Mass, Weight and Gravitational Field Strength
Two quantities that everyday language treats as the same thing and physics does not. Almost every mark lost in this topic comes from using one word for the other.
1. The difference
| Mass | Weight | |
|---|---|---|
| What it is | the amount of matter in an object | the force of gravity acting on it |
| Unit | kilogram (kg) | newton (N) |
| Type | scalar | vector (acts downwards) |
| Changes with location? | NO — constant everywhere | YES — depends on gravity |
| Measured with | balance | newtonmeter / spring balance |
Mass is measured in kilograms; weight is measured in NEWTONS. Giving the unit of mass as N, or being unable to name the unit of weight, were both recorded errors. Weight is a force, so its unit is the force unit.
Mass never changes. Weight does. An astronaut has the same mass on the Earth, on the Moon and in deep space, but a different weight in each — because weight depends on the gravitational field.
“How much do you weigh?” is a question about MASS in everyday speech. In physics, if you say 60 kg you have given a mass; the corresponding weight on Earth is about 600 N.
2. The equation
W = m g weight (N) = mass (kg) × gravitational field strength (N/kg)
Example: a 60 kg person on Earth, with g = 9.8 N/kg.
- W = 60 × 9.8 = 588 N
Rearranged: m = W / g
Example: an object weighing 45 N on Earth.
- m = 45 ÷ 9.8 = 4.6 kg
Multiply mass by g to get weight — don’t divide. Dividing mass by g was a specific recorded error. Check the direction: weight in newtons should be a bigger number than the mass in kg on Earth.
Mass must be in kilograms. Convert grams first (÷ 1000).
Take care with standard form and powers of ten — calculating weight with the wrong power was recorded, and large masses make this easy to slip on.
3. Which value of g?
On Earth, g ≈ 9.8 N/kg (some sources use 9.81; many questions allow 10 for simplicity).
Use the value the question gives you. Tutors were explicit on this: if the paper states g = 10 N/kg, use 10 even if you normally use 9.81. If nothing is stated, use 9.8 (or 9.81) and say which you used.
Elsewhere in the Solar System:
| Location | g (N/kg) |
|---|---|
| Earth | 9.8 |
| Moon | 1.6 (about 1/6 of Earth’s) |
| Mars | 3.7 |
| Jupiter | 24.8 |
| Deep space | ≈ 0 |
Example: a 60 kg astronaut on the Moon.
- W = 60 × 1.6 = 96 N — about one sixth of their Earth weight
- Their mass is still 60 kg
In deep space, weight is very nearly zero but mass is unchanged. Thinking weight stays the same in space was a recorded error. Astronauts float because there is no significant gravitational field, not because they have lost mass.
4. Gravitational field strength
Gravitational field strength (g) is the force per unit mass acting on an object in a gravitational field.
g = W / m, in N/kg
Numerically, g in N/kg equals the acceleration of free fall in m/s² — both are 9.8 on Earth. They are the same quantity expressed two ways.
A gravitational FIELD is the region where a mass experiences a force; the gravitational FORCE is the weight itself. Confusing the field with the force was recorded — the field is the cause, the weight is the effect on a particular mass.
Field strength is larger on planets with more mass, and smaller further from the planet’s centre.
5. Measuring mass and weight
Mass — use a balance, which compares the object with known masses. A balance gives the same reading anywhere, because it compares two masses in the same field.
Weight — use a newtonmeter (spring balance), which measures the force stretching the spring. It reads less on the Moon.
A bathroom “scale” reads mass in kg but actually measures force. It is calibrated for Earth’s gravity, so it would read incorrectly on the Moon.
6. Mistakes that cost marks
Using “weight” when you mean mass, or vice versa.
Giving the unit of mass as N, or weight as kg.
Dividing by g when you should multiply.
Saying mass changes on the Moon.
Saying weight stays the same in space.
Using the wrong value of g when the question specified one.
Leaving mass in grams.
Confusing the gravitational field with the gravitational force.
Standard form and power-of-ten slips.
Frequently asked questions
What is the difference between mass and weight? Mass is the amount of matter (kg, scalar, constant). Weight is the force of gravity on it (N, vector, varies with location).
What is the unit of weight? The newton (N).
What is the equation for weight? W = m g.
What is g on Earth? About 9.8 N/kg — use whatever value the question gives.
What is g on the Moon? About 1.6 N/kg, roughly one sixth of Earth’s.
Does mass change on the Moon? No — only weight changes.
Why do astronauts float in space? Because the gravitational field strength is almost zero, so their weight is almost zero. Their mass is unchanged.
What is gravitational field strength? The force per unit mass, g = W/m, in N/kg.
How do I measure mass and weight? Mass with a balance; weight with a newtonmeter.
Is g the same as the acceleration of free fall? Numerically yes — 9.8 N/kg and 9.8 m/s² are the same quantity.
Quick revision checklist
- I can define mass and weight and state the difference
- I know the units: kg and N
- I know mass is scalar and weight is a vector
- I know mass is constant and weight varies
- I can use W = mg and rearrange it
- I multiply, not divide, to find weight
- I use the value of g given in the question
- I know g on Earth and on the Moon
- I can explain why astronauts float
- I can define gravitational field strength
- I don’t confuse the field with the force
- I know how each is measured
- I convert grams to kilograms
These notes cover mass, weight and gravitational field strength in the Cambridge IGCSE Physics (0625) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Physics lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus and formula list for your own exam series.
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