# Mass and time

Instruments used to measure mass (scales) and time (clock).

It was in the year 1960 that an international committee established rules to define the standards of fundamental quantities. The system since established is a generalization of the metric system and is called the International System of Units, whose abbreviation is ( **SI** ). In this system, the units of mass and time are the kilogram and the second, respectively.

**Mass**

It is common in our daily lives to use some units of measurement, such as the unit of mass. When we buy fruits, vegetables, meats, etc., all this purchase is measured by a scale that provides the mass of each item, whether in kilograms or grams.

The unit of mass kilogram, whose abbreviation is * kg*, was defined as the mass of a platinum-iridium cylinder, deposited at the International Bureau of Weights and Measures in Sèvres, outside Paris. This mass standard was made in 1887 and has never been altered.

For all practical purposes, it is equal to the mass of 10 ^{-3} m ^{3} of distilled water at a temperature of 4°C. The mass of 1 m ^{3} of water is therefore 10 ^{3} kg at that temperature.

For historical reasons, the name of the SI unit of mass contains the prefix kilo, so the multiples and submultiples of that unit are formed based on the gram. The metric ton, abbreviated t, corresponds to 10 ^{3} kg.

The most commonly used mass units in our daily lives are:

► 1 ton = 10 ^{3} kg = Tonne

► 1 kg = Kilogram

► 1 g = 10 ^{-3} kg = Gram

► 1 mg = 10 ^{-6} kg = Milligram

**Time**

Before the year 1960, time was recorded in a standardized way , using the second as a unit, whose abbreviation was s. The **second** was defined as

of the mean solar day (something that could not be defined exactly). In 1967, the second unit of time was redefined as a function of a characteristic frequency of a particular kind of transition of the cesium atom, which became the “frequency clock” and, today, the second is the duration of 9,192 .631,770 periods of oscillation corresponding to the transition between the two hyperfine levels of the ground state of the resting cesium-133 atom.

We use minutes and hours as practical submultiples (outside SI):

► 1 min = 60 s

► 1 h = 60 min

► 60 min = 3,600 s