# Mass and weight

Mass and weight

The mass of a body is the quantity of matter it contains, and the basic SI unit of mass is the kilogram. (Fig. 1.7.) The standard kilogram is the mass of a certain cylindrical piece of platinum-iridium alloy kept at Sevres, Its various multiples and sub multiples are given below:

1 tonne (t) = 1000 kg
1 kg = 1000 grams (g)
1 g = 1000 milligrams (mg)
1 g = 1000000 micro grams (g)

The kilogram was originally intended to be equal to the mass of 1000 cubic centimeters (cm ‘) of pure water at the temperature of its maximum density, 4 C. But a slight error was made at the time and the kilogram is actually equal to the s of 1000.028 cm ‘ of water.

The weight of a body is the force it exerts on anything which freely supports it and, normally, it exerts this force owing to the fact that it is itself being attracted towards the earth by the race a/gravity. This will be discussed in more detail in later chapters. In everyday conversation the distinction between mass and weight is relatively unimportant: a butcher who had not studied physics would doubtless be surprised if a housewife who had done so, asked him what was the mass of her weekend joint. In science, however, we must be careful to distinguish between them. The mass and weight of a body are different and are measured in different units. As we have already said the unit of mass is the kilogram, but the unit of weight which is a force is the newton (N) (see page 37). An important distinction between mass and weight is that the mass of a body does not depend on where the body happens to be, whereas the weight of a body can vary from place to place. Changes which can occur in the weight of a body are discussed in chapter 2.