IGCSE Physics Mass and Weight: Complete Guide for Cambridge IGCSE Physics (0625)

Mass and weight are fundamental concepts in Cambridge IGCSE Physics (0625) that students often confuse. Understanding the difference between mass and weight, mastering the weight formula, and learning about gravitational field strength are essential skills for achieving top grades in your IGCSE Physics exams.

This comprehensive IGCSE Physics Mass and Weight guide covers everything you need to know, including the key differences between mass and weight, gravitational field strength, the weight formula (W = mg), measurement instruments, step-by-step worked examples, common exam questions, and expert tips from Tutopiya’s IGCSE Physics tutors. We’ll also show you how to avoid the most common mistakes that cost students valuable marks.

🎯 What you’ll learn: By the end of this guide, you’ll know how to distinguish between mass and weight, calculate weight using the formula, understand gravitational field strength, and apply these skills confidently in IGCSE Physics exams.

Already studying with Tutopiya? Practice these skills with our dedicated IGCSE Physics Mass and Weight practice deck featuring exam-style questions, videos, and instant feedback.

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Why Mass and Weight Matter in IGCSE Physics

Mass and weight are foundational concepts that appear throughout the IGCSE Physics syllabus. Here’s why they’re so important:

• High exam frequency: Mass and weight questions appear in almost every IGCSE Physics paper
• Exam weight: Typically worth 4-8 marks per paper, making them crucial for grade boundaries
• Foundation for advanced topics: Essential for understanding forces, energy, momentum, and pressure
• Real-world applications: Used in engineering, space science, sports, and everyday life
• Common confusion: Students often lose marks by confusing mass and weight
• Practical connections: Links theory to practical experiments and measurements

Key insight from examiners: Students often lose marks not because they don’t understand the concepts, but because they confuse mass with weight or forget to include units. This guide will help you avoid these pitfalls.

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Mass: The Amount of Matter

Mass is a measure of the amount of matter in an object. It’s a fundamental property that doesn’t change with location.

Key Characteristics of Mass

Mass is a scalar quantity:

• Has magnitude (size) only
• No direction
• Always positive (or zero)

Mass is constant:

• The same everywhere in the universe
• Doesn’t change with location
• A 10 kg object on Earth has the same 10 kg mass on the Moon, Mars, or in space

Units of mass:

• SI unit: kilogram (kg)
• Other common units: grams (g), tonnes (t)
• 1 kg = 1000 g
• 1 tonne = 1000 kg

Example: A textbook has a mass of 2.5 kg. This mass is the same whether the book is on Earth, the Moon, or floating in space.

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Weight: The Force of Gravity

Weight is the force exerted on a mass due to gravity. It’s the pull of gravity acting on an object.

Key Characteristics of Weight

Weight is a vector quantity:

• Has both magnitude and direction
• Direction is always downward (toward the center of the Earth or planet)
• Can be measured in newtons (N)

Weight depends on location:

• Changes with gravitational field strength
• Different on different planets
• Different at different altitudes (slightly)

Units of weight:

• SI unit: newton (N)
• Named after Sir Isaac Newton
• 1 N = 1 kg m/s²

Example: A 10 kg object weighs approximately 98 N on Earth, but only 16 N on the Moon because the Moon’s gravity is weaker.

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Mass vs Weight: The Complete Comparison

Aspect	Mass	Weight
Definition	Amount of matter in an object	Force exerted on mass due to gravity
Type	Scalar quantity	Vector quantity
SI Unit	Kilogram (kg)	Newton (N)
Changes with location?	No - constant everywhere	Yes - depends on gravitational field strength
Measurement tool	Balance or scale	Force meter (newton meter) or spring balance
Symbol	m	W
Formula	Mass is measured directly	W = m × g

Common Confusion to Avoid

❌ Wrong: “My mass is 60 kg, so my weight is 60 kg”
✅ Correct: “My mass is 60 kg, so my weight is approximately 588 N (on Earth)”

❌ Wrong: “Mass and weight are the same thing”
✅ Correct: “Mass is the amount of matter; weight is the force of gravity acting on that mass”

❌ Wrong: “An object has no weight in space”
✅ Correct: “An object has no weight in space (microgravity), but its mass remains the same”

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Gravitational Field Strength (g)

Gravitational field strength (symbol: g) is the force per unit mass exerted by a gravitational field. It tells us how strong gravity is at a particular location.

