Primary 2 · Mathematics

P2 length

A wavy string can end well short of a straight one and still be longer. P2 questions draw exactly this, and the answer that comes back is the string that reaches further.

Level
P2
Usually taught
Term 1
Covers
1 concept

What changes at P2

Primary 2 adds the metre. Your child measures in metres (m) and centimetres (cm), compares and orders lengths, and solves problems such as how much longer one thing is, or how tall a pile of identical boxes is.

Comparing is where the marks go. Two straight things are easy to compare: line them up and see which end is further. That habit fails as soon as one of them bends.

The sentence that costs the mark

String A is wavy. String B is straight and reaches further. Which string is longer?

What the working usually does

longer string:where it ends=String B

Compares where the two strings end, as if both were straight.

What the question asked

longer string:pulled straight=String A

Pictures each string pulled straight, then compares.

Comparing where two things end is exactly how your child learned to compare lengths, and it is right for straight things. The bends in String A hide length that only shows once it is pulled straight.

AB
Pulled straight, String A is longer.

The same question, worked properly

  1. Look at the ends

    B reaches further

    Both strings start on the same line, and B ends further to the right. That is the answer that looks right.

  2. Pull A straight

    every bend adds length

    This is the step that fails. Each bend in String A is length going up and down instead of across. Pulled straight, it stretches past the end of B.

  3. Compare

    String A is longer

    String A is the longer string. To check, lay a piece of real string along each one, then straighten both side by side.

How can you tell it has clicked?

Draw a wavy line and a straight line that ends further, and ask which is longer. "The straight one, it goes further" means the ends are being compared.

A child who asks to measure, or traces the wavy line with a finger, has understood that length is about the whole string, not about where it stops.

One thing to try tonight

Lay two shoelaces side by side, one straight and one wiggled so it ends first. Ask which is longer.

Then pull the wiggly one straight beside the other. Let your child do the pulling: seeing it overtake the straight lace does more than any explanation.

What your child will meet in this topic

  • Length in cm and m

Most Singapore schools introduce this in Term 1, 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 adds the kilometre and converts between units: 1 m is 100 cm, so 2 m 35 cm is 235 cm. Each comparison still needs both lengths in the same form first.

The same idea returns with perimeter in P3: the length around a shape, which is a bent line pulled straight.

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.

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