Measurement (including Time)

How much tutoring
does your child need?

Overview

Here’s another important topic you will come across in year 5 maths – measurement. You will learn about the measurements of length, mass, time, perimeter, and volume, among other things. You will also learn about the different units of measurement.

Learning Outcomes

By the time you finish reading this article, you will

• Learn about the Metric System of units
• Understand why certain units of measurement are appropriate in some cases while others are not
• Know how to find the perimeter of rectangles
• Know how to find the area of rectangles
• Be able to convert time between the 12-hour format and the 24-hour format

Metric System of Units

We need to measure length, mass, and capacity in our everyday lives. For example, we measure the length of a room, the mass of fruit, or the capacity of a bottle. To do so, we use metric units. In the metric system of units, each unit is ten times larger than the previous smaller unit. We have milliunit (smallest), centiunit, deciunit, unit, decaunit, hectounit, and kilounit (largest), but as of now, we will focus only on kilounit, unit, centiunit, and milliunit.

For length, the basic unit is the metre (m). One kilometre (km) is equal to 1000 metres, and one metre is equal to 100 centimetres (cm). There are 10 millimetres (mm) in a cm.

Likewise, for mass, the basic unit is the gram (g). One kilogram (kg) is equal to 1000 grams, and one gram is equal to 100 centigrams (cg). There are 10 milligrams (mg) in a cg.

In the exact same way, for capacity, we have litre (L) as the basic unit. One kilolitre (kL) equals 1000 litres, and one litre equals 100 centilitres (cL). Finally, 1 cL = 10 millilitres (mL).

To convert between units, all you need to do is multiply (when converting a larger unit to a smaller unit) or divide (when converting a smaller unit to a larger unit) by 10, 100 or 1000.

Which Unit Is Appropriate?

Why do we have so many units for each physical quantity?

It would be impractical to use kilometres to measure the width of a piece of paper, kilograms to measure the mass of a grain of salt, or litres to measure the capacity of a small medicine dropper!

The choice of metric units for length, mass, or capacity depends on the size of the object or quantity we want to measure. Let’s take a closer look at some examples:

• We use millimetres (mm) to measure small objects, such as the thickness of a sheet of paper or the diameter of a pencil.
• We use centimetres (cm) to measure the length of a book.
• We use metres (m) to measure the length of a room or the height of a building.
• We use kilometres (km) to measure the distance between cities or the length of a marathon race.
• We use milligrams (mg) to measure the mass of small objects such as a grain of salt or a vitamin tablet.
• We use grams (g) to measure the mass of fruits, vegetables, or small packages.
• We use kilograms (kg) to measure the mass of a person or a large package.
• We use millilitres (mL) to measure the capacity of a small container, such as a test tube or a medicine dropper.
• We use centilitres (cL) to measure the capacity of a small bottle or a shot glass.
• We use litres (L) to measure the capacity of a large container such as a jug.

Perimeter

You have learnt in year 4 that perimeter is the total distance around the outside of a shape. In other words, it is the length of the boundary or the sum of all the sides of the shape.

To find the perimeter of a rectangle, you need to add up the lengths of all four sides. Since opposite sides of a rectangle are equal, you only need to know the length and width of the rectangle to calculate its perimeter.

Perimeter of rectangle = 2\(\times \) (length + width)

Here’s an example: find the perimeter of the rectangle shown below.

 

Here. the perimeter = 2 \(\times \) (12 cm + 8 cm) = 2 \(\times \) 20 cm = 40 cm

Area

You have also learnt that area is the space enclosed within a shape. You can think of a shape as made up of small squares, each measuring 1 unit on each side. Then, the area is the total number of such squares inside the shape. Here’s an example:

Find the area of the shape shown below.

As you can see, there are 5 rows of squares, with 9 squares in each row. Therefore, there are 5 \(\times \) 9 = 45 squares in all.

Therefore, to find the area of a rectangle, you need to multiply its length and width.

Area = length \(\times \)width

Using this, the area of a rectangle with length = 12 cm and width = 8 cm = 12 x 8 = 96 square cm. Note the unit is square units.

So then, when do you use perimeter and when do you use area? Remember, the perimeter is the length around a shape. On the other hand, the area is the space inside a shape. Therefore, if you want to find out how much fencing is needed for a piece of land, use the perimeter. If you need to find the amount of carpet needed to cover a floor, use the area.

Time – 12-Hour and 24-Hour Format

Time is displayed in either the 12-hour format (AM/PM) or the 24-hour format (military time). The 24-hour format is commonly used in places where it is essential to avoid ambiguity, such as railway stations, airports, and hospitals, so that you don’t get confused between AM and PM! 12:00 and above is PM. For example, 18:00 is 6 PM.

To convert a given time from the 12-hour format to the 24-hour format, keep the same time if it’s AM, and add 12 hours if it’s PM. For example, 3:30 PM = 3 hours + 12 hours + 30 minutes = 15:30 hours in the 24-hour format. Remember, you should add 12 hours only when the time is PM. As another example, 6:45 AM is 06:45 hours. There is no need to add 12 hours in this case.

To convert a given time from the 24-hour format to the 12-hour format, first, check if the hour is more than 12. If so, it means the time is in the afternoon or evening (PM). In that case, subtract 12 from the hours, and write the time as PM.

If, however, the hour is 12 or less, write down the same time with ‘AM’ next to it.

Therefore, 17:20 hours is 5:20 PM (17 – 12 = 5), but 09:30 hours is 9:30 AM.

Wrap Up

Remember, we only discussed the most commonly used units of measurement in this article. There are more units that are less frequently used. While solving a problem related to length, perimeter, area, mass, capacity or time, pay attention to the units given in the problem. Don’t forget to put the right unit in your answer. If you need any help, feel free to contact us.