Mass - How Much Matter Is in an Object
Mass is the measure of how much matter is contained in an object. Unlike weight, mass does not change wherever you go – on Earth, the Moon, or in space your mass remains the same. It is one of the seven base quantities in science and everyday measurement.
For 130 years, the kilogram was defined by a single physical object: a small cylinder of platinum-iridium alloy, nicknamed “Le Grand K,” locked in a vault outside Paris since 1889, with official copies distributed to countries around the world. The trouble was that Le Grand K's mass could drift very slightly over time due to microscopic contamination or cleaning, meaning the entire world's definition of a kilogram was quietly changing too. In 2019, scientists retired the physical cylinder for good and redefined the kilogram using a fixed value of a fundamental constant of nature (Planck's constant), measured with an extraordinarily precise device called a Kibble balance – so today, a kilogram means exactly the same thing everywhere in the universe, with no vault required.
What Is Mass?
Mass is the total amount of matter in an object. It is measured in kilograms (kg) – the SI base unit – or in grams for smaller quantities. Mass does not depend on gravity.
Metric Units of Mass
| Unit | Symbol | Relation | Typical use |
|---|---|---|---|
| Milligram | mg | 1 g = 1,000 mg | Medicine doses |
| Gram | g | 1 kg = 1,000 g | Food ingredients, letters |
| Kilogram | kg | SI base unit | Body mass, bags of flour |
| Tonne | t | 1 t = 1,000 kg | Cars, lorries, heavy cargo |
Converting Metric Units of Mass
| Conversion | Operation | Example |
|---|---|---|
| kg → g | × 1,000 | 2.5 kg = 2,500 g |
| g → kg | ÷ 1,000 | 750 g = 0.75 kg |
| g → mg | × 1,000 | 3 g = 3,000 mg |
| mg → g | ÷ 1,000 | 500 mg = 0.5 g |
| t → kg | × 1,000 | 4.2 t = 4,200 kg |
| kg → t | ÷ 1,000 | 3,500 kg = 3.5 t |
Measuring Instruments
- Balance scale – compares two masses directly; used in science labs.
- Digital scale – common in kitchens and post offices.
- Spring balance – calibrated to show mass in grams.
- Bathroom scale – measures body mass in kg or stones and pounds.
Worked Examples
3.8 × 1,000 = 3,800 g.
4,250 ÷ 1,000 = 4.25 kg.
2.4 × 1,000 = 2,400 kg.
350 + 220 = 570 g. 570 ÷ 1,000 = 0.57 kg.
Key Takeaways
- Mass measures the amount of matter in an object and is constant everywhere.
- 1 kg = 1,000 g | 1 t = 1,000 kg | 1 g = 1,000 mg.
- Multiply to convert to a smaller unit; divide to convert to a larger unit.
- Mass and weight are different: mass is in kg, weight is a force in Newtons.