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1.8.1States of Matter

Moments — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 12 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Section 1 of 5The Turning Effect

A force can make an object rotate, and the size of this turning effect is called the moment.

Moment
  • Moment The turning effect of a force about a pivot, equal to the force multiplied by the perpendicular distance from the pivot.
Pivot
  • Pivot A fixed point about which an object rotates, such as the hinge of a door or the bolt turned by a spanner.
Clockwise and anticlockwise
  • Clockwise and anticlockwise The two directions of rotation;
    • a moment is described by the direction in which it would turn the object about the pivot.
Perpendicular distance
  • Perpendicular distance The shortest distance from the pivot to the line of action of the force, which is the distance used in the moment equation.

Section 2 of 5Calculating a Moment

Converting every quantity into SI units before multiplying avoids most mistakes.

1
Identify the force in newtons and the perpendicular distance from the pivot in metres.
2
Convert any distance given in centimetres into metres by dividing by 100.
3
Multiply the force by the perpendicular distance to obtain the moment, using M = F × d.
4
State whether the moment is clockwise or anticlockwise, based on the direction in which the force turns the object.

Section 3 of 5Moments in Everyday Life

Moments in Everyday Life
Door
  • Door Pushing close to the hinge needs a larger force, because the perpendicular distance from the pivot is small.
Spanner
  • Spanner A longer spanner gives a larger moment for the same force, which makes a rusted bolt easier to turn.
Force direction
  • Force direction Only the part of a force at right angles to the lever arm produces a moment about the pivot.
Distance and force
  • Distance and force Moments depend on both quantities, so a larger force does not always give a larger moment.

Section 4 of 5Worked Example

Question

  • A spanner of length 0.25 m is used to turn a nut.
  • A force of 40 N is applied perpendicular to the spanner at its far end.
  • Calculate the moment of the force about the nut.

  1. Identify the valuesF = 40 N and the perpendicular distance d = 0.25 m.
  2. Write the equation M = F × d.
  3. SubstituteM = 40 × 0.25 = 10 N m.

Answer The moment is 10 N m.

Section 5 of 5Exam Checklist & Tips

1.8.1 – Key Points for the Exam

  • Moment The turning effect of a force about a pivot, in N m.
  • Pivot The fixed point about which an object turns.
  • Perpendicular distance The shortest distance between the pivot and the line of action of the force.
  • Newton metre (N m) The unit of moment, equal to a force of 1 N acting at a perpendicular distance of 1 m.
  • Turning effect The tendency of a force to rotate an object about a pivot.

Common Mistakes

  • Using the total length of the spanner when the force is applied at a different point;
    • the perpendicular distance from the pivot must be used.
  • Leaving distances in centimetres, which gives a moment that is 100 times too large in N m.
  • Forgetting to state whether the moment is clockwise or anticlockwise.
  • Assuming a larger force always gives a larger moment; the distance from the pivot matters too.

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