Welcome to our Physics lesson on Mass, this is the second lesson of our suite of physics lessons covering the topic of Length, Mass and Time, you can find links to the other lessons within this tutorial and access additional physics learning resources below this lesson.
Mass is the quantity of matter a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field (see the concepts of force, acceleration and gravitational field in the corresponding articles).
For now, we can only limit in the "quantity of matter" in explaining the meaning of mass. Thus, since all materials are composed by tiny particles (atoms and molecules), their number contributes in making a certain object more massive (heavier) or less massive (lighter). A massive (heavier) object has a very large number of atoms than a less massive (lighter) one.
In the SI system, the unit of mass is kilogram (kg). But why is kilogram (a multiple of gram) used as a base unit for mass instead of gram? The answer is: for political reasons.
Kilogram is also an arbitrary established unit of mass. The mass of a cylinder of platinum-iridium alloy kept in the International Bureau of weights and measures preserved at Serves near Paris, is called one kilogram.
In the original metric system, the unit of mass was the grave, which was equal to 1000 grams. Since the gram itself was too small to be measured reliably, it was not practically used as the standard, so the larger grave was the unit of mass.
Grave was not used anymore after the French Revolution for political reasons. The old grave unit was replaced with its equivalent: 1000 grams, which could also be named a kilogram using the system of prefixes.
The units of mass also change by the powers of ten. We use one of the prefixes discussed in the article "Units. Systems of Units. Fundamental and Derived SI Units" to express multiples and submultiples of gram (the main unit kg is one of them). They are:
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