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1.6: International System of Units

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Mechanical Engineering

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International System of Units

1.6: International System of Units

The International System of Units, known as the SI system, is a universally accepted measurement system recognized and used worldwide. The SI system is based on a set of three base units considered absolute, and their values do not change with location. These base units are meters, kilograms, and seconds.

Prefixes are used to define both larger and smaller quantities in the SI system. For example, milli, micro, and nano define smaller quantities, while kilo, mega, and giga are used to define larger quantities. The kilogram is the only base unit defined with a prefix among the SI system's base units.

Several rules must be followed while expressing units and prefixes. Firstly, dots are used to differentiate units into composite units. Secondly, a prefix is avoided in the denominator and is only expressed in the numerator of a composite unit. Thirdly, an exponential power on a unit with a prefix applies to the prefix and its unit. Finally, results are represented using a single prefix.

It is essential to note that, unlike the metric system, the SI system does not use multiples of deca or centi. The SI system's prefixes are based on powers of ten, with the base unit having no prefix. SI units are intended to simplify and standardize scientific measurements by leveraging a universal system.

One of the primary reasons the SI system is used worldwide is its accuracy and precision in measurement. Even the most minuscule and massive quantities can be quantified accurately with their base units and prefixes.

The SI system's base unit of length is the meter, defined as the distance light travels in a vacuum during a specific time frame. Other measurements, such as a centimeter, a kilometer, and a millimeter, can be derived from the meter using the prefix system.

The SI system's base unit of mass is the kilogram, defined as the amount of mass of a set of foundational objects known as the International Prototype of the Kilogram. Other mass measurements, such as grams and milligrams, can be derived from the kilogram using the prefix system.

The SI system's base unit of time is the second, defined as the duration of 9,192,631,770 cycles of the radiation associated with a specific transition between energy levels of a cesium-133 atom. From the second, other time measurements, such as a minute and an hour, can be derived using the prefix system.

In addition to these units, four more are defined as the fundamental units of measurement. They are the ampere, Kelvin, mole, and candela.

Suggested Reading


SI System International System Of Units Base Units Meters Kilograms Seconds Prefixes Milli Micro Nano Kilo Mega Giga Composite Units Exponential Power Metric System Deca Centi Accuracy Precision

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