Mass and Weight

This lesson explains Mass and Weight for Cambridge IGCSE Physics 0625. It covers the Core knowledge required by every candidate. Focus on the cause-and-effect explanation and the exact quantities being compared. The goal is to explain the idea accurately and apply it in an unfamiliar context.

What is the difference between mass and weight?

Mass is the quantity of matter in an object. It is measured in kilograms and never changes with location. Weight is the gravitational force acting on a mass. It is a force, measured in newtons, and it changes with the strength of the gravitational field.

In words: weight equals mass multiplied by gravitational field strength. In symbols: $W = mg$.

Quantity	Symbol	Unit	Scalar or vector	Measured with
Mass	$m$	kilogram (kg)	scalar	beam balance
Weight	$W$	newton (N)	vector (acts downward)	spring balance / newton meter
Gravitational field strength	$g$	N/kg	vector	(calculated, not read off)

On Earth, $g = 9.8\ \text{N/kg}$, the 0625 standard value. Some papers use 10 N/kg, so read the question and use the value given. On the Moon, $g$ is about $1.6\ \text{N/kg}$, so a 60 kg astronaut keeps a mass of 60 kg but a weight of only 96 N instead of 588 N.

Why does a balance read the same on the Moon?

A beam balance compares two masses, and gravity pulls equally on both sides, so it reads the same anywhere. A spring balance measures the pull of gravity directly, so its reading drops on the Moon. This comparison is a favourite 2-mark explain question, and it checks whether you understand the definitions rather than just reciting them.

Worked example

A student has a mass of 55 kg. Gravitational field strength on Earth is 9.8 N/kg. (a) Calculate the student’s weight on Earth. [2] (b) On Mars, $g = 3.7\ \text{N/kg}$. State the student’s mass on Mars and calculate her weight there. [2]

Solution (a). Equation: $W = mg$. Substitute: $W = 55 \times 9.8$. Answer: $W = 539\ \text{N} \approx 540\ \text{N}$.

Solution (b). Mass is unchanged: 55 kg. Weight: $W = 55 \times 3.7 = 203.5 \approx 200\ \text{N}$ (2 s.f.).

Original marking points

• M1 (a): $W = mg$ or $55 \times 9.8$ seen.
• A1 (a): 539 N or 540 N with unit.
• B1 (b): mass = 55 kg (unchanged).
• A1 (b): 203.5 N, accept 200-204 N with unit.

These marking points belong to this original example. They are not an official Cambridge mark scheme.

Common errors and how to correct them

• Writing weight in kilograms. “Weight = 55 kg” may not earn credit. Fix: weight is a force, always in newtons.
• Saying mass changes on the Moon. Fix: mass is constant everywhere; only weight changes.
• Defining weight as “how heavy something is”. Fix: write “the gravitational force acting on a mass”.
• Using 10 N/kg when the paper states 9.8. Fix: scan the question stem for the given $g$ before substituting.
• Quoting $g$ without units or as 9.8 N. Fix: $g = 9.8\ \text{N/kg}$ (newtons per kilogram).

How to apply this in an exam

Whenever a question mentions another planet, write two headed lines before calculating: “mass = unchanged” and “weight $= mg$ with new $g$”. The unchanged-mass statement is often a separate B1 mark, and students who jump straight to the calculation forfeit it.

Where this skill matters

Mass and weight is Core content, so every 0625 candidate meets it. Papers 1 and 2 use MCQs on units and Moon-versus-Earth comparisons. Papers 3 and 4 set short $W = mg$ calculations, then reuse weight inside density, pressure and energy questions, so an error here cascades. Paper 6 touches it through spring balance readings and balance comparisons. Definitions matter as much as numbers: learn the exact wording above, because “state what is meant by weight” is a one-mark gift only if your sentence contains “gravitational force”.