Mass Measurements
Primary 4 · Mathematics
Mass Measurements
Lesson 1 of 4
15 min
Let's begin
Gatete and Ishimwe are at the market in Musanze with their aunt. She wants to buy potatoes. Gatete lifts one bag. Ishimwe lifts another bag.
"Mine is heavier!" they both shout. Who is right? The seller smiles and puts the two bags on her balance.
Today you will learn how we measure how heavy things are.
The mass of an object tells us how heavy it is. A sack of rice has a big mass. A pencil has a small mass.
Lifting things in your hands is not a good test. Two bags can feel almost the same. We need to measure.
A balance has two pans. Put one object in each pan. The heavier side goes down. The lighter side goes up.
If the beam stays level, the two masses are equal.
The standard units of mass are the kilogram (kg) and the gram (g).
1 kg = 1 000 g
Hold these in your mind to help you estimate:
Put the object in one pan. Put standard masses (1 kg, 500 g, 200 g, 100 g ...) in the other pan until the beam is level. Then add up the masses.
Worked example
A pawpaw balances with a 1 kg mass, a 200 g mass and a 100 g mass.
Step 1 Add the grams: 200 g + 100 g = 300 g.
Step 2 Put it together: 1 kg and 300 g, written 1 kg 300 g.
Step 3 In grams: 1 000 g + 300 g = 1 300 g.
Before you weigh, sort things into groups: less than 100 g, between 100 g and 1 kg, and more than 1 kg. A pencil is less than 100 g. A mango is between 100 g and 1 kg. A goat is more than 1 kg.
Careful! A big object is not always heavy. A big bag of popcorn can be lighter than a small stone.
Tip
The heavy side sinks. On a balance, the pan that goes down holds more mass. When the beam is level, the two sides are equal.
Work each one in your exercise book first. Then tap Show answer to check.
Which has the bigger mass: a sack of rice or a pencil?
Answer: The sack of rice. It is much heavier.
Would you measure a goat in grams or in kilograms?
Answer: In kilograms. A goat is much heavier than 1 kg.
A bag of beans balances with a 2 kg mass and a 500 g mass. What is its mass?
Answer: 2 kg 500 g, which is 2 500 g.
Mangoes are in one pan and a 1 kg mass is in the other. The mango pan goes down. Is the mass of the mangoes more or less than 1 kg?
Answer: More than 1 kg. The heavier side goes down.
Put in order from lightest to heaviest: an egg, a bag of sugar, a paper clip.
Answer: Paper clip (about 1 g), egg (about 50 g), bag of sugar (about 1 kg).
What you learned today
Sources and credits
The stories, Rwandan examples, drawings and practice questions were written by Ganzaa.
Quick check
On a balance, the pan with beans goes down. The pan with rice goes up. Which has the bigger mass?
