Measurement progression, 4th step
The purpose of the activity
In this activity, the learners develop an understanding of weight and mass. The learners also develop their skill at using benchmarks of 1 kilogram and 500 grams to aid in estimating the weight of given objects.
The teaching points
- The learners will understand the need for having and using standard measures of weight (kilograms and grams).
- The learners will know the conversions between kilograms and grams (1,000 grams = 1 kilogram)
- The learners will develop and then use benchmarks (for example, visualising the weight of 1 kilogram) to carry out estimation tasks.
- Discuss with the learners relevant or authentic situations where the understanding of kilograms and grams is necessary.
- Weighing devices for objects less than 3 kilograms (as applicable: electronic, analogue, spring, balance scales).
- Objects that weigh 1 kilogram (for example, 1 litre of milk, a 1-kilogram block of cheese) or 1/2 kilogram (for example, a block of butter, a tub of margarine).
- A range of objects.
- Supermarket bags.
The guided teaching and learning sequence
1. Begin by showing the learners three ‘closed’ supermarket bags (labeled a, b, c) and posing the questions:
“Which is the heaviest?”
“Which is the lightest?”
Give the learners the opportunity to lift the bags and make their decision.
Check that everyone agrees on the heaviest and lightest bags. If not, ask those who disagreed or who were uncertain to directly compare two of the bags by holding one in each hand. Ask the learners if they have any idea about the weight of the heaviest bag.
“How heavy do you think the heaviest bag is?”
“Why do you think that?”
2. Discuss the difficulty of estimating the weight of objects. This is usually due to the fact that we don’t have much practice at estimating weight. Also we don’t have the same ‘personal’ means with which to benchmark like we do for length (for example, fingertip to shoulder is about 1 metre).
3. Ask the learners to suggest objects that could be used as benchmarks for a 1-kilogram weight. Possibilities include: 2 tubs of margarine, 2 blocks of butter, a litre of milk or juice, a kilogram of cheese.
4. Check the learners understand that the metric prefix ‘kilo’ means 1,000.
“What does kilo stand for in kilogram?”
“How many kilograms in 2,000 grams?”
“How many grams in 6 kilograms?”
5. Distribute a 1 kilogram and 500 gram object to the learners working in groups. Give the learners the opportunity to handle the objects.
6. Prepare a number of labelled bags filled with objects that in total weigh between 200 grams and 3 kilograms. Ask the learners to estimate the weight of each bag using the following ranges:
- under 500 grams
- 500 grams to 1 kilogram
- 1–1.5 kilograms
- 1.5–2 kilograms.
7. Examine with the learners the weighing devices that you have available, discussing the level of accuracy permitted by each device. Also discuss with the learners the purpose of having standard units of measure so weights can be accurately communicated.
8. Ask for volunteers to select one of the bags from above and weigh this on a set of scales. Use this to check the accuracy of estimated weights and to reinforce the correct use of the weighing device.
“What would you use to estimate 20 grams if you didn’t have a measuring device?”
10. Next ask learners to work in pairs or groups to fill a plastic bag with rice so that it weighs 20 grams.
11. Give the learners the opportunity to check their estimates on the measuring devices.
Ask the learners to fill a bag with objects until they estimate that the bag weighs 1 kilogram. Use measuring devices to find the ‘winner’.
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