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1.4.2States of Matter

The Principle of Moments — Cambridge IGCSE Chemistry 0620 Revision Notes & Topical Questions

5 min read 5 parts Lesson 29 of 30 Unit quiz

📚 Cambridge IGCSE Chemistry 0620 Hub / Topical Revision
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Part 1 of 5The Principle

1.4.2 The Principle of Moments

When an object is balanced about a pivot, the clockwise and anticlockwise moments are equal.

Principle of moments
  • For an object in equilibrium, the sum of clockwise moments about a pivot equals the sum of anticlockwise moments.
Equilibrium
  • An object is in equilibrium when it is balanced and does not rotate or accelerate.
Clockwise moment
  • The total turning effect of forces that tend to rotate the object in a clockwise direction.
Anticlockwise moment
  • The total turning effect of forces that tend to rotate the object in an anticlockwise direction.
Resultant moment
  • When the object is balanced, the resultant moment about the pivot is zero.

Part 2 of 5Solving a Balance Problem

A clear list of forces and distances avoids missing a moment.

1
Draw a diagram and label the pivot, every force and each perpendicular distance.
2
Write down the clockwise moments and the anticlockwise moments separately.
3
Set the sum of the clockwise moments equal to the sum of the anticlockwise moments.
4
Rearrange the equation to find the unknown force or distance, and check the units.

Part 3 of 5Uses of the Principle

Uses of the Principle
Seesaw
  • A seesaw balances when the product of weight and distance from the pivot matches on both sides.
Crane
  • A counterweight is placed on one side of a crane so that its moment balances the load on the other.
Beam balance
  • A beam balance compares an unknown mass with known masses placed at known distances.
Lever
  • A lever allows a small force to balance a larger load at a greater distance from the pivot.

Part 4 of 5Worked Example

Question

  • A child weighing 300 N sits 1.5 m from a pivot on one side of a balanced plank.
  • An adult weighing 500 N sits on the other side.
  • Calculate the distance of the adult from the pivot.

  1. Clockwise moment = 300 N × 1.5 m = 450 N m.
  2. Anticlockwise moment = 500 N × d.
  3. Set the moments equal500 × d = 450.
  4. d = 450 ÷ 500 = 0.9 m.

Answer The adult must sit 0.9 m from the pivot.

Part 5 of 5Exam Checklist & Tips

1.4.2 – Key Points for the Exam

  • Principle of moments In equilibrium, total clockwise moment about a pivot equals total anticlockwise moment.
  • Equilibrium A state in which all forces and moments on an object balance.
  • Pivot The fixed point about which the balance of moments is taken.
  • Weight The gravitational force on an object, which acts vertically downwards.

Common Mistakes

  • Using mass instead of weight in the moment calculation; the force in the moment must be in newtons.
  • Leaving out one of the forces or moments, which gives an incorrect balance equation.
  • Measuring the distance along the beam rather than the perpendicular distance from the pivot.
  • Assuming there is no moment when an object is balanced; the moments are equal and opposite.

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