1
Heavy and light
Learner’s Book 1: Unit 12, pp. 166–168
GoalSay which of two things is heavier and which is lighter, and use balance scales to check.
New words
mass · how heavy something isheavy · has a lot of masslight · has only a little massbalance scales · scales with two pans that show which side is heavier
Explanation
We can measure how long something is, how heavy it is, how much it can hold and how hot it is. Today we look at how heavy things are. How heavy something is, is called its mass. A watermelon has a lot of mass, so it is heavy. A grape has only a little mass, so it is light. Balance scales help us compare. We put one thing in each pan. The heavier thing makes its pan go down, and the lighter thing goes up.
Worked examples
The key is in the left pan and the bike is in the right pan. The right pan goes down, so the bike is heavier than the key. The key is lighter than the bike.
Big does not always mean heavy! A big balloon is full of air and has very little mass. A small stone has more mass, so the stone’s pan goes down.
You can feel mass with your hands. Hold a full school bag in one hand and an empty bag in the other. The full bag pulls your hand down, so it is heavier.
Class activity
Make your own balance scales: hang a paper cup on a string from each end of a coat hanger, then hang the coat hanger from a hook. Put one small thing in each cup. Which side goes down? Say ‘… is heavier than …’.
Practice
1
Which is heavier, the leaf or the table?
the table
2
Which is lighter, the bus or the ant?
the ant
3
A basket of apples and one apple are on a balance. Which has more mass?
the basket of apples
4
Sami has a big empty box and a small bag of sugar. Big things are not always heavy. How can he find out which one is heavier?
Put them on balance scales (or hold one in each hand). The side that goes down is heavier.