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 wildly as gravity changes. But the beam balance (measuring mass) stays perfectly level, because the change in gravity pulls equally on both the object and the reference brass weights.
| 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 strictly exactly 9.8 N/kg. Notice this is numerically the same as the acceleration of free fall, 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 exactly 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.