You have measured length with a ruler. Two other things we measure are mass (how heavy something is) and liquid volume (how much liquid a container holds). Word problems about them use the same operations as always.
| Unit | Measures | About as much as |
|---|---|---|
| 1 gram (g) | mass | a paper clip |
| 1 kilogram (kg) | mass | a big book; 1,000 grams |
| 1 liter (L) | liquid volume | a large water bottle |
Why it works
Mass is the amount of stuff in an object, not its size. A pillow is big but has little mass; a rock the size of your fist has much more. A scale or a balance measures mass. Liquid volume is the space a liquid fills, measured in liters with marks on a container.
Key idea
Mass: grams and kilograms. Liquid volume: liters. Read the scale: find what each mark is worth, then read where the needle or the water reaches.
Worked example: Read the marks
A container is marked 2, 4, 6, 8, 10 liters, with one small mark between each pair. The water is at the small mark between 6 and 8. How much water?
- Between 6 and 8 there is one small mark, so each small mark is 1 liter.
- The mark between 6 and 8 is 7.
Answer: 7 liters
Worked example: Equal groups
Each bag of flour has a mass of 3 kg. What is the mass of 4 bags?
- 4 groups of 3 kg: $4 \times 3 = 12$.
Answer: 12 kg
Worked example: Sharing liquid
12 liters of water are poured equally into 3 buckets. How much is in each?
- Share 12 into 3 equal groups: $12 \div 3 = 4$.
Answer: 4 liters
Common mistake
Thinking bigger means heavier. A beach ball is bigger than a brick but has much less mass. Check by thinking about lifting it, or put both on a balance: the side that goes down has more mass. And a kilogram of feathers has exactly the same mass as a kilogram of rocks.