion, objects can also rotate. For an object to be in complete equilibrium, it must also not be changing its rotational motion. This means the net 'torque' acting on it must be zero.

Torque is essentially the rotational equivalent of force; it's what causes an object to rotate or change its rotational speed. If all the torques trying to rotate an object clockwise are perfectly balanced by all the torques trying to rotate it counter-clockwise, then the net torque is zero.

This condition ensures 'rotational equilibrium'. A seesaw perfectly balanced with children of equal weight at equal distances from the pivot is an example of rotational equilibrium.\n\nSo, in summary, for an object to be in complete equilibrium, two conditions must be met simultaneously: \n1.

The net force acting on it must be zero (translational equilibrium). \n2. The net torque acting on it must be zero (rotational equilibrium). \n\nThese two conditions ensure that the object's linear velocity and angular velocity remain constant, whether that constant velocity is zero or some non-zero value.