CIE AS Physics (9702) exams from 2022

Revision Notes

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)

Worked example – Beam in equilibrium

Worked example - beam in equilibrium, downloadable AS & A Level Physics revision notes

Coplanar Forces in Equilibrium

  • Coplanar forces can be represented by vector triangles
  • In equilibrium, these are 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:

Vector triangle in equilibrium, downloadable AS & A Level Physics revision notes

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


Worked example – Forces in equilibrium

Worked example - forces in equilibrium, downloadable AS & A Level Physics revision notes

Exam Tip

The diagrams in exam questions about this topic tend to be drawn to scale, so make sure you have a ruler handy!

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