Take a 2 kg bag to the Moon and it is still 2 kg of matter. But it weighs about six times less, because weight is the pull of gravity, and the Moon pulls more gently. Mass and weight are not the same quantity, and the exam rewards keeping them strictly apart.
Mass is the amount of matter in an object, measured in kilograms, and is the same everywhere. Weight is the gravitational force on the object, measured in newtons, and is found from W = mg, where g is the gravitational field strength in N/kg.
Switch worlds and watch the two instruments. The spring balance (measuring weight) stretches as gravity changes. But the beam balance (measuring mass) stays level, because the change in gravity pulls equally on both the object and the reference brass weights.
Four quick checks on the distinction and W = mg. Each correct answer earns XP and lights this skill on your star map.
Mass is measured in kilograms, and weight is measured in...
An astronaut takes a tool to the Moon. Compared with Earth, on the Moon the tool has...
The weight of an object is calculated from...
Mass is best described as...
| Mass | Weight | |
|---|---|---|
| What it is | the amount of matter | the gravitational force on the object |
| Unit | kilogram (kg) | newton (N) |
| Type | scalar | vector (acts downward) |
| Changes with location? | no, same everywhere | yes, depends on g |
| Measured with | a beam balance | a spring balance (force meter) |
Gravitational field strength g is the weight per unit mass, g = W / m, measured in N/kg. On Earth it is about 9.8 N/kg. Notice this is numerically the same as the acceleration of free fall, about 9.8 m/s².
The most common error here is using mass where a force is needed. If a question asks for a force, such as the weight, the answer must be in newtons, found with W = mg. Saying an object weighs 5 kg mixes the two up: 5 kg is its mass, and its weight would be about 49 N on Earth. In any equation that needs weight, substitute mg, not the mass.
A rock has a mass of 6.0 kg. Taking g = 9.8 N/kg on Earth and g = 1.6 N/kg on the Moon, find its weight in each place and state its mass on the Moon.
Unlocks once the four checks above are done. Worth more XP, written in the style of Paper 1.
A 4.0 kg object rests on Earth, where g = 9.8 N/kg. Its weight is...
A student writes that a box weighs 5 kg. The correct statement is that...
A fixed mass is taken to several planets. Its weight is largest on the planet with the...
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