Section 4

Working with weights


Learning Outcomes

By the end of this section, you will have:
  • helped pupils develop their understanding of weight by using progressive practical activities;
  • explored ways to establish pupils’ understanding of the need and use of standard units for weight;
  • explained different ways to organise your class.

When exploring weight with pupils it is important to use a lot of practical hands on activities in the early stages so that they are able to build up mental models which will help their understanding in later stages. In this section you will plan ways to introduce your pupils to the concept of weight by following three stages:
  • comparing weights of two or more objects by holding them at the same time
  • estimating and measuring weight of objects using non-standard units such as stones
  • measuring and comparing the weight of objects using standard units
Central to this work is the use of simple balances and using standard units that can be made from cheap and readily available materials, to enable practical group work and active learning.

Page 1

Estimation is an important skill in both mathematics and science and a useful skill to cultivate for all pupils. Simple balances can be made with very modest resources that allow pupils to approach measuring and estimating weight through practical investigation. You may like to make simple balances and plan and carry out these activities jointly with a science teacher in your school. This can be done by helping pupils to compare and contrast weights in different ways.

Case Study 1: Estimating weight

Mrs Nkumu in Nigeria was on a teachers’ course at her local district offices and as part of the day’s lesson on numeracy the facilitator told the following story to them. Then she asked them what they thought the girls knew and what would they do next with these pupils if they were in their class.Two girls, Ranke and Ade, were discussing the quantity of popcorn in two packets, A and B which looked the same shape and size. Ronke picked up the packets one after the other and was surprised that B felt heavier than A. She told Ade that B appeared to be heavier than A. Ade decided to put the two packets in the two pans of a simple balance. (See Resource 1: Simple balance). She observed that on the scale, packet B went down and so B is heavier than A. Ronke was right.’ The teachers worked in pairs and devised activities that encouraged estimation of heavier/lighter, then using a balance or scales to test their ideas. Each pair tried their lesson out with their class and reported back next session. Mrs Nkumu found that her class enjoyed the lesson but that she did not have enough different objects for the pupils to use. Next time she said she would spend more time collecting objects and she would use smaller groups of 4-6 rather than over 10 each.

Activity 1: Comparing weight

You will need 5 simple balances (see Resource 1) to carry out this activity and 5 sets of common objects e.g. stones, balls, tins, bottle tops etc that could be used with the balances. Write instructions for your pupils on the board (see Resource 2: Worksheets) and show the class what you want them to do using any two objects. Ask them to estimate which is heavier by giving the objects to 2 pupils to answer. Now ask a pupil to test their idea out by putting them on the pan and let them decide. Ask them which is the heaviest object and why they think so. Organise your pupils into 5 groups, giving each group a set of objects and a balance. Ask pupils to find which object is heavier by estimating its weight and then by using the balance. (See Key Resource: Using group work in your classroom) Ask them to fill in a table of their results to share with the class to see if everyone agrees. You could challenge older or more able pupils to see if they can order their objects from heaviest to lightest before testing. How could they test their answers using the simple balance?

Page 2

When developing understanding of the idea of weighing it is better if non-standard units are used to measure first. If pupils compare and contrast weights against non-standard unit bottle tops or beans they will quickly understand this is not sensible as the weight of different bottle tops and seeds vary. This is made easier by making sure they have sufficient experience weighing objects against different non standard units. Only when pupils understand the need for a common unit should the introduction of standard units like grams or kilograms begin.

Case Study 2: Using a ‘standard’ unit to measure

Lizzy, a Primary school teacher, felt that having taught her pupils how to use a simple balance to compare weights of objects, they should now compare the weight of any object with that of a given chosen ‘standard’ object. She assembled different objects and chose dry beans to be her chosen measure. Using the balance she asked two pupils to place an object on one pan and put enough beans on the other until it balanced. They counted the beans for each object and recorded their results. Next she used some longer beans and weighed the same objects and recorded these results. She talked with the class about the difference in numbers between the two kinds of bean and how difficult it was to compare the weight of a stone and wood if one person used one set of beans and the other used the bigger beans.

Activity 2: Data presentation

Before doing this, read Resource 3: Pupil instructions and collect together the following resources – enough for your size class.
  • simple balances
  • collect objects of similar weights to use as measures (e.g. bottle tops and beans)
  • objects of different weights to measure (e.g. small bottles, tins or stones).
You could just collect enough for one group and have each group take turns to do the activity while the others do different work. Write the instructions for the groups on the board and explain what they have to do. Resource 4: Traditional weights used in Ghana. These are examples of Akan weights used in trading. Notice that they are fashioned very beautifully. (See Key Resource: Using group work in your classroom) At the end, ask them to compare how it was different using beans or bottle tops to measure the weight, rather than just comparing pairs of objects. Note their answers on the board. Ask if they think this is a fairer way to measure. Ask pupils to list the objects in order from heaviest to lightest – is this harder or easier than before? Why?

Page 3

The previous activities should have shown your pupils that ‘standard units‘ of weight are needed because without these it is not possible to accurately compare or know how much something weighs. This next part explores how you can introduce terms and develop their understanding of kilogram (kg), and gram (g) (1000 gram = 1 kilogram). You may want to bring bags of sugar and rice to class, to show their weight is recorded in grams or kilograms and for them to feel the actual weights or make some mock ones i.e. plastic bags filled with sand, stones etc to the correct weight. If you can, borrow a pair of scales to do this.
If you do not have access to calibrated weighing scales or weights at school, it may still be possible to make approximate measurements of weight using your simple balances, and using some every day objects that have their weight on them to test them against. The second focus in this part is on understanding the terms gram and kilogram and being able to convert one to the other. This kind of activity should only be undertaken when pupils are confident at weighing in grams and kilograms.

Case Study 3: Using homemade standard units

Mr.Adu wants his pupils to estimate, measure and compare weights of 1 object in grams and kilograms. He asked permission from a secondary school science department to use their balances to make bean-bags weighing 100g, 50g and 10g (using different coloured cloth for each weight). He asked some of the parents who work in the local sewing shop to help him sew several sets for his class. He demonstrated the weighing of objects in grams using the improvised weights and a simple balance, and then asked pupils to weigh objects to the nearest 10g, and record their results in a table.

The pupils were very enthusiastic and weighed nearly everything they could find in the classroom. Mr Adu listened to their talk as they weighed and was pleased to hear them using the correct terms easily

Key Activity: Weighing in grams

Before the lesson, collect a number of objects that have their weights shown – tinned or packet foods and other goods (you only need the wrapper, not the whole good). Try to have enough to give each group of pupils at least two or three labels. It would also be good to have some labels for weights in kilograms as well as grams. Ask groups to write down the name of the product, and its weight – ensure that they include the correct units (grams or kilograms). They could do this by using the actual bags and putting them in order on their table. Pupils could arrange and re-arrange the packages by weight from highest to lowest or lowest to highest or sort into groups:
  • > 500 grams or
  • < 500 grams
Then ask pupils to convert each weight from kilograms to grams or vice versa. When they have finished ask each group to swap their sheets with another group and they can check each others answers. Remind them that 1000 gram = 1 kilogram. Discuss with your pupils why they might need to be able to convert weights in their everyday lives. Display their work on the wall to show each group their achievements. What did the groups learn and how do you know this? You could ask them to tell you what they think they have learnt.

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