4.1.4 Conditions for Equilibrium

• A system is in equilibrium when all the forces are balanced. This means:
  • There is no resultant force
  • There is no resultant torque

• An object in equilibrium will therefore remain at rest, or at a constant velocity, and not rotate
• The system is in an equilibrium state when applying the principle of moments (see The Principle of Moments)

• Coplanar forces can be represented by vector triangles
• Forces are in equilibrium if an object is either
  • At rest
  • Moving at constant velocity

• In equilibrium, coplanar forces are represented by closed vector triangles
  • The vectors, when joined together, form a closed path

• The most common forces on objects are
  • Weight
  • Normal reaction force
  • Tension (from cords and strings)
  • Friction

• The forces on a body in equilibrium are demonstrated below:

Three forces on an object in equilibrium form a closed vector triangle