Moments (OCR A Level Physics)

What is a Moment?

• A moment is the turning effect of a force

• Moments occur when forces cause objects to rotate about some pivot

• The moment of a force is given by

               Moment (N m) = Force (N) × perpendicular distance from the pivot (m)

• The SI unit for the moment is Newton metres (N m). This may also be Newton centimetres (N cm) depending on the units given for the distance

The force might not always be perpendicular to the distance

• An example of moments in everyday life is opening a door

• The door handle is placed on the other side of the door to the hinge (the pivot) to maximise the distance for a given force and therefore provide a greater moment (turning force)

  • This makes it easier to push or pull it

The Principle of Moments

• The principle of moments states:

  For a system to be in equilibrium, the sum of clockwise moments about a point must be equal to the sum of the anticlockwise moments (about the same point)

Diagram showing the moments acting on a balanced beam

• In the above diagram:

  • Force F₂ is supplying a clockwise moment;

  • Forces F₁ and F₃ are supplying anticlockwise moments

• Hence: F₂ × d₂ = (F₁ × d₁) + (F₃ × d₃)