What is the difference between mass and weight in Physics?

In Physics, mass is a measure of the amount of matter in an object and is typically measured in kilograms (kg). It is a scalar quantity and does not change regardless of location. Weight, on the other hand, is the force exerted by gravity on an object and is measured in newtons (N). Weight is a vector quantity and can vary depending on the gravitational field strength, such as on different planets.

How do you calculate weight from mass?

Weight can be calculated using the formula: Weight (W) = Mass (m) × Gravitational Field Strength (g). On Earth, the gravitational field strength is approximately 9.8 N/kg. Therefore, if you know the mass of an object, you can calculate its weight by multiplying the mass by 9.8 N/kg.

Why does mass remain constant while weight changes?

Mass remains constant because it is an intrinsic property of an object, representing the amount of matter it contains. Weight changes because it is dependent on the gravitational field strength, which can vary in different locations. For example, an object will weigh less on the Moon than on Earth due to the Moon's weaker gravitational pull.

How does the concept of mass and weight relate to the Cambridge IGCSE Physics curriculum?

In the Cambridge IGCSE Physics curriculum, understanding the distinction between mass and weight is crucial for topics related to Motion, Forces, and Energy. Students learn how to calculate weight from mass and explore how these concepts apply to real-world scenarios, such as understanding the effects of gravity on different planets.

What are some common misconceptions about mass and weight?

A common misconception is that mass and weight are the same, but they are distinct concepts. Another misconception is that weight is constant, when in fact it varies with gravitational field strength. Understanding these differences is essential for solving problems in Physics, particularly in the context of the Cambridge IGCSE curriculum.

Why is "Stating mass in newtons (or weight in kg)" a common mistake in Mass and Weight?

Why it happens: Treating the two as the same. How to avoid it: Mass: kg. Weight: N. Always check the unit matches the quantity. Source: 0625/42 — recurring

Why is "Saying mass changes on the Moon" a common mistake in Mass and Weight?

Why it happens: Confusing with weight. How to avoid it: Mass is constant; only weight changes when g changes.

Why is "Using $W = m \cdot 10$ on the Moon" a common mistake in Mass and Weight?

Why it happens: Default assumption. How to avoid it: Use the LOCAL value of g. On the Moon g 1.6\,N/kg.

Why is "Saying a digital scale measures weight" a common mistake in Mass and Weight?

Why it happens: Scales display kg. How to avoid it: A spring or digital scale measures the FORCE on it (weight) and converts to a kg readout assuming Earth gravity.

Motion, Forces and EnergyCambridge IGCSE Physics (0625)

Mass and Weight

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Mass and Weight — Cambridge IGCSE 0625 Physics Extended (2026)

Mass is how much matter; weight is the force of gravity on that matter. $W = mg$ . Cambridge mark schemes are strict: don't say weight is in $\text{kg}$ .

What you’ll learn

Mapped to the Cambridge IGCSE 0625 syllabus (2026-2028).

1.4 — State that mass is a measure of the amount of matter.
1.4 — State that weight is the force of gravity acting on the mass.
1.4 — Calculate weight using $W = mg$ .
1.4 — Recall that $g$ is the gravitational field strength.

Mass vs weight — the difference

Mass: matter, in $\text{kg}$ . Weight: force, in $\text{N}$ . They are NOT the same.

Mass ( $m$ ). The amount of matter in an object. Measured in kilograms ( $\text{kg}$ ). Scalar. Constant everywhere in the universe — your mass on the Moon is the same as on Earth.

Weight ( $W$ ). The force exerted on a mass by gravity. Measured in newtons ( $\text{N}$ ). Vector — points DOWN. Depends on the gravitational field, so weight changes from planet to planet.

Quantity	Symbol	Unit	Type	Constant?
Mass	$m$	$\text{kg}$	scalar	Yes, everywhere
Weight	$W$	$\text{N}$	vector	No, depends on $g$

Cambridge wording. Always include the unit: "weight is $58.8\,\text{N}$ ", NOT "weight is $6\,\text{kg}$ " — that's MASS.

Mass: amount of matter, $\text{kg}$ .
Weight: gravitational force, $\text{N}$ .
Mass is the same anywhere; weight changes with $g$ .
Always quote weight in $\text{N}$ , not $\text{kg}$ .

$W = mg$

Weight is mass times gravitational field strength.

Formula. $W = mg.$

$W$ : weight ( $\text{N}$ ).
$m$ : mass ( $\text{kg}$ ).
$g$ : gravitational field strength ( $\text{N/kg}$ ). Numerically equal to acceleration of free fall ( $\text{m/s}^{2}$ ).

Earth. $g \approx 9.8\,\text{N/kg}$ at the surface. Cambridge usually says use $9.8$ , but sometimes $10$ .

