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Mass

What is a Mass?

Definition Mass vs. Weight
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's 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. In this chapter, we will use the correct word and focus on measuring mass.
Method Comparing Mass
We can compare the mass of two objects using a balance scale.
  • The side with the object of greater mass goes down.
  • The side with the object of lesser mass goes up.
  • If the scale is balanced, the objects have equal mass.

Standard Units of Mass


To measure mass accurately, we must use standard units that are the same for everyone. The most common standard units of mass are the tonne (t), kilogram (kg), gram (g), and milligram (mg). These units allow us to measure everything from a tiny amount of medicine to a large vehicle.

Definition Units of 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 candy is about 1 g.
  • Milligram \(\left(\mathrm{mg}\right)\): Used for very tiny amounts, such as the amount of medicine in a tablet.

Conversion of Mass Units

Definition Conversion of Mass Units
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.
  • \(1 \, \text{tonne} = 1\,000 \, \text{kilograms}\)
  • \(1 \, \text{kilogram} = 1\,000 \, \text{grams}\)
  • \(1 \, \text{gram} = 1\,000 \, \text{milligrams}\)
Method Converting Using Multiplication or Division
  • Use multiplication to go from a bigger unit to a smaller one (like tonnes to kilograms, kilograms to grams, or grams to milligrams).
  • Use division to go from a smaller unit to a bigger one (like milligrams to grams, grams to kilograms, or kilograms to tonnes).
  • Each step to the right in the diagram means “\(\times 1\,000\)”. Each step to the left means “\(\div 1\,000\)”.