Mass Units of Work

The mass of an object is a measure of the amount of matter in it. Weight is the force that gravity exerts on an object and so can vary from place to place. The terms mass and weight are used loosely, and inaccurately, in everyday speech to mean the same thing. The NZ curriculum document reflects the correct use of the terminology and uses the term mass not weight.

Stage One: Identifying the Attribute

Early experiences must develop an awareness of what mass is, and of the range of words that can be used to describe it. A mass need to be brought to many students' attention as it is not an attribute that can be seen. They should learn to pick up and pull objects to feel their heaviness. Initially young students describe objects as heavy or not heavy. They gradually learn to compare and use more meaningful terms such as lighter and heavier.

Stage Two: Comparing and Ordering

Comparing the masses of objects is the second stage in developing an understanding of mass. Balance scales and see-saws can be used to directly compare the mass of two objects.

Young students are influenced by what they see and can be easily deceived by the shape or the size of an object. For example, students who do not yet conserve the property of mass will think that if the shape of an object changes so does its mass.

Two pieces of plasticine can be compared and found to be the same mass. The student can then change the shape of one of the pieces. Many young students will now say that the longer object has greater mass. Students who are able to conserve the property of mass can reason that because nothing has been added or taken away they both have the same mass

Many students require lots of practical experience to disassociate mass from size and to accept that small object can be heavy and large objects light.

Stage 3: Non-Standard units

Measuring the mass of objects using non-standard or informal units is the third stage in the learning sequence. Beginning with non-standard but familiar units allows the students to focus on the process of repeatedly using a unit as a measuring device. Students should be given lots of opportunities to use balance scales or "home-made" beam balances and objects such as blocks, marbles and felt pens to measure a wide range of objects.

Home-made beam balances can be constructed, for example, from a shoe-box lid fixed to a can. The can be be kept in place with rolls of plasticene on each side of the can.

From the earliest of these experiences, students should be encouraged to estimate. Initially these estimates may be no more than guesses, but estimating involves the students in developing a sense of the size of the unit. As everyday life involves estimates at least as frequently as exact measures the skill of estimating is important.

At this stage students can also be introduced to the appropriateness of measuring units. For example, a block is more appropriate than a paper clip for measuring the mass of a book.

Although non-standard units reinforce most of the basic measuring principles students need to realise that they are limited as a means of communication. This can be highlighted through activities that involve the students measuring a single object using non-standard units, for example, books.

Stage 4: Standard Units

When students can measure lengths effectively using non-standard units, they are ready to move to the use of standard units. The motivation for moving to this stage often follows from experiences where the students have used different non-standard units for the same mass and have realised that consistency in the units used would allow for the easier and more accurate communication of mass measures.

Students’ measurement experiences must enable them to:

1. Develop an understanding of the size of the standard unit
2. Estimate and measure using the unit

It is sensible to begin with the kilogram as the gram is too small to "feel". An appreciation for the feel of a kilogram needs to be built up with lots of examples of 1 kilogram mass, for example, 1 kilogram bags of stones, polystyrene, sand, butter and nails. The students should compare a standard 1 kilogram mass with other objects first by holding and then by using a balance scale.

The usual sequence is then to divide the kilogram into smaller parts, for example, ½ a kilogram (500 grams), ¼ of a kilogram (250 grams) and 1/10 of a kilogram (100 grams). Students can focus on 100 grams as many familiar items are packaged in 100 grams.

Students should also be introduced to the use of spring balances to measure the mass of objects.

As the students become familiar with the size of the kilogram and gram they should be given many opportunities to estimate before measuring.

Stage 5: Applying and Interpreting

Many measuring devices for mass have scales that the students need to interpret. For example, bathroom scales usually show a simple scale with the main intervals being in kilograms whereas the main interval on kitchen scales is 100 grams.

Students should also be aware of the difference between gross and net weight. Later on they can investigate the density of objects by measuring masses and volumes.