Worked. Mass $= 6\,\text{kg}$ . Find weight on Earth.

$W = 6 \times 9.8 = 58.8\,\text{N}$ .

Worked. Same mass on the Moon ( $g_{\text{Moon}} = 1.6\,\text{N/kg}$ ).

$W = 6 \times 1.6 = 9.6\,\text{N}$ .

Mass stays the same; weight is roughly $1/6$ .

$W = mg$ .
$g$ on Earth: $9.8\,\text{N/kg}$ .
$g$ on the Moon: $\sim 1.6\,\text{N/kg}$ .
Mass unchanged across planets; weight changes.

Measuring mass and weight

Mass: balance (compares masses). Weight: newtonmeter (calibrated spring).

Mass: balance / electronic scales. A balance compares the unknown mass to known masses (or measures the force needed to support it and converts back to mass using the local $g$ ). Result: in $\text{kg}$ .

Weight: newtonmeter (force meter). A spring inside the meter stretches in proportion to the force pulling on it (Hooke's law). Calibrated to read directly in newtons.

Tip. A newtonmeter reads weight WHERE IT IS. The same object on the Moon would give a smaller reading.

Worked (lab). A student weighs a $200\,\text{g}$ mass on a newtonmeter at home (Earth).

Mass: $0.200\,\text{kg}$ .
Weight: $W = 0.200 \times 9.8 = 1.96\,\text{N}$ . Newtonmeter reads $\sim 2\,\text{N}$ .

Balance → mass.
Newtonmeter (spring force meter) → weight.
A newtonmeter reads less on the Moon for the same mass.

How it’s examined

Mass-vs-weight appears every Paper 2 as a 1-2 mark item, and underlies most Paper 4 force calculations. Examiner reports flag stating weight in $\text{kg}$ as the recurring lost mark — Cambridge mark schemes accept neither $\text{kg}$ nor missing units.

Sources: Cambridge IGCSE Physics 0625 syllabus 2026-2028 (1.4); 0625/22 May/Jun 2024 — Q4 (mass vs weight); 0625 Examiner Reports 2022-2024. Last reviewed 2026-05-06.

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Step-by-step worked examples — Mass and Weight

Step-by-step solutions to past-paper-style questions on mass and weight, written exactly the way a tutor would explain them at the board.

