# Year 10+ Measurement

### Chapters

### Converting Between Units

# Converting Between Units

Sam is a bit confused. He's been given the speed of a fast moving tortoise as \(2 \text{ km/h}\) and needs to convert this to metres per second. He knows that there are \(1,000\) metres in one kilometre, \(60\) minutes in an hour and \(60\) seconds in a minute. So, he can work out that there are \(3,600\) seconds in an hour, but now he's not sure how to combine these together to get a speed in metres per second. He wonders whether he should calculate \(\dfrac{2 \times 1000}{3600} = 0.55555 \dots \) m/s or \(\dfrac{3600}{2000} = 1.8 \) m/s. What should Sam do?

It's understandable that Sam might be a bit confused. One way to resolve his confusion would be to think a bit and realise that he has to put the metres over the seconds because he needs to find the
tortoise's speed in **metres per second**. So, the correct answer is \(\dfrac{2 \times 1000}{3600} = 0.55555 \dots \) m/s.

If that doesn't help, Sam can always try the technique discussed in this article. I've never seen this technique taught in an Australian school, but then I've only taught in NSW. I have been doing some online teaching for an American company, and this is the way they teach unit conversion. You might have watched an American video about problem solving, say in science, where they've spoken about "setting up a proportion" to solve the problem. They're actually talking about fractions, and this article will show you how to use them to solve unit conversion problems. You can then decide which method you like the best.

The best way to learn any new technique is to work through lots of examples, so that's what we'll do. Let's start with an easy one. I know this one isn't likely to cause you confusion, but it's just to give you an idea of how the method works before we try anything more complicated.

## Example 1

Suppose you are asked to convert \(5\) m to centimetres (cm). We all know this is easy: there are \(100\) cm in one metre, so the answer is \(5 \times 100 = 500 \) cm, but let's see how the other method works.

There are three steps to the new method:

- Set up the "proportion": write the conversion as a fraction and set it equal to \(1\).
- Multiply the quantity to be converted by this fraction (this is OK - you're just multiplying by \(1\))
- Cancel any units that occur in both the numerator and denominator (top and bottom) of the answer

**Step 1**

We know there are \(100\) cm in \(1\) m, so we write

**Step 2**

Now multiply the quantity we need to convert by our new form of \(1\):

**Step 3**

Cancel the units that are both on the top and the bottom (those are the metres)

If you still get confused and write the conversion the wrong way around, you'll end up not being able to cancel out. You'll know you're wrong and will be able to go back and fix it up. For example, if we wrote our conversion fraction as

## Example 2

Let's use this technique to solve Sam's tortoise problem from the beginning of the article. To avoid further confusion for Sam, we'll do it in two steps, but when Sam gets good at this method (should he decide to use it), he can do it all in one line. I'll show you how to do that at the end.

Sam has a bit of trouble with unit conversions, so I think he should break it up as follows:

- Convert from km/h to m/h
- Convert from m/h to m/s

**Note:** If you got the hours to seconds conversion fraction wrong and put it up the other way, you'd end up with

## Example 3

There's time for one last example. This time, we'll use the method to convert an imperial unit into a metric one. Gertie the snail is doing some low flying along the garden path at a speed of \(0.25\) inches per hour. What is her speed in centimetres per second?

We know that \(1 \text{ inch}\) is equal to \(2.54 \) centimetres, so we can set up the following product of fractions for the conversion, making sure that the unwanted units will eventually cancel each other out:

## Summary

If you decide to use this method, make sure you write down all your working. The steps are:

- Set up the "proportion": write the conversion as a fraction and set it equal to \(1\).
- Multiply the quantity to be converted by this fraction (this is OK - you're just multiplying by \(1\))
- Cancel any units that occur in both the numerator and denominator (top and bottom) of the answer

### Description

In these chapters, you will learn more about measurement topics such as

- Metric numbers
- Speed
- Conversions

and several other topics

### Audience

Year 10 or higher, some chapters are suitable for Year 8+ students

### Learning Objectives

Learn about measurement and related topics

Author: Subject Coach

Added on: 28th Sep 2018

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