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Year 4 Maths Mass

Measure mass in kilograms & grams

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Theory

Measuring mass in kilograms and grams is a Year 4 measurement skill: choosing the right unit, using the fact that 1 kilogram = 1000 grams, and reading a kitchen scale or a pan balance. This Year 4 maths page explains grams and kilograms, how to read a dial, and how to compare and add masses. In the Australian Curriculum this is Year 3 content and in the Victorian Curriculum it is Level 3; it is taught in Year 4 in Western Australia. Curriculum: VC2M3M02 (Level 3) / AC9M3M02 (Year 3).

Mass is how heavy something is. We measure mass in grams (g) and kilograms (kg). Grams are small, so we use them for light things like a strawberry or a pencil. Kilograms are bigger, so we use them for heavy things like a melon or a school bag.

The two units are joined by one important fact: 1 kilogram is the same as 1000 grams, written \(1\text{ kg} = 1000\text{ g}\). This lets you swap a mass between the two units.

A scale weighs an object and shows its mass with a pointer on a dial. A pan balance compares two masses: the heavier object pushes its pan down, and the lighter pan lifts up. When the pans are level, the masses are equal.

Kitchen scale dial reading 600 grams A round kitchen scale marked from 0 to 900 grams in steps of 100. The red pointer rests on the 600 mark, so the mass is 600 grams. 0 100 200 300 400 500 600 700 800 900 grams
A kitchen scale: the pointer on the 600 mark means the mass is 600 g.
Pan balance with the heavier object lower A pan balance. The left pan holds a large melon and is lower, so the melon is heavier. The right pan holds a small apple and is higher, so the apple is lighter. heavier lighter
On a balance, the heavier object pulls its pan down.

Mass does not need formulas, but these facts let you swap between kilograms and grams.

\[ 1 \text{ kg} = 1000 \text{ g} \]
1 kg =1000 g
\[ \dfrac{1}{2} \text{ kg} = 500 \text{ g} \]
12 kg =500 g
\[ 2 \text{ kg} = 2000 \text{ g} \]
2 kg =2000 g
\[ 1 \text{ kg } 750 \text{ g} = 1750 \text{ g} \]
1 kg 750 g =1750 g
Kilograms to grams: multiply by 1000. Grams to kilograms: every 1000 g makes 1 kg. To write a mass in kilograms and grams, split off each whole 1000 g as 1 kg.
Benchmarks for estimating. A paper clip is about \(1\text{ g}\), a banana about \(120\text{ g}\), a \(1\) litre bottle of water about \(1\text{ kg}\) and a large dog about \(30\text{ kg}\). Compare an object with these to choose a sensible mass.

How to measure and read mass

  1. Choose the unit. Light objects are measured in grams, heavy objects in kilograms.
  2. Read the scale. Find the size of each mark from the numbers, then count on to where the pointer stops. If the pointer is halfway between two numbers, the mass is halfway between them too.
  3. Rename if you need to. Use \(1\text{ kg} = 1000\text{ g}\) to change kilograms into grams, or grams into kilograms.
Example 1 — Choose the unit
Which unit is best for the mass of a strawberry: grams or kilograms?
Solution

A strawberry is very light, only a few tens of grams. Light objects are measured in grams, so the best unit is grams.

Example 2 — Change kg to g
Write \(6\text{ kg } 250\text{ g}\) in grams.
Solution
\(6\text{ kg}\)\(=\)\(6 \times 1000 = 6000\text{ g}\)
\(6000 + 250\)\(=\)\(6250\text{ g}\)
3000+250=3250

The mass is \(6250\text{ g}\).

Example 3 — Read a scale
Each mark is 100 g. What mass does the scale show?
Kitchen scale dial reading 300 grams A round kitchen scale marked from 0 to 900 grams in steps of 100. The red pointer rests on the 300 mark, so the mass is 300 grams. 0 100 200 300 400 500 600 700 800 900 grams
Solution
\(\text{each mark}\)\(=\)\(100\text{ g}\)
\(3\text{ marks}\)\(=\)\(300\text{ g}\)
3×100=300

The pointer is on the 300 mark, so the mass is \(300\text{ g}\).

