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1.5.1Motion, Forces and Energy

Moments — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

5 min read 8 parts Lesson 44 of 91 1 quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.5.1 Moments — IGCSE PHYSICS notes: Calculate the moment of a force about a pivot and explain how force and distance change the turning effect.A force can make an object turn about a fixed point as well as push or pull it.Pivot (fulcrum) = the fixed point about which the object …

  • What the symbols mean
  • Perpendicular distance
  • Increase the force
  • Increase the distance from the pivot
  • Net moment
  • Unit conversion

Part 1 of 6What a moment is

A force can make an object turn about a fixed point as well as push or pull it.
Pivot (fulcrum)
Pivot (fulcrum) the fixed point about which the object turns.
Moment
Moment the turning effect of a force about a pivot.
Examples:
  • A door turns about its hinges.
  • A spanner turns a nut about its axis.
  • A see-saw plank turns about its central support.
The turning effect depends on the size of the force and where it is applied.

Part 2 of 6The moment equation

Equation
  • Equation moment = force × perpendicular distance from the pivot to the line of action of the force (M = F × d).
    What the symbols mean: moment (N m) · F = force (N) · d = perpendicular distance from the pivot to the line of the force (m)
  • M is the moment in newton metres (N m).
  • F is the force in newtons (N).
  • d is the perpendicular distance in metres (m).
Perpendicular distance
  • Perpendicular distance it is measured at 90° to the direction of the force.
A force pointing along the line to the pivot has zero perpendicular distance and no turning effect.
Pushing a door at the hinge side, or along its edge, will not open it.

Part 3 of 6What changes the size of a moment

What changes the size of a moment
Increase the force
  • Increase the force doubling F doubles the moment if d is unchanged.
Increase the distance from the pivot
  • Increase the distance from the pivot doubling d doubles the moment if F is unchanged.
Door handles are placed far from the hinges for this reason.
A long spanner loosens a stubborn nut more easily than a short one.
A small force far from the pivot can produce the same moment as a large force close to it.

Part 4 of 6Clockwise and anticlockwise moments

Every moment turns the object either clockwise or anticlockwise.
Moments that turn the same way add together.
Moments that turn opposite ways partly or fully cancel.
Net moment
Net moment total clockwise moment − total anticlockwise moment.
  • If it is not zero, the object turns in the direction of the larger moment.
  • If it is zero, the turning effects are balanced.

Part 5 of 6Calculating a moment

1
Mark the pivot on a sketch.
2
Draw the line of action of the force and find the perpendicular distance from the pivot to it.
3
Convert distances to metres by dividing centimetres by 100.
4
Multiply F by d and state the unit N m and the direction of turning.
Unit conversion
  • 60 N cm is the same as 0.60 N m.
Convert kilonewtons to newtons before multiplying.

Part 6 of 6More from the specification

1Rotational equilibrium and balanced forces
  • Rotational equilibrium and balanced forces equal opposing turning moments exert balanced rotational forces across closed mechanical linkage loops.
2Gravity
  • A force of attraction between objects with mass.
3Lamina
  • A thin flat sheet.
4Equilibrium
  • State with zero net force.
5Resultant
  • Vector sum of two or more vectors.
6Topple
  • Tip over because the weight acts outside the base.
7Resultant moment
  • The net turning effect after clockwise and anticlockwise moments are combined.
8Pivot at the centre of mass
  • Pivot at the centre of mass if the pivot is elsewhere, the beam's own weight also produces a moment that must be included.

Key terms

TermMeaning
MomentThe turning effect of a force about a pivot; M = F × d.
PivotThe fixed point about which an object can turn.
Perpendicular distanceShortest distance from the pivot to the line of action of the force.
Newton metre (N m)The SI unit of moment.
Clockwise / anticlockwiseThe two possible directions of turning.
Line of actionThe straight line along which a force acts.

Avoid these mistakes

1
Measuring the distance along the lever when the force is at an angle — it must be perpendicular to the line of action.
2
Using centimetres in M = Fd and giving the answer in N m — convert to metres first.
3
Thinking a moment is a force — it is a turning effect with unit N m.
4
Mixing clockwise and anticlockwise moments — keep them separate before balancing.
5
Believing a bigger force always gives a bigger moment — a force through the pivot gives zero moment.

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