Equilibrium in Physics

Forces and Moments

Forces produce changes in translational motion, while the moment of force produces changes in rotational motion. Equilibrium is the state where the net force and net torque on an object are zero.

Conditions of Equilibrium

First Condition of Equilibrium

  • The vector sum of all forces acting on the body is zero.
  • The force polygon must close.

Second Condition of Equilibrium

  • The vector sum of all torques acting on the body is zero.
  • Also called the Principle of Moments.
  • The sum of clockwise and anti-clockwise moments about a pivot must be zero.

Principle of Moments

The sum of the clockwise moments about a pivot must be equal to the sum of the anti-clockwise moments. The perpendicular distance from the axis of rotation plays an important role. Anti-clockwise torques are positive, while clockwise torques are negative.

Types of Equilibrium

Static Equilibrium

A body is at rest under the action of several forces and torques.
Example: A book resting on a table.

Dynamic Equilibrium

A body is moving at uniform velocity under the action of several forces. Dynamic equilibrium is further divided into:

  • Dynamic Translational Equilibrium: Body moving with uniform linear velocity.
  • Dynamic Rotational Equilibrium: Body moving with uniform rotation.