Mass

In science, mass and weight mean two different things.

• Mass is the amount of matter, or “stuff”, inside an object. An object's mass is the same everywhere, whether it is on Earth or on the Moon. We measure mass in grams (g) and kilograms (kg).
• Weight is the force of gravity pulling on an object's mass. Because the Moon has less gravity than Earth, an object's weight is less on the Moon, but its mass stays the same.

In everyday language, we often use “weight” when we really mean mass. The correct word is mass: a scale marked in grams and kilograms gives the mass.

• Tonne \(\left(\mathrm{t}\right)\): Used for very large masses. The mass of a small car is about 1 tonne.
• Kilogram \(\left(\mathrm{kg}\right)\): Used for everyday objects. The mass of a big book is about 1 kg.
• Gram \(\left(\mathrm{g}\right)\): Used for small objects. The mass of a small sweet is about 1 g.
• Milligram \(\left(\mathrm{mg}\right)\): Used for very tiny amounts, such as a grain of sand or the medicine in a tablet.

In the metric system, mass units are related by groups of \(1\,000\). Each unit is \(1\,000\) times larger than the next smaller one.

• \(1 \, \text{tonne} = 1\,000 \, \text{kilograms}\)
• \(1 \, \text{kilogram} = 1\,000 \, \text{grams}\)
• \(1 \, \text{gram} = 1\,000 \, \text{milligrams}\)