Measures in the primary curriculum

Mass - part 3

An important point to note is that the balance-type weighing devices do actually measure mass. We put the book in one pan, balance it with a mass of 200 grams in the other pan, and because the book ‘weighs the same as’ a mass of 200 grams we conclude that it also has a mass of 200 grams. Note that we would get the same result using the balance on the Moon. However, the pointers on spring-type weighing devices, such as many kitchen scales and bathroom scales, actually respond directly to weight. This means that they would give a different reading if we took them to the Moon, for example. But, of course, they are calibrated for use on the Earth’s surface, so when I stand on the bathroom scales and the pointer indicates 72 kilograms I can rely on that as a measurement of my mass. On the Moon it would point to 12 kilograms, this would be totally wrong.

Remember to refer to the things we use for weighing objects on a balance as masses and use the language weighs the same as a mass of so many grams. Then encourage pupils to say ‘the mass is so many grams’, whilst acknowledging that most people incorrectly say ‘the weight is so many grams’

Because weight is a force, it should be measured in the units of force. The standard unit of force in the metric system is the Newton, appropriately named after Sir Isaac Newton (1642-1727), the mathematical and scientific genius who first articulated this distinction between mass and weight. A Newton is defined as the force required to increase the speed of a mass of 1 kilogram by 1 metre per second every second. A Newton is actually about the weight of a small apple and a mass of a kilogram has a weight of nearly 10 newtons. You probably don’t need to know this, although you may come across spring-type weighing devices with a scale graduated in newtons.

One way to introduce the word ‘mass’ to primary-school pupils is to refer to those plastic or metal things we use for weighing objects in a balance as ‘masses’ (rather than weights’). So we would have a box of 10-gram masses and a box of 100-gram masses, and so on. Then when we have balanced an object against some masses, we can say that the object weighs the same as a mass of so many grams, as a step towards using the correct language, that the mass of the object is so many grams.