Revision Notes

1.5.6 Conditions For Equilibrium

Equilibrium Defined

  • The term “equilibrium” means that an object keeps doing what it’s doing, without any change
  • Therefore:
    • If the object is moving it will continue to move (in a straight line)
    • If it is stationary it will remain stationary
    • The object will also not start or stop turning
  • The above conditions require two things:
    • The forces on the object must be balanced (there must be no resultant force)
    • The sum of clockwise moments on the object must equal the sum of anticlockwise moments (the principle of moments)


Equilibrium, IGCSE & GCSE Physics revision notes

When the forces and moments on an object are balanced, the object will remain in equilibrium

  • If the above two conditions are met, then the object will be in equilibrium


Extended Only

Demonstrating Equilibrium

  • A simple experiment to demonstrate that there is no net moment on an object in equilibrium involves taking an object, such as a beam, and replacing the supports with newton (force) meters:

Demonstrating equilibrium, IGCSE & GCSE Physics revision notes

Several forces act on a supported beam, including the mass of the beam and the mass of an object suspended from it


  • The beam in the above diagram is in equilibrium
  • The various forces acting on the beam can be found either by taking readings from the newton meters or by measuring the masses (and hence calculating the weights) of the beam and the mass suspended from the beam
  • The distance of each force from the end of the ruler can then be measured, allowing the moment of each force about the end of the ruler to be calculated
  • It can then be shown that the sum of clockwise moments (due to forces F2 and F3) equal the sum of anticlockwise moments (due to forces F1 and F4)


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