Measures in the primary curriculum

Mass - part 2

Of course, the weight and the mass are directly related: the greater the mass, the greater the weight, and therefore the heavier the object feels when I hold it in my hand. However, the big difference between the two is that, whereas the mass of an object is invariant, the weight changes depending on how far you are from the centre of the earth (or whatever it is that is exerting the gravitational pull on the object).

We are all familiar with the idea that an astronaut’s weight changes in space, or on the Moon, because the gravitational pull being exerted on the astronaut is less than it is on the Earth’s surface. In some circumstances, for example when in orbit, this gravitational pull can effectively be cancelled out and the astronaut experiences ‘weightlessness’. The astronaut can then place a book on the palm of his or her hand and it weighs nothing. On the Moon’s surface the force exerted on the book by gravity, that is, the weight of the book, is about one-sixth of what it was back on the Earth’s surface. But throughout all this the mass of the astronaut and the mass of the book remain unchanged. The book is still 200 grams, as it was on Earth, even though its weight has been changing constantly. (So a good way of losing weight is to go to the Moon, but this does not affect your waist size because what you really want to do is lose mass!)

Remember that with older pupils in the primary range you can discuss the effect of gravity and space travel on weight and the idea that mass does not change.