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The SI base units are the standard units of measurement defined by the International System of Units (SI) for the seven base quantities of what is now known as the International System of Quantities: they are notably a basic set from which all other SI units can be derived.
- Second – Unit of Time. Symbol: s. The second is defined by taking the fixed numerical value of the cesium frequency ∆ν, the unperturbed ground-state hyperfine transition frequency of the cesium-133 atom, to be 9,192,631,770 when expressed in the unit Hz, which is equal to s.
- Meter – Unit of Length. Symbol: m. The meter is defined by taking the fixed numerical value of the speed of light in vacuum c to be 299,792,458 when expressed in the unit m s, where the second is defined in terms of ∆ν.
- Kilogram – Unit of Mass. Symbol: kg. The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.62607015 ×10 when expressed in the unit J s, which is equal to kg m2 s, where the meter and the second are defined in terms of c and ∆ν.
- Ampere – Unit of Electric Current. Symbol: A. The ampere is defined by taking the fixed numerical value of the elementary charge e to be 1.602176634 × 10 when expressed in the unit C, which is equal to A s, where the second is defined in terms of ∆ν.
In the International System of Units (SI), the unit of time is the second (symbol: s). It has been defined since 1967 as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", and is an SI base unit . [ 12 ]
The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 billion oscillations of the caesium atom.
NameLengthNotes5.39 × 10−44 sThe amount of time light takes to travel ...10−30 sOne nonillionth of a second.10−27 sOne octillionth of a second.10−24 sOne septillionth of a second.- Length: Meter (m) The meter is the metric unit of length. It's defined as the length of the path light travels in a vacuum during 1/299,792,458 of a second.
- Mass: Kilogram (kg) The kilogram is the metric unit of mass. It's the mass of the international prototype of the kilogram: a standard platinum/iridium 1 kg mass housed near Paris at the International Bureau of Weights and Measures (BIPM).
- Time: Second (s) The basic unit of time is the second. The second is defined as the duration of 9,192,631,770 oscillations of radiation corresponding to the transition between the two hyperfine levels of cesium-133.
- Electric current: Ampere (A) The basic unit of electric current is the ampere. The ampere is defined as the constant current that, if maintained in two infinitely long straight parallel conductors with a negligible circular cross-section and placed 1 m apart in a vacuum, would produce a force between the conductors equal to 2 x 10-7 newtons per meter of length.
Units of Time, Length, and Mass: The Second, Meter, and Kilogram. The initial chapters in this textmap are concerned with mechanics, fluids, and waves. In these subjects all pertinent physical quantities can be expressed in terms of the base units of length, mass, and time.
resources. Discussion. what is time? Zeit ist das, was man an der Uhr abliest. [Time is what a clock measures.] Albert Einstein (1879–1955) Nam Tempus, Spatium, Locum & Motum, ut omnibus notiſſima, non definio. [I do not define time, space, place, and motion, as being well known to all.] Isaac Newton (1642–1727)
