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### Quantity of Matter

Materials quite obviously take up space; we say they have volume. In Britain we buy petrol by the gallon, on the continent we buy it by the .litre. The gallon, the litre, are all units of volume measures of a quantity of material bought or sold. But it is also quite common to buy and sell things by weighing them in ounces, pounds, kilograms, or tons.

The simplest form of weighing machine consists of a balanced lever with equal arms. When two identical lumps of material are hung from the ends of the arms, they exactly balance each other. If one of the lumps, say the coin is replaced by something quite different but which still balances it, then we say that the two things have the same quantity of matter. To quantity of matter measured in this way we give the name mass.

In this balancing method we are really balancing two forces, the weights of the objects. It is important to distinguish between the mass which we measure this way and the weight which helps us to do so. The heaviness of objects is due to the attraction which our planet, the Earth, has for them. This heaviness is different at other places. Thus it has been calculated that objects on the Moon have only one-sixth of their earth-weight; a 10-stone boy would weigh only 23 pounds on the Moon, would find it possible to jump 30 ft. and throw a cricket ball a quarter of a mile. The space traveller of the future will find that 50 pounds of luggage become less heavy the further he goes from the Earth. But the quantity of it - its mass - will not change; it will still balance 50 pounds on a lever balance.

The standards of mass on the metric system and on the British system are the kilogram and the pound respectively. The abbreviations for these are kg. and Ib. The abbreviations for the f.orces which the Earth has on them are Kg (for kilogram-weight) and Lb (for pound-weight).

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