7.01 Units of length

Lesson

Can you estimate lengths that use centimetres or metres?

Examples

Example 1

Choose the best estimate for the length of a classroom.

A
4 metres
B
60 metres
C
100 metres
D
10 metres
Worked Solution
Create a strategy

Think about 1 metre as 1 long step. How many steps do you need to take to go from one end of a classroom to the other?

Apply the idea

It would take about 10 long steps to go from one end of a classroom to the other. So the best answer is option D.

Idea summary

When estimating length, 1 \text{ cm} (centimetre) is about as big as your thumbnail, and 1 \text{ m} (metre) is about as long as two normal steps (for an average Year 3 student).

Estimate length

This video has a look at the units of millimetres \text{(mm)}, centimetres \text{(cm)}, metres \text{(m)}, and kilometres \text{(km)} and how to tell which are bigger or smaller.

Examples

Example 2

A bee is 11 \text{ mm} long. A fly is 14 \text{ mm} long. Which insect is longer?

Worked Solution
Create a strategy

Plot the given numbers on a number line and choose the number on the right.

Apply the idea

We can see from the number line that 14 is on the right of 11, which means that 14 \text{ mm} is longer than 11 \text{ mm}.

So, the longer insect is a fly.

Idea summary

When estimating length, we can use \text{mm} (millimetres) for very small objects,\text{ cm} (centimetres) for small objects, \text{m} (metres) for big objects, and \text{km} (kilometres) for much bigger objects or longer distances.

Length problems

This video shows you how to use some strategies to tackle problems.

Examples

Example 3

Choose the best estimate for the length of a fingernail.

A
100 millimetres
B
60 millimetres
C
10 millimetres
D
1 millimetre
Worked Solution
Create a strategy

Use a ruler. The gap between each line is 1 millimetre.

Apply the idea

A sultana has almost the same length of a fingernail. We can see below that a sultana is 9 millimetres.

The closest estimate from the options is 10 millimetres, so the answer is option C.

Idea summary

To estimate the length of an object, we can compare it to an object where we have an idea already of its estimated length.

Choose the right unit

In this video we look at some familiar things, to get an idea of which unit of measurement we need to use.

Examples

Example 4

Choose the most appropriate unit of measure for the height of a tree.

A
Metres
B
Kilometres
C
Centimetres
Worked Solution
Create a strategy

Imagine objects that have lengths in each unit.

Apply the idea

We can use comparisons such as a person is 1.6 \text{ m} tall, two cities can be 500 \text{ km} apart, and a ruler that is 30 \text{ cm}.

It is more appropriate to compare the height of a tree to the height of a person in metres, the answer is option A.

Idea summary

1 \text{ m} is about the same as two steps.

1 \text{ km} is about the size of a one and a half laps around the school oval.

Outcomes

MA3-9MG

selects and uses the appropriate unit and device to measure lengths and distances, calculates perimeters, and converts between units of length