1Weight on Earth
Core• weight
Question
Find the weight of a $4\,\text{kg}$ rock on Earth. Take $g = 10\,\text{N/kg}$ .
Step-by-step solution
1. Step 1
  $W = mg$ .
  $W = 4 \times 10 = 40\,\text{N}$
Answer
$40\,\text{N}$
2Weight on the Moon
Extended• Adapted from 0625/22 May/Jun 2024 Q4• gravity
Question
An astronaut has mass $80\,\text{kg}$ . On the Moon $g = 1.6\,\text{N/kg}$ . Find the weight on the Moon and on Earth.
Step-by-step solution
1. Step 1
  Moon weight.
  $W_{M} = 80 \times 1.6 = 128\,\text{N}$
2. Step 2
  Earth weight.
  $W_{E} = 80 \times 10 = 800\,\text{N}$
Answer
Moon $128\,\text{N}$ , Earth $800\,\text{N}$
Examiner tip
Mass is the SAME everywhere. Only weight changes when $g$ changes.
3Find mass from weight
Core• rearrange
Question
A rock has a weight of $24\,\text{N}$ on Earth where $g = 10\,\text{N/kg}$ . Find its mass.
Step-by-step solution
1. Step 1
  Rearrange $W = mg$ .
  $m = \dfrac{W}{g} = \dfrac{24}{10} = 2.4\,\text{kg}$
Answer
$2.4\,\text{kg}$
4Compare masses with a beam balance
Extended• balance
Question
A beam balance shows the same mass on Earth and on the Moon. Explain why.
Step-by-step solution
1. Step 1
  A beam balance compares two WEIGHTS, but both are subject to the SAME $g$ .
2. Step 2
  When $g$ changes location, both pans change by the same factor, so the comparison still gives the correct MASS.
Answer
Both objects experience the same $g$ , so the equal-arm balance still measures mass correctly.
5Weight on Jupiter compared with Earth
Core• gravity, comparison
Question
A probe has mass $250\,\text{kg}$ . On Jupiter the gravitational field strength is $g_{J} = 25\,\text{N/kg}$ . Find its weight on Jupiter and on Earth ( $g_{E} = 10\,\text{N/kg}$ ), and the ratio.
Step-by-step solution
1. Step 1
  Earth weight.
  $W_{E} = 250 \times 10 = 2500\,\text{N}$
2. Step 2
  Jupiter weight.
  $W_{J} = 250 \times 25 = 6250\,\text{N}$
3. Step 3
  Ratio.
  $\dfrac{W_{J}}{W_{E}} = \dfrac{6250}{2500} = 2.5$
Answer
$W_{E} = 2500\,\text{N}$ , $W_{J} = 6250\,\text{N}$ . The probe weighs $2.5\times$ more on Jupiter.
6Inertia — why passengers lurch when a bus brakes
Extended• Adapted from 0625/22 Oct/Nov 2023 Q5• inertia, concept
Question
A bus suddenly brakes and standing passengers lurch forward. Explain in terms of inertia, naming Newton's law involved.
Step-by-step solution
1. Step 1
  Before braking, both bus and passenger move together at the same velocity.
2. Step 2
  When the bus decelerates, the brake-pad friction acts on the BUS only. No corresponding backward force acts on the passenger.
3. Step 3
  By Newton's first law, the passenger continues at their original velocity (resists change in motion) — this is inertia.
Answer
Inertia is the tendency of an object to resist a change in motion (Newton's first law). The passenger keeps moving forward at the original velocity while the bus slows.
Examiner tip
The examiner report flags candidates often write 'momentum carries them forward' — Cambridge wants 'inertia' and a reference to Newton's first law for full marks.
7Show that mass is conserved but weight is not, between Earth and Moon
Extended• concept, conservation
Question
A $6\,\text{kg}$ rock is moved from Earth ( $g = 10\,\text{N/kg}$ ) to the Moon ( $g = 1.6\,\text{N/kg}$ ). State the mass and weight in each location and explain which is conserved.
Step-by-step solution
1. Step 1
  Mass measures the amount of matter — independent of location.
  $m_{E} = m_{M} = 6\,\text{kg}$
2. Step 2
  Weight depends on local $g$ .
  $W_{E} = 6 \times 10 = 60\,\text{N};\ W_{M} = 6 \times 1.6 = 9.6\,\text{N}$
3. Step 3
  Mass is conserved (the rock contains the same matter). Weight is NOT conserved — it changes with $g$ .
Answer
Mass = $6\,\text{kg}$ everywhere (conserved). Weight: $60\,\text{N}$ on Earth, $9.6\,\text{N}$ on the Moon — not conserved.
8Beam balance vs spring scale taken to the Moon
Extended• balance, spring-scale
Question
A $2.0\,\text{kg}$ bag of sugar is weighed on Earth and on the Moon using (a) a beam balance with calibrated $1\,\text{kg}$ standard masses, (b) a spring scale calibrated on Earth in newtons. State what each reads in each location. Use $g_{E} = 10\,\text{N/kg}$ , $g_{M} = 1.6\,\text{N/kg}$ .
Step-by-step solution
1. Step 1
  Beam balance compares weights, both arms feel the same $g$ . Reading is unchanged.
  $\text{beam reading} = 2.0\,\text{kg in both places}$
2. Step 2
  Spring scale reads the local weight directly.
  $W_{E} = 2.0 \times 10 = 20\,\text{N};\ W_{M} = 2.0 \times 1.6 = 3.2\,\text{N}$
Answer
Beam balance: $2.0\,\text{kg}$ in both locations. Spring scale: $20\,\text{N}$ on Earth, $3.2\,\text{N}$ on the Moon.
Examiner tip
The 2024 mark scheme awards method marks for noting that the beam balance reading is unchanged BECAUSE both pans are pulled by the same local $g$ .
9Weight in a moving lift — apparent vs true
Extended• Adapted from 0625/42 Oct/Nov 2022 Q3• lift, concept
Question
A passenger of mass $70\,\text{kg}$ stands on bathroom scales inside a lift. Take $g = 10\,\text{N/kg}$ . State the scale reading in newtons when the lift (a) is stationary, (b) accelerates upward at $2\,\text{m/s}^{2}$ .
Step-by-step solution
1. Step 1
  (a) Stationary: net force is zero, so normal force $N = mg$ .
  $N_{a} = 70 \times 10 = 700\,\text{N}$
2. Step 2
  (b) Accelerating up: $N - mg = ma$ , so $N = m(g + a)$ .
  $N_{b} = 70 \times (10 + 2) = 840\,\text{N}$
Answer
(a) $700\,\text{N}$ ; (b) $840\,\text{N}$ . True weight is unchanged ( $700\,\text{N}$ in both); only the apparent weight on the scales rises.
Examiner tip
The examiner report flags candidates often say the passenger's mass increases in the lift — mass is invariant; the contact force changes.
10A* — apparent weight of a submerged object
Challenge• apparent weight, buoyancy, synoptic
Question
A $0.40\,\text{kg}$ metal ball hangs from a spring scale. When fully submerged in water it experiences an upward buoyancy force of $0.50\,\text{N}$ . Find the scale reading. Use $g = 10\,\text{N/kg}$ .
Step-by-step solution
1. Step 1
  Real weight from $W = mg$ .
  $W = 0.40 \times 10 = 4.0\,\text{N}$
2. Step 2
  The spring scale measures the tension it provides — which balances weight minus buoyancy.
  $T = W - F_{B} = 4.0 - 0.50 = 3.5\,\text{N}$
Answer
Spring scale reads $3.5\,\text{N}$ — the apparent weight. The TRUE weight is still $4.0\,\text{N}$ .
Examiner tip
The examiner report flags candidates often say the ball's weight has decreased — only the apparent weight changes; the gravitational pull (true weight) is unchanged.

