Measuring matter Matter is measured in units of mass called kilograms. Small quantities of matter are measured in grams and milligrams. There are 1,000 grams in a kilogram. Very large quantities of matter are measured in tonnes. A tonne is also a unit of mass. One tonne is 1,000 kilograms. Scientists often use abbreviations for these units: Milligram = mg Gram = g Kilogram = kg Tonne = t
1 g = 1000 mg
1 kg = 1000 g 1 tonne = 1000 kg
These units of mass are part of an international system of units called the Système International (SI system). The SI system is used in most countries of the world. Fig. 1 shows a special cylinder of metal which is kept at the International Bureau of Weights and Measures in Paris. Scientists have agreed that this particular piece of metal has a mass of exactly 1 kg. It is called the international prototype kilogram (IPK). Instruments that are used to measure mass make measurements based on the amount of matter in the original IPK. This means that when the mass of an object is measured the result should always be the same no matter where in the world the measurement is made.
Fig. 1 The international prototype
kilogram (IPK).
Mass and weight Mass and weight are not the same. Gravity causes matter to have weight. Without gravity, matter does not have any weight. With gravity, objects with a large mass have a large weight and objects with a small mass have a small weight. This means that the weight of an object is proportional to its mass. If the force of gravity changes, then the weight of objects change. For example, the force of gravity is less on the moon than it is on Earth. Therefore, objects weigh less on the moon than they do on Earth. Because weight is proportional to mass, scientists can calculate the mass of an object by measuring its weight. This can be written as a
formula: Weight = Mass x Gravity
A spring balance can be used to measure both the weight and the mass of an object. The extension of the spring is proportional to the weight of the object. The scale on the spring balance can show the weight or the mass or both. An electronic balance works in a similar way. However, the display usually shows only the mass of the object.
