Equilibrium (OCR A Level Physics)

Revision Note

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Katie M

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Katie M

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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

Worked example

Four beams of the same length each have three forces acting on them.

Which beam has both zero resultant force and zero resultant torque acting?

Conditions for Equilibrium

  • 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:

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

A weight hangs in equilibrium from a cable at point X. The tensions in the cables are T1 and T2 as shown.WE - Forces in equilibrium question image 1, downloadable AS & A Level Physics revision notesWhich diagram correctly represents the forces acting at point X?WE - Forces in equilibrium question image 2, downloadable AS & A Level Physics revision notes

Equilibrium Worked Example (3), downloadable AS & A Level Physics revision notes

Examiner 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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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.