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1.8.2States of Matter

Law of Moments and Stability — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 13 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
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Part 1 of 5The Principle of Moments

An object that is not rotating has balanced moments about any chosen pivot.

Principle of moments
  • Principle of moments For an object that is not turning, the total clockwise moment about any pivot equals the total anticlockwise moment about the same pivot.
Balanced object
  • Balanced object A balanced object has a zero resultant moment, so it does not rotate about the pivot.
Choice of pivot
  • Choice of pivot The principle holds for any point chosen as the pivot, so the pivot can be chosen to remove an unknown force from the working.
Weight of a beam
  • Weight of a beam A uniform beam's own weight acts at its centre, so this weight must be included in any moment calculation.

Part 2 of 5Solving Balance Problems

A systematic method makes it clear which moments act in which direction.

1
Identify the forces that cause clockwise moments about the pivot, and those that cause anticlockwise moments.
2
Calculate each moment as M = F × d, converting every distance into metres first.
3
Write the principle of moments: the sum of the clockwise moments equals the sum of the anticlockwise moments.
4
Rearrange the equation to find the unknown force or distance, and check that the answer is physically sensible.

Part 3 of 5Stability and Moments

Stability and Moments
Toppling moment
  • Toppling moment Whether an object topples depends on the moment of its weight about the edge of its base.
Wide base
  • Wide base A wider base moves the edge further from the centre of mass, so a larger tilt is needed before toppling.
Car and bus
  • Car and bus A car can tilt further than a bus, because its centre of mass is lower and its base is wider.
Changing loads
  • Changing loads Moving a load outwards or adding mass beyond the base changes the moments and can make an object topple.

Part 4 of 5Worked Example

Question

  • A child of weight 300 N sits 2.0 m from the pivot of a see-saw on one side.
  • A parent of weight 600 N sits on the other side.
  • Calculate the distance of the parent from the pivot for the see-saw to balance.

  1. Clockwise moment from the childM = 300 × 2.0 = 600 N m.
  2. Anticlockwise moment from the parentM = 600 × d.
  3. Principle of moments600 × d = 600.
  4. d = 600 ÷ 600 = 1.0 m.

Answer The parent must sit 1.0 m from the pivot.

Part 5 of 5Exam Checklist & Tips

1.8.2 – Key Points for the Exam

  • Principle of moments For an object in balance, the total clockwise moment equals the total anticlockwise moment about any pivot.
  • Clockwise moment A moment that turns the object in the clockwise direction.
  • Anticlockwise moment A moment that turns the object in the anticlockwise direction.
  • Pivot The fixed point about which the object can turn.
  • Balance A state in which the resultant moment about a pivot is zero.

Common Mistakes

  • Comparing a force with a moment without multiplying the force by its distance from the pivot.
  • Mixing units, such as using centimetres for one distance and metres for another.
  • Omitting the weight of a uniform beam when calculating its moments.
  • Assigning the wrong direction to a moment; check which way the force would turn the object.

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