Mass and weight
In everyday speech, people say "weight" for how heavy something is. Scientists make a distinction:
- Mass is how much matter is in an object. It’s the same everywhere: on Earth or on the Moon.
- Weight is the force of gravity pulling on that mass. On the Moon you’d weigh about one sixth as much, but your mass wouldn’t change.
In maths class, we usually measure mass in grams and kilograms.
Metric units of mass
| Unit | Symbol | Equals | About as heavy as |
|---|---|---|---|
| milligram | mg | 1/1,000 g | a grain of sand |
| Gram | g | — | a paper clip |
| Kilogram | kg | 1,000 g | a litre of water |
| Tonne | t | 1,000 kg | a small car |
Tools for measuring mass
| Tool | How it works |
|---|---|
| Pan balance | Put the object on one side and known masses on the other until they balance. |
| Kitchen scale | Shows the mass in grams on a dial or screen. |
| Bathroom scale | Shows body mass in kilograms (or pounds). |
| Truck scale | Weighs whole trucks in tonnes. |
Using a scale
- Check it reads 0 before you start (press "tare" or "zero").
- Place the object gently in the centre.
- Wait until the number stops changing.
- Read the number and unit.
To weigh something in a container, put the empty container on first and press tare, or subtract its mass afterwards.
Reading a dial
Find what each small mark is worth. If there are 10 marks between 0 and 100 g, each mark is 10 g.
Choosing a unit
| Object | Unit |
|---|---|
| A pencil | g |
| An apple | g (about 150 g) |
| A dog | kg |
| A truck | t |
Converting
- kg → g: × 1,000. 2.5 kg = 2,500 g
- g → kg: ÷ 1,000. 750 g = 0.75 kg
Big doesn’t mean heavy
A large pillow can have less mass than a small rock. Size and mass are different attributes.
Worked examples
Example 1: A bag of flour is 2 kg. How many grams?
2 × 1,000 = 2,000 g.
Example 2: Three apples have masses of 140 g, 155 g and 165 g. What is the total in kg?
460 g = 0.46 kg.
Example 3: A box on a scale reads 3.2 kg. The empty box is 400 g. What is the mass of the contents?
3,200 − 400 = 2,800 g (2.8 kg).
Common mistakes
- Not zeroing the scale before weighing.
- Forgetting to subtract the container.
- Mixing up units: writing 2 g for a 2 kg bag.
Try it yourself
- Convert 4,300 g to kg.
- Convert 0.6 kg to g.
- Which unit for the mass of an elephant?
- 5 identical books have a total mass of 3 kg. What is the mass of one book in grams?
Answers: 1) 4.3 kg 2) 600 g 3) tonnes (or kg) 4) 600 g
Quick recap
- Mass is the amount of matter; weight is the pull of gravity.
- 1,000 g = 1 kg; 1,000 kg = 1 t.
- Zero the scale before you measure.