Mass and Weight: The Key Difference

Mass and weight are different quantities. Confusing them is a frequent error on IGCSE Physics papers, and examiners test the distinction directly.

What is mass?

Mass is the amount of matter in an object. It is a scalar quantity measured in kilograms (kg). Mass does not change with location. A 60 kg person has a mass of 60 kg whether they are on Earth, on the Moon, or floating in space.

Mass is measured using a balance (which compares with known masses).

What is weight?

Weight is the gravitational force acting on an object. It is a vector quantity measured in newtons (N). Weight depends on the gravitational field strength, so it changes with location.

Weight is measured using a newton meter (spring balance).

$W = mg$

where $W$ is weight in N, $m$ is mass in kg, and $g$ is gravitational field strength in N/kg.

How do they compare?

Property	Mass	Weight
Definition	Amount of matter	Gravitational force
Type	Scalar	Vector
Unit	kg	N
Changes with location?	No	Yes
Measured with	Balance	Newton meter

A 60 kg person on Earth ($g = 9.8\ \text{N/kg}$) weighs $60 \times 9.8 = 588\ \text{N}$. On the Moon ($g = 1.6\ \text{N/kg}$), the same person weighs $60 \times 1.6 = 96\ \text{N}$, but their mass is still 60 kg.

Common errors and how to correct them

• Saying “my weight is 60 kg”. Weight is measured in newtons, not kilograms. The correct statement is “my mass is 60 kg” or “my weight is about 590 N.”
• Saying mass changes on the Moon. Mass is constant everywhere. Only weight changes because $g$ is different.
• Using a spring balance to measure mass. A spring balance measures weight (force). A beam balance or electronic balance measures mass.

How to apply this in an exam

When asked to “state the difference between mass and weight”, give three clear contrasts: scalar vs vector, kg vs N, and constant vs location-dependent. A single sentence is rarely enough for full marks.