Mass: Tiny to Heavy

Overview

Students explore mass as how much matter an object contains, then practise choosing and reading milligrams, grams and kilograms. They connect familiar objects to appropriate metric units and explain their reasoning using measurement language.

Learning intentions

• WALT explain what mass means.
• WALT identify and use milligrams, grams and kilograms.
• WALT choose a sensible unit for measuring different objects.
• WALT read and record mass using the correct unit.

Success criteria

• I can explain that mass describes how much matter is in an object.
• I can name and use mg, g and kg correctly.
• I can select a sensible unit for an object and explain why.
• I can record a measurement with a number and its unit.

Curriculum links

• Mathematics and Statistics — Measurement: understanding and measuring mass using standard metric units.
• Mathematics and Statistics — Mathematical practices: communicating, representing, explaining and checking mathematical thinking.
• Mathsteasers: developing higher-order thinking through reasoning and challenging measurement questions.
• Mathsteasers / Alignment: connecting measurement ideas with relevant mathematical learning and challenge.

Lesson structure (40 minutes)

1. 0–5 min · Hook and prior knowledge. Display an image of a feather, an apple and a school bag using the opening comparison slides. Ask: “Which has the greatest mass? Which unit would help us measure each one?” Students make a silent prediction, then share with a partner. Accept everyday language such as “light” and “heavy”, while explaining that mass is measured precisely using units.

2. 5–12 min · Direct teaching. Use the mass and units teaching slides to define mass and introduce milligrams (mg), grams (g) and kilograms (kg). Explain that a milligram is very small, a gram is suitable for many small objects, and a kilogram is suitable for heavier objects. Show the relationships: 1,000 mg = 1 g and 1,000 g = 1 kg. Emphasise that the unit must always be written with the measurement. Students repeat and discuss examples, such as medicine measured in milligrams, a pencil measured in grams and a bag measured in kilograms.

3. 12–20 min · Unit sorting investigation. Place several safe classroom objects where all students can see them, such as an eraser, paper clip, book, drink bottle and school bag. Open the unit-choice investigation slides for the instructions and model one example. In groups of four, students record whether each object would most sensibly be measured in mg, g or kg, and justify one choice. Groups share one answer. Clarify that the chosen unit should make the number useful and easy to understand; for example, a school bag is more sensibly measured in kilograms than grams.

4. 20–30 min · Guided worksheet practice. Distribute the mass units practice worksheet. Students complete questions identifying suitable units, matching objects to units, converting between 1,000 mg and 1 g or 1,000 g and 1 kg, and recording measurements in a number-and-unit format. Pause after the first two questions to check that students understand the difference between “the amount” and “the unit”. Confer with groups and ask, “How do you know that unit is sensible?”

5. 30–36 min · Reasoning challenge. Display the mass reasoning challenge slides. Present: “A packet is labelled 2,000 g. Is it heavier, lighter or the same as 2 kg?” Students solve independently, explain their reasoning to a partner, then share. Follow with: “Would 5 mg, 5 g or 5 kg be sensible for a small button? Why?” Highlight that equivalent measurements can look different but describe the same mass.

6. 36–40 min · Plenary and exit check. Use the final review slides to revisit the three units and their relationships. Students complete the final three questions on the mass units practice worksheet: define mass in their own words, choose a unit for a classroom object, and complete “1 kg = ___ g”. Invite two students to explain different answers before collecting the worksheets.

Resources

• the mass and measurement slide deck
• the mass units practice worksheet
• Safe classroom objects: paper clip, eraser, pencil, book, bottle and school bag
• Demonstration balance or digital scale, if available
• Board and markers
• Pencils and rulers
• Optional kilogram, gram and milligram labels for teacher modelling

Assessment

• Listen during predictions and group sorting for correct use of “mass”, “light”, “heavy”, “mg”, “g” and “kg”.
• Check worksheet responses, particularly unit selection, the two 1,000-unit relationships and recording a number with its unit.
• Use the exit check to identify whether students need further teaching on the meaning of mass, choosing units or equivalence between units.

Differentiation

• Support students with a visible reference showing mg → g → kg, the relationships 1,000 mg = 1 g and 1,000 g = 1 kg, and sentence starters such as “I chose ___ because…”.
• Provide a reduced selection of familiar objects and allow students to explain answers orally before writing.
• Pair students strategically and pre-teach the words “mass”, “measure”, “unit”, “equal”, “lighter” and “heavier” for English language learners.
• Extend confident students by asking them to give two equivalent ways to describe a mass, or explain why measuring a school bag in milligrams would be impractical.