Primary 2 · Mathematics
P2 mass
A packet of biscuits has a mass of about 200 g. Write 200 kg and the number is right while the packet now outweighs a fridge. A bag of sugar in the hand is what fixes it.
- Level
- P2
- Usually taught
- Term 2
- Covers
- 1 concept
What changes at P2
Primary 2 measures mass in kilograms (kg) and grams (g). Your child reads a kitchen scale, compares and orders masses, and picks the right unit for everyday things.
The word at school is mass, where most of us grew up saying weight. Picking the unit takes a feel for how heavy a gram and a kilogram are, and a worksheet cannot give that.
The sentence that costs the mark
A packet of biscuits has a mass of about 200 ___. Write g or kg.
What the working usually does
a packet of biscuits:200 kg=heavier than a fridge
Picks kg, the unit printed on the rice bag at home.
What the question asked
a packet of biscuits:200 g=a light packet
Compares the packet with 1 kg first, then picks the unit.
The 200 is right, and kg is a real unit of mass. On paper both answers look equally possible. Only a feel for how heavy 1 kg is shows that one of them describes a packet you would need a trolley to move.
| Figure | Reading |
|---|---|
| bag on a kitchen scale | read the mass off the scale |
The same question, worked properly
Picture 1 kg
1 kg ≈ a bag of sugarA kilogram is about the mass of a bag of sugar or a packet of flour. A gram is about a paper clip.
Lighter or heavier?
biscuits: much lighter than 1 kgThis is the step that fails. A packet of biscuits is much lighter than a bag of sugar, so its mass is in grams, not kilograms.
Write the unit
about 200 gThe packet has a mass of about 200 g. Light things are usually in grams; heavy things, like a bag of rice, in kilograms.
How can you tell it has clicked?
Name a few things, say a pencil, a watermelon and a bag of rice, and ask "g or kg?" before anything is weighed. Quick, correct answers mean both units have a size in your child's head.
Also watch a balance question. "The big box is heavier" before looking at the pans means size is standing in for mass; on a balance, the side that goes down is heavier.
One thing to try tonight
Put a 1 kg bag of sugar in your child's hand, then a packet of biscuits. Ask which is heavier and by how much, roughly.
Then read both labels together. The sugar says kg and the biscuits say g, and your child has just felt why.
What your child will meet in this topic
- Mass in g and kg
Most Singapore schools introduce this in Term 2, though schools do differ by a term either way. If your child’s school runs a different order, you can set it in the schedule on your dashboard.
What it leads to
P3 writes masses in kilograms and grams together, such as 2 kg 350 g, and converts them to grams. That only makes sense once each unit has a weight your child can feel.
Estimating first is also the check that catches a wrong answer later. A child who expects about 200 g will notice when a sum comes out at 200 kg.
The same topic in other years
It comes back each year and gets harder, so it helps to see what came before and what is coming.
More in Primary 2
Other topics in the same year, so you can see what sits either side of this one.
See how your child is doing on this topic
StudyLah marks each step separately, so you can see whether it was the setup or the arithmetic that cost the mark.