Understanding Gravitational Field Strength

Formula:

g = Weight ÷ Mass

Or rearranged:

g = W ÷ m

Units: Gravitational field strength is measured in newtons per kilogram (N/kg)

Key values:

• On Earth: g ≈ 9.8 N/kg (often rounded to 10 N/kg for calculations)
• On the Moon: g ≈ 1.6 N/kg
• On Mars: g ≈ 3.7 N/kg
• On Jupiter: g ≈ 24.8 N/kg (much stronger!)

What it means:

• On Earth, every kilogram of mass experiences a force of 9.8 N due to gravity
• A stronger gravitational field means objects weigh more
• A weaker gravitational field means objects weigh less

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The Weight Formula: W = mg

The relationship between weight, mass, and gravitational field strength is given by:

Weight Formula:

W = m × g

Where:

• W = weight (in newtons, N)
• m = mass (in kilograms, kg)
• g = gravitational field strength (in N/kg)

Rearranging the Formula

You can rearrange this formula to find any of the three quantities:

To find weight:

W = m × g

To find mass:

m = W ÷ g

To find gravitational field strength:

g = W ÷ m

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Worked Example 1: Calculating Weight on Earth

A student has a mass of 50 kg. Calculate their weight on Earth. (Take g = 9.8 N/kg)

Solution:

1. Identify known values:

   • Mass (m) = 50 kg
   • Gravitational field strength (g) = 9.8 N/kg

2. Use the formula:

   • W = m × g
   • W = 50 × 9.8
   • W = 490 N

3. State answer: The student’s weight on Earth is 490 N.

Examiner tip: Always include units in your final answer. Weight must be in newtons (N), not kilograms!

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Worked Example 2: Weight on Different Planets

An object has a mass of 20 kg. Calculate its weight: a) On Earth (g = 9.8 N/kg) b) On the Moon (g = 1.6 N/kg) c) On Mars (g = 3.7 N/kg)

Solution:

a) Weight on Earth:

• W = m × g
• W = 20 × 9.8 = 196 N

b) Weight on the Moon:

• W = m × g
• W = 20 × 1.6 = 32 N

c) Weight on Mars:

• W = m × g
• W = 20 × 3.7 = 74 N

Key observation: The mass is the same (20 kg) everywhere, but the weight changes because gravitational field strength is different on each planet.

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Worked Example 3: Finding Mass from Weight

An object weighs 245 N on Earth. Calculate its mass. (Take g = 9.8 N/kg)

Solution:

1. Identify known values:

   • Weight (W) = 245 N
   • Gravitational field strength (g) = 9.8 N/kg

2. Rearrange the formula:

   • W = m × g
   • m = W ÷ g

3. Calculate:

   • m = 245 ÷ 9.8
   • m = 25 kg

Answer: The object’s mass is 25 kg.

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Worked Example 4: Comparing Weight on Earth and Moon

Isabelle has a mass of 60 kg. Calculate her weight on Earth and on the Moon. Take the Earth’s gravitational field strength as 9.8 N/kg, and the Moon’s gravitational field strength as 1.6 N/kg.

