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
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.
Section 1 of 5The Principle
When an object is balanced about a pivot, the clockwise and anticlockwise moments are equal.
Principle of moments
- Principle of moments For an object in equilibrium, the sum of clockwise moments about a pivot equals the sum of anticlockwise moments.
Equilibrium
- Equilibrium An object is in equilibrium when it is balanced and does not rotate or accelerate.
Clockwise moment
- Clockwise moment The total turning effect of forces that tend to rotate the object in a clockwise direction.
Anticlockwise moment
- Anticlockwise moment The total turning effect of forces that tend to rotate the object in an anticlockwise direction.
Resultant moment
- Resultant moment When the object is balanced, the resultant moment about the pivot is zero.
Section 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.
Section 3 of 5Uses of the Principle
Uses of the Principle
Seesaw
- Seesaw A seesaw balances when the product of weight and distance from the pivot matches on both sides.
Crane
- Crane A counterweight is placed on one side of a crane so that its moment balances the load on the other.
Beam balance
- Beam balance A beam balance compares an unknown mass with known masses placed at known distances.
Lever
- Lever A lever allows a small force to balance a larger load at a greater distance from the pivot.
Section 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.
- Clockwise moment = 300 N × 1.5 m = 450 N m.
- Anticlockwise moment = 500 N × d.
- Set the moments equal500 × d = 450.
- d = 450 ÷ 500 = 0.9 m.
Answer The adult must sit 0.9 m from the pivot.
Section 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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