Forces: Moments

Forces: Moments

Definition of a Moment:

  • A moment is the turning effect of a force.
  • The size of a moment is equal to the force causing it times the distance from the force to the turning point (the pivot), and is measured in newton metres (Nm).

Formula for Moments:

  • The formula for calculating a moment is: Moment (Nm) = Force (N) x Distance to the pivot (m).
  • The larger the force or the distance to the pivot, the greater the moment.

Principle of Moments:

  • The Principle of Moments states that for any object in equilibrium (not turning or rotating), the total sum of the clockwise moments must be equal to the total sum of the anti-clockwise moments.
  • In other words, when an object is balanced, the total turning effect in both directions will be the same.


  • A lever is a simple machine that uses the principle of moments to make work easier.
  • The pivoting point of a lever is called the fulcrum.
  • Levers work by reducing the amount of force needed to create a moment. By increasing the distance from the force to the pivot, less force is required to generate the same moment.

Centre of Mass:

  • The centre of mass is the single point in an object where its mass can be thought of as being concentrated.
  • If an object is freely suspended, it will come to rest with its centre of mass directly beneath the point of suspension. This is because this is the position of minimal potential energy.


  • An object is stable if, when displaced, it returns to its original position. It is unstable if it does not return to its original position. It is neutrally stable if it stays in its new position.
  • The lower the centre of mass of an object, the more stable it tends to be. This is because a lower centre of mass often means a larger moment causing a restoring effect.
  • Additionally, an object is more stable if its line of action (the vertical line through the centre of mass) falls within its base. If the line of action falls outside its base, the object will topple.