Solution:

Step 1: List the known values

• Mass (m) = 60 kg
• Gravitational field strength on Earth (g_Earth) = 9.8 N/kg
• Gravitational field strength on Moon (g_Moon) = 1.6 N/kg

Step 2: Calculate weight on Earth

• W_Earth = m × g_Earth
• W_Earth = 60 × 9.8 = 588 N

Step 3: Calculate weight on Moon

• W_Moon = m × g_Moon
• W_Moon = 60 × 1.6 = 96 N

Step 4: Compare the two values

• Weight on Earth = 588 N
• Weight on Moon = 96 N
• Weight on Earth is greater than weight on Moon
• This is because Earth has a larger gravitational field strength than the Moon

Answer:

• Weight on Earth: 588 N
• Weight on Moon: 96 N
• The weight force is greater on Earth because the Earth has a larger gravitational field strength.

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Measuring Mass: Using a Balance

Balances are used to measure mass. They compare the mass of an object with standard masses.

Types of Balances

Digital balance:

• Modern, electronic balance
• Displays mass directly in kg or g
• Very precise and easy to read

Analogue balance (beam balance):

• Traditional balance with pans
• Uses standard masses for comparison
• More time-consuming but still accurate

Key points about balances:

• Measure mass, not weight
• Work by comparing masses
• Give the same reading regardless of location (Earth, Moon, etc.)
• Because gravitational field strength is constant on Earth, balances effectively measure mass

Example: A digital balance shows 2.5 kg. This is the mass of the object, and it will show 2.5 kg whether you’re on Earth, the Moon, or Mars.

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Measuring Weight: Using a Force Meter

Force meters (also called newton meters or spring balances) are used to measure weight (force).

How Force Meters Work

Components:

• Spring that stretches when force is applied
• Hook to hang the object
• Scale calibrated in newtons (N)

How to use:

1. Hang the object from the hook
2. The spring stretches due to the weight force
3. Read the weight from the scale in newtons

Key points about force meters:

• Measure weight (force), not mass
• Reading changes with location (different on different planets)
• Calibrated in newtons (N)
• The spring stretches more when weight is greater

Example: A force meter shows 49 N. This is the weight of the object. On the Moon, the same object would show approximately 8 N because the Moon’s gravity is weaker.

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Balance vs Force Meter: When to Use Which

Instrument	Measures	Unit	Changes with Location?	Use When
Balance	Mass	kg or g	No	You need to know the amount of matter
Force Meter	Weight	N	Yes	You need to know the force of gravity

Common exam question: “State which instrument measures mass and which measures weight.”

Answer:

• Balance measures mass (in kg)
• Force meter measures weight (in N)

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Common Student Misconceptions and Errors

Misconception 1: Mass and Weight Are the Same

❌ Wrong: “Mass and weight mean the same thing”

✅ Correct: Mass is the amount of matter (scalar, constant), while weight is the force of gravity (vector, changes with location).

Why students get confused: In everyday language, we often say “I weigh 60 kg” when we mean “I have a mass of 60 kg.” In physics, we must be precise.

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Misconception 2: Weight Is Measured in Kilograms

❌ Wrong: “My weight is 60 kg”

✅ Correct: “My mass is 60 kg, so my weight is approximately 588 N”

Common exam error: Students write weight in kilograms instead of newtons. This loses marks!

Examiner tip: Always check your units. Weight must be in newtons (N), mass in kilograms (kg).

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Misconception 3: Mass Changes with Location

❌ Wrong: “An object has less mass on the Moon”

✅ Correct: Mass is constant everywhere. An object has the same mass on Earth, Moon, or in space. Only weight changes.

Why students get confused: They see that objects weigh less on the Moon and think the mass has changed too.

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Misconception 4: Objects Have No Weight in Space

❌ Wrong: “Astronauts have no weight in space because there’s no gravity”

✅ Correct: Astronauts experience microgravity (very weak gravity), not zero gravity. Their mass remains the same, but their weight is very small (almost zero) due to the weak gravitational field.

Clarification: There is still gravity in space (from Earth, Moon, etc.), but it’s very weak, so weight is nearly zero.

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Common Exam Errors to Avoid

Error 1: Using Wrong Units

❌ Wrong: “The weight is 50 kg”
✅ Correct: “The weight is 490 N” (for a 50 kg object on Earth)

Examiner tip: Weight must always be in newtons (N). If you get an answer in kg, you’ve calculated mass, not weight!

