All revision notes topics

MomentsEdexcel IGCSE Physics: Revision notes

Section 1

What is a moment?

A moment is the turning effect of a force about a pivot (also called a fulcrum). Anything that can rotate — a see-saw, a spanner, a door — is being acted on by a moment whenever a force is applied away from its pivot.

moment = force × perpendicular distance from the pivot

Moments are measured in newton-metres (N m). The distance used must always be the perpendicular distance from the line of action of the force to the pivot — not just any distance measured along the object.

Key termsmomentpivotperpendicular distance
Exam tip

Always state the formula and substitute values with units — examiners award a mark for correct substitution even if the final answer is wrong.

Example

A force of 20 N is applied 0.5 m from a pivot on a spanner. Moment = 20 × 0.5 = 10 N m.

Section 2

Centre of gravity

The centre of gravity of an object is the single point through which its entire weight can be considered to act. For a uniform, regularly-shaped object, the centre of gravity is at its geometric centre.

When calculating moments due to weight, always treat the weight as acting straight down through the centre of gravity, regardless of the object's actual shape.

Key termscentre of gravity
Common mistake

Students often forget that weight always acts vertically downwards through the centre of gravity, not along the object's surface.

Section 3

The principle of moments

For an object in equilibrium (not turning), the principle of moments states:

sum of clockwise moments = sum of anticlockwise moments

This applies to any system of parallel forces acting in one plane, such as a see-saw or a beam balanced on a pivot. To use it:

  1. Identify the pivot.
  2. Identify every force and its perpendicular distance from the pivot.
  3. Decide whether each moment is clockwise or anticlockwise.
  4. Set the sum of clockwise moments equal to the sum of anticlockwise moments and solve for the unknown.
Key termsprinciple of momentsequilibrium
Example

A 2 m see-saw is pivoted at its centre. A child weighing 400 N sits 1.0 m from the pivot. Where must a 500 N child sit to balance it? 400 × 1.0 = 500 × d, so d = 0.8 m from the pivot.

Section 4

Beams supported at both ends

A light (weightless) beam resting on two supports, with a heavy object placed somewhere along it, has an upward supporting force at each end. As the object moves along the beam:

  • The support closer to the object carries a larger share of the upward force.
  • The support further from the object carries a smaller share.
  • The two upward forces always add up to the total weight resting on the beam.

This can be solved by taking moments about one of the supports, which eliminates that support's own force from the equation (since its perpendicular distance from itself is zero).

Key termssupport force
Exam tip

Taking moments about one support point is the standard exam technique — it removes that support's unknown force from the calculation entirely.

Must Know

  • moment = force × perpendicular distance from the pivot (units: N m)
  • weight acts through the centre of gravity
  • principle of moments: sum of clockwise moments = sum of anticlockwise moments for a system in equilibrium
  • always use the perpendicular distance to the pivot, not the distance along the object
  • for a beam on two supports, the support nearer a load takes more of the upward force
  • take moments about one support to eliminate its unknown force from the calculation

That's the notes covered.

Carry on to the next subtopic.