Example 4 — Total on a balance
A jar balances with three \(500\text{ g}\) masses and one \(100\text{ g}\) mass. Find its mass in kilograms and grams.
Balanced pan scale with a jar and gram masses A level pan balance. The left pan holds a jar. The right pan holds three 500 gram masses and one 100 gram mass, which together balance the jar. jar 500 g 500 g 500 g 100 g
Solution
\(3 \times 500\)\(=\)\(1500\text{ g}\)
\(1500 + 100\)\(=\)\(1600\text{ g}\)
\(1600\text{ g}\)\(=\)\(1\text{ kg } 600\text{ g}\)
1500+100=1600

The jar has a mass of \(1\text{ kg } 600\text{ g}\).

Example 5 — Halfway between numbers
The pointer stops halfway between 600 and 700. What mass does the scale show?
Kitchen scale dial reading 650 grams A round kitchen scale marked from 0 to 900 grams in steps of 100. The red pointer is halfway between 600 and 700, so the mass is 650 grams. 0 100 200 300 400 500 600 700 800 900 grams
Solution
\(700 - 600\)\(=\)\(100\text{ g}\)
\(\text{half of } 100\)\(=\)\(50\text{ g}\)
\(600 + 50\)\(=\)\(650\text{ g}\)

The scale shows \(650\text{ g}\).

Example 6 — Take some away
A bag of potatoes has a mass of \(3\text{ kg}\). Mum uses \(400\text{ g}\). What mass is left?
Solution
\(3\text{ kg}\)\(=\)\(3000\text{ g}\)
\(3000 - 400\)\(=\)\(2600\text{ g}\)
\(2600\text{ g}\)\(=\)\(2\text{ kg } 600\text{ g}\)

There is \(2600\text{ g}\), or \(2\text{ kg } 600\text{ g}\), left. Change to grams before you subtract.

Common pitfalls

Kilograms are heavier than grams. Watch out for mixing them up. \(1\text{ kg}\) is \(1000\text{ g}\), so 1 kg is much heavier than 500 g, not lighter.
Check the size of each mark. Marks may count by 50, 100 or 200 g. Read the numbers on the scale before you say the mass.
Match the unit to the object. A pencil is measured in grams, not kilograms, and a bag of potatoes is measured in kilograms, not grams.

Frequently asked questions

What is the difference between grams and kilograms?

Grams and kilograms both measure mass, which is how heavy something is. Grams are small units for light things like a banana, and kilograms are larger units for heavy things like a school bag. \(1\text{ kg} = 1000\text{ g}\).

How many grams are in a kilogram?

There are \(1000\) grams in \(1\) kilogram. To change kilograms into grams you multiply by \(1000\), so \(7\text{ kg} = 7000\text{ g}\).

How do you read a kitchen scale?

Look at where the pointer stops. Find the size of each mark by reading the numbers, then count on. If each mark is \(100\text{ g}\) and the pointer is on the third mark, the mass is \(300\text{ g}\).

Do you measure a banana in grams or kilograms?

A banana is light, only about \(120\text{ g}\), so you measure it in grams. Kilograms are used for heavier objects like a bag of potatoes.

How do you change kilograms into grams?

Multiply the kilograms by \(1000\). For a mass in kilograms and grams, change the kilograms first, then add the grams. So \(2\text{ kg } 300\text{ g} = 2000 + 300 = 2300\text{ g}\).

What does a balance scale tell you?

A pan balance compares two masses. The pan holding the heavier object goes down and the lighter pan goes up. If the pans are level, the two masses are equal.

How do you put masses in order?

Change every mass to grams, then order the numbers. For \(700\text{ g}\), \(1\text{ kg } 300\text{ g}\), \(1\text{ kg}\) and \(1150\text{ g}\): \(700 < 1000 < 1150 < 1300\), so the order from lightest is \(700\text{ g}\), \(1\text{ kg}\), \(1150\text{ g}\), \(1\text{ kg } 300\text{ g}\).

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