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Error 2: Confusing Mass and Weight in Calculations

❌ Wrong: Using mass value where weight is needed (or vice versa)
✅ Correct: Always identify what you’re asked to find and use the correct formula

Example: If asked for weight, use W = m × g, not just the mass value.

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Error 3: Forgetting to State the Location

❌ Wrong: “The weight is 98 N” (without saying where)
✅ Correct: “The weight on Earth is 98 N”

Examiner tip: When comparing weights on different planets, always state which planet you’re referring to.

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Error 4: Using Wrong Value of g

❌ Wrong: Using g = 10 N/kg when the question specifies g = 9.8 N/kg
✅ Correct: Always use the value of g given in the question

Common mistake: Students memorize g = 10 N/kg but the question asks for g = 9.8 N/kg.

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Error 5: Not Rearranging the Formula Correctly

❌ Wrong: Trying to find mass using W = m × g without rearranging
✅ Correct: Rearrange to m = W ÷ g before substituting values

Examiner tip: Show your working. Even if you make a calculation error, you’ll get method marks for the correct formula.

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IGCSE Physics Practice Questions: Mass and Weight

Test your understanding with these exam-style questions:

Question 1: Basic Weight Calculation

A box has a mass of 15 kg. Calculate its weight on Earth. (Take g = 9.8 N/kg)

Solution:

1. Identify known values:

   • Mass (m) = 15 kg
   • Gravitational field strength (g) = 9.8 N/kg

2. Use the formula:

   • W = m × g
   • W = 15 × 9.8 = 147 N

Answer: The box’s weight on Earth is 147 N.

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Question 2: Weight on Different Planets

An astronaut has a mass of 70 kg. Calculate their weight: a) On Earth (g = 9.8 N/kg) b) On the Moon (g = 1.6 N/kg) c) Explain why the weight is different

Solution:

a) Weight on Earth:

• W = m × g
• W = 70 × 9.8 = 686 N

b) Weight on Moon:

• W = m × g
• W = 70 × 1.6 = 112 N

c) Explanation:

• The mass is the same (70 kg) on both Earth and Moon
• Weight is different because gravitational field strength is different
• Earth has stronger gravity (9.8 N/kg) than the Moon (1.6 N/kg)
• Therefore, the weight force is greater on Earth

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Question 3: Finding Mass from Weight

An object weighs 392 N on Earth. Calculate its mass. (Take g = 9.8 N/kg)

Solution:

1. Identify known values:

   • Weight (W) = 392 N
   • Gravitational field strength (g) = 9.8 N/kg

2. Rearrange the formula:

   • W = m × g
   • m = W ÷ g

3. Calculate:

   • m = 392 ÷ 9.8 = 40 kg

Answer: The object’s mass is 40 kg.

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Question 4: Comparing Mass and Weight

A student says: “My mass is 55 kg, so my weight is 55 kg.” Explain why this statement is incorrect.

Solution:

The statement is incorrect because:

1. Mass and weight are different:

   • Mass is the amount of matter (measured in kg)
   • Weight is the force of gravity (measured in N)

2. Wrong units:

   • Weight cannot be measured in kilograms
   • Weight must be measured in newtons (N)

3. Correct statement:

   • “My mass is 55 kg, so my weight on Earth is approximately 539 N” (using g = 9.8 N/kg)

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Question 5: Instrument Selection

State which instrument you would use to measure: a) The mass of a book b) The weight of a book

Explain your choices.

