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1.5Motion, Forces and Energy

Moments — Cambridge IGCSE Physics 0625 Revision Notes & Topical Questions

15 min read 23 parts Lesson 38 of 91 Unit quiz

📚 Cambridge IGCSE Physics 0625 Hub / Topical Revision
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Lesson summary

1.5 Moments (Part 1) — IGCSE PHYSICS notes: Calculate the moment of a force and use the principle of moments to find an unknown force or distance on a balanced object.Definition — the moment of a force is its turning effect about a pivot.A force can make an object rotate clockwis…

  • Everyday examples
  • Deciding the direction
  • Greater distance, smaller force
  • Door example
  • Spanner example
  • Several forces

Part 1 of 8Moment of a force

Definition
  • Definition the moment of a force is its turning effect about a pivot.
  • A force can make an object rotate clockwise or anticlockwise about a fixed pivot.
Everyday examples
  • Everyday examples a child on a see-saw, turning a spanner, opening a door, a crane lifting supplies, a screwdriver opening a paint tin, a tap, a wheelbarrow and scissors.
Deciding the direction
  • Deciding the direction imagine only that force acting on the object about the pivot.
  • If the object turns like the hands of a clock the moment is clockwise.

Part 2 of 8The moment equation

Equation
  • Equation moment = force × perpendicular distance from the pivot, or M = Fd.
  • The distance must be measured at right angles to the line of the force.
  • On a horizontal beam the forces that cause moments are the ones acting up or down.
Units
  • Units newton metres (N m), or N cm if the distance is in centimetres.
  • Convert distances to metres unless told otherwise.
  • Avoid N/m, which suggests division, and joules, which are never accepted for a moment.

Part 3 of 8Why distance matters

Why distance matters
Greater distance, smaller force
  • Greater distance, smaller force a larger distance from the pivot needs less force for the same moment.
Door example
  • Door example pushing next to the hinge needs a large force, while pushing at the handle needs much less.
Spanner example
  • Spanner example a longer spanner makes a stiff bolt easier to turn.

Part 4 of 8Principle of moments (core)

Statement
  • Statement for a balanced object the total clockwise moment about a pivot equals the total anticlockwise moment about that pivot.
Core use
  • Core use one force on each side of the pivot, so F1d1 = F2d2.

Part 5 of 8Principle of moments (extended)

Several forces
  • Several forces add all the moments on each side separately.
  • Sum of clockwise moments = sum of anticlockwise moments.
  • For example F2d2 = F1d1 + F3d3.
Weight of the beam
  • Weight of the beam a beam pivoted at its centre contributes no moment because its distance from the pivot is zero.
  • If the pivot is off-centre, the beam's own weight must be included.

Part 6 of 8Method for balance problems

1
Draw the beam, the pivot and every force with its distance.
2
Label each moment clockwise or anticlockwise.
3
Convert all distances to the same unit, preferably metres.
4
Write the principle: total clockwise = total anticlockwise.
5
Substitute Fd for each force and solve for the unknown.
6
Check the answer is sensible, for example the lighter person sits further out.

Part 7 of 8Moments in everyday devices

Lever
  • Lever a long handle gives a large moment from a small effort.
Wheelbarrow
  • Wheelbarrow the load is close to the wheel pivot, so a small lifting force at the handles balances it.
Crane
  • Crane a counterweight on the short arm balances the load on the long arm.
Door
  • Door the handle is placed far from the hinges to give a large moment.

Part 8 of 8More from the specification

More from the specification
Static equilibrium in structural loops
  • Static equilibrium in structural loops equating clockwise and anticlockwise turning moments prevents rotation and ensures static stability throughout closed structural loops.
Principle of Moments and Equilibrium
  • An object is in rotational equilibrium when there is no resultant turning effect,
  • meaning total clockwise moments equal total anticlockwise moments about any pivot (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)
Locating Center of Gravity of a Plane Lamina
  • In a laboratory experiment, an irregular plane lamina is suspended from a clamp;
    • vertical lines drawn using a plumb line intersect to locate its center of gravity.
Balancing Masses on a Meter Ruler
  • Suspending calibrated masses at various distances along a balanced meter ruler demonstrates how varying force and perpendicular distance maintains equilibrium.
Center of Gravity Position and Stability
  • Lowering the position of the center of gravity and broadening the base of an object increases physical stability against toppling.
Gravity
  • A force of attraction between objects with mass.
Lamina
  • A thin flat sheet.
Equilibrium
  • State with zero net force.
Position
  • Location relative to a reference point.
Resultant
  • Vector sum of two or more vectors.
Topple
  • Tip over because the weight acts outside the base.

Key terms

TermMeaning
MomentThe turning effect of a force about a pivot, force × perpendicular distance.
PivotThe fixed point about which an object turns.
Principle of momentsFor a balanced object, total clockwise moment equals total anticlockwise moment.
ClockwiseTurning in the direction of a clock's hands.
AnticlockwiseTurning against the direction of a clock's hands.
Newton metreThe unit of moment.

Avoid these mistakes

1
Measuring the distance along the force instead of perpendicular to it.
2
Leaving distances in centimetres in one moment and metres in another.
3
Stating the principle as "the moments are equal" with no clockwise or anticlockwise.
4
Giving the unit of a moment as N/m or J — use N m.

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