What Is Mass?

Mass, a Quantity of Matter

The mass of an object represents the quantity of matter it contains: atoms, molecules, everything that makes it up. The more matter an object contains, the greater its mass. A suitcase full of books has a greater mass than the same suitcase empty.

Unit and Measuring Instrument

Mass is measured in kilograms (kg), with submultiples such as the gram (g) or the milligram (mg). A scale is used to measure a mass (kitchen scale, electronic scale, beam balance...).

Object	Approximate Mass
An apple	150 g
A filled backpack	5 kg
A car	1200 kg

Mass Does Not Change with Location

This is a fundamental property: the mass of an object stays the same, whether on Earth, on the Moon, or in a space station. A 70 kg astronaut on Earth still has a mass of 70 kg in space, even while floating in weightlessness.

Common Pitfall

Don't confuse mass and weight! In everyday language we say "I weigh 50 kg," but in physics the kilogram is the unit of mass, not of weight. Weight, on the other hand, is measured in newtons, as you will discover in the next lesson.