Solution:

a) To measure mass: Use a balance

• Balances measure mass in kilograms
• They compare the object’s mass with standard masses
• The reading is in kg or g

b) To measure weight: Use a force meter (newton meter)

• Force meters measure weight (force) in newtons
• They work by measuring how much a spring stretches
• The reading is in N

Explanation:

• Mass and weight are different quantities
• Mass is measured with a balance (kg)
• Weight is measured with a force meter (N)

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Key Formulas Summary

Weight Formula

• Weight: W = m × g
• Where: W = weight (N), m = mass (kg), g = gravitational field strength (N/kg)

Rearranged Formulas

• Mass: m = W ÷ g
• Gravitational field strength: g = W ÷ m

Key Values

• Earth: g ≈ 9.8 N/kg (often rounded to 10 N/kg)
• Moon: g ≈ 1.6 N/kg
• Mars: g ≈ 3.7 N/kg

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Measurement Instruments Summary

Instrument	Measures	Unit	Principle
Balance	Mass	kg or g	Compares with standard masses
Force Meter	Weight	N	Measures spring extension due to weight force

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Examiner Tips for Mass and Weight Questions

1. Always include units: Weight in N, mass in kg

2. Use the correct formula: W = m × g (not m = W × g!)

3. Check the value of g: Use the value given in the question (usually 9.8 N/kg or 10 N/kg)

4. State the location: When comparing weights, always say “on Earth” or “on the Moon”

5. Distinguish mass and weight: Remember - mass is constant, weight changes with location

6. Show your working: Even if you make a calculation error, method marks are awarded for correct formulas

7. Read the question carefully: Make sure you know whether you’re being asked for mass or weight

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Frequently Asked Questions About IGCSE Physics Mass and Weight

What is the difference between mass and weight?

Mass is the amount of matter in an object (scalar, constant, measured in kg). Weight is the force of gravity acting on that mass (vector, changes with location, measured in N). Mass stays the same everywhere; weight changes depending on the gravitational field strength.

How do I calculate weight from mass?

Use the formula: W = m × g

• W = weight (N)
• m = mass (kg)
• g = gravitational field strength (N/kg)

On Earth, g ≈ 9.8 N/kg, so a 10 kg object weighs: 10 × 9.8 = 98 N

Why does weight change but mass doesn’t?

Mass is the amount of matter - this never changes. Weight is the force of gravity, which depends on the gravitational field strength (g). Since g is different on different planets, weight changes even though mass stays the same.

What is gravitational field strength?

Gravitational field strength (g) is the force per unit mass. It tells us how strong gravity is at a location. On Earth, g ≈ 9.8 N/kg, meaning every kilogram experiences a force of 9.8 N due to gravity.

How do I measure mass and weight?

• Mass: Use a balance (digital or analogue) - gives reading in kg or g
• Weight: Use a force meter (newton meter) - gives reading in N

What is the value of g on Earth?

On Earth, g ≈ 9.8 N/kg. In IGCSE exams, this is often rounded to 10 N/kg for simplicity, but always use the value specified in the question.

Do objects have weight in space?

Objects in space experience microgravity (very weak gravity), so their weight is nearly zero, but not exactly zero. Their mass remains the same. Astronauts appear “weightless” because they’re in freefall, not because there’s no gravity.

Can weight be negative?

No, weight cannot be negative. Weight is the magnitude of the gravitational force, which is always positive. The direction is always downward (toward the center of the planet), but the magnitude (value) is always positive.

How do I avoid common mistakes in mass and weight questions?

1. Always include units - weight in N, mass in kg
2. Use the correct formula: W = m × g
3. Check the value of g given in the question
4. State the location when comparing weights
5. Remember: mass is constant, weight changes with location
6. Don’t confuse mass and weight - they’re different quantities!

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Related IGCSE Physics Resources

Strengthen your IGCSE Physics preparation with these comprehensive guides:

• IGCSE Physics Motion - Speed, velocity, acceleration, and motion graphs
• IGCSE Physics Physical Quantities and Measurement Techniques - Foundation concepts before studying mass and weight
• IGCSE Physics Motion, Forces and Energy - Complete topic coverage with videos, slides, and quizzes
• IGCSE Physics Revision Notes and Syllabus - Complete syllabus overview and revision strategies
• IGCSE Past Papers Guide - Access free IGCSE Physics past papers and exam resources

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