Measures in the primary curriculum

Mass - part 1

What’s the difference between mass and weight?

There is a real problem here about the language we use to describe what we are measuring when, for example, we put a book in one pan of a balance and equalise it with, say, 200 grams in the other, Colloquially, most people say that what we have found out is that the book weighs 200 grams, or that its weight is 200 grams. This is technically incorrect. What we have discovered is that the book weighs the same as a mass of 200 grams, or that the mass of the book is 200 grams. This conflict between everyday language usage and the scientifically correct usage is not resolved simply by using the two words, mass and weight, interchangeably.

Remember that Pupils should develop skills and understanding of measurement through practical, purposeful activities. They should learn to choose and use appropriate measuring devices, discussing the ideas of accuracy and approximation.

The units we use for weighing, such as grams and kilograms, are actually units for measuring the mass of an object, not its weight. The mass is a measurement of the quantity of matter there is in the object. Note that this is not the same thing as the amount of space it takes up – that is, the volume of the object. A small lump of lead might have the same mass as the 200-gram book, but it would take up much less space, because the molecules making up the piece of lead are much more tightly packed together than those in the book.

The problem with this concept of mass is that we cannot actually experience it directly. I cannot see the mass of the book, feel it or perceive it in any way. When I hold the book in my hand what I experience is the weight of the book. The weight is the force exerted on the book by the pull of gravity. I can feel this, because I have to exert a force myself to hold the book up.