Cogito
Grade 2 Math · Chapter 2 · Lesson 5
Estimating Length
A good guess uses something you already know.
10 problems · about 12 minutes · 2.MD.A.3
What this lesson teaches
The child estimates a length in a named unit by comparing to a known benchmark, then checks by measuring.
- An estimate is a careful guess before measuring.
- Use a length you know: a fingernail, a thumb, a step.
- Close is good enough for an estimate.
Warm-Up Review
From earlier lessons. Loosen up before the new work.
3 problemsReview — The Same Object, Two Units: Which gives the bigger number, centimetres or metres?
Answer Centimetres.
Why Smaller unit, bigger number, same length.
Review — Inches and Feet: How many inches are in a foot?
Answer 12
Why 1 foot = 12 inches.
Review — Centimetres and Metres: How many centimetres are in a metre?
Answer 100
Why 1 metre = 100 centimetres.
Warm Up
Straightforward practice. Get the method working first.
3 problemsAbout how long is one big step?
Answer About 1 metre.
Why A big step is roughly a metre.
Does an estimate have to be exactly right?
Answer No. It has to be close.
Why Close is what an estimate is for.
A pencil is about how long?
Answer About 18 centimetres.
Why It fits in a pencil case.
Build It Up
The same ideas with more to keep track of.
1 problemA bus is about how long?
Answer About 10 metres.
Why You can walk down the inside of it.
Stretch Yourself
Mixed problems. Work out what kind of question it is before you start.
3 problemsAbout how many centimetres wide is your fingernail?
Answer 1
Why It is the benchmark from the lesson.
The Door: About how tall is a classroom door?
Answer About 2 metres.
Why A door is about 2 metres tall.
The Spoon: About how long is a teaspoon?
Answer About 12 centimetres.
Why A teaspoon is roughly 12 centimetres long.