Key Formulae — Mass and Weight

The formulae you need to memorise for mass and weight on the Cambridge IGCSE 0625 paper, with every variable defined in plain English and a note on when to use it.

Weight
$W = mg$
$W$
weight in newtons (N)
$m$
mass in kg
$g$
gravitational field strength in N/kg ( $\approx 10$ on Earth)
When to use
Convert between mass and weight.

Key Definitions and Keywords — Mass and Weight

Definitions to memorise and the exact keywords mark schemes credit for mass and weight answers — sharpened from recent examiner reports for the 2026 0625 sitting.

Mass
Examiner keyword
A measure of the amount of matter in an object — also a measure of its INERTIA. SI unit kilogram (kg). Constant everywhere.
Weight
Examiner keyword
The gravitational force on an object. SI unit newton (N). Changes with $g$ .
Gravitational field strength ( $g$ )
Examiner keyword
Force per unit mass on a small test mass at that point. On Earth $g \approx 10\,\text{N/kg}$ .
Inertia
Examiner keyword
The tendency of an object to resist a change in its motion. Larger mass → more inertia.

Common Mistakes and Misconceptions — Mass and Weight

The traps other students keep falling into on mass and weight questions — taken from recent Cambridge IGCSE 0625 examiner reports and mark schemes — and how to avoid them.

✕Stating mass in newtons (or weight in kg)
0625/42 — recurring
Why it happens
Treating the two as the same.
How to avoid it
Mass: kg. Weight: N. Always check the unit matches the quantity.
✕Saying mass changes on the Moon
Why it happens
Confusing with weight.
How to avoid it
Mass is constant; only weight changes when $g$ changes.
✕Using $W = m \cdot 10$ on the Moon
Why it happens
Default assumption.
How to avoid it
Use the LOCAL value of $g$ . On the Moon $g \approx 1.6\,\text{N/kg}$ .
✕Saying a digital scale measures weight
Why it happens
Scales display kg.
How to avoid it
A spring or digital scale measures the FORCE on it (weight) and converts to a kg readout assuming Earth gravity.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

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Short walkthrough of the concepts students most often get stuck on.

Mass and Weight — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Quantity

Symbol

Unit

Type

Constant?

Mass

m

\text{kg}

scalar

Yes, everywhere

Weight

W

\text{N}

vector

No, depends on

g

Mass and Weight

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Weight on Earth

2Weight on the Moon

3Find mass from weight

4Compare masses with a beam balance

5Weight on Jupiter compared with Earth

6Inertia — why passengers lurch when a bus brakes

7Show that mass is conserved but weight is not, between Earth and Moon

8Beam balance vs spring scale taken to the Moon

9Weight in a moving lift — apparent vs true

10A* — apparent weight of a submerged object

Weight

Mass

Weight

Gravitational field strength (ggg)

Inertia

✕Stating mass in newtons (or weight in kg)

✕Saying mass changes on the Moon

✕Using W=m⋅10W = m \cdot 10W=m⋅10 on the Moon

✕Saying a digital scale measures weight

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Mass and Weight — frequently asked questions

Mass and Weight

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Weight on Earth

2Weight on the Moon

3Find mass from weight

4Compare masses with a beam balance

5Weight on Jupiter compared with Earth

6Inertia — why passengers lurch when a bus brakes

7Show that mass is conserved but weight is not, between Earth and Moon

8Beam balance vs spring scale taken to the Moon

9Weight in a moving lift — apparent vs true

10A* — apparent weight of a submerged object

Weight

Mass

Weight

Gravitational field strength (ggg)

Inertia

✕Stating mass in newtons (or weight in kg)

✕Saying mass changes on the Moon

✕Using W=m⋅10W = m \cdot 10W=m⋅10 on the Moon

✕Saying a digital scale measures weight

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Mass and Weight — frequently asked questions

Gravitational field strength ( $g$ )

✕Using $W = m \cdot 10$ on the Moon

Gravitational field strength ( $g$ )

✕Using $W = m \cdot 10$ on the Moon