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In physics and metrology, units are standards for measurement of physical quantities that need clear definitions to be useful. Reproducibility of experimental results is central to the scientific method. To facilitate this we need standards, and to get convenient measures of the standards we need a system of units. Scientific systems of units are a formalization of the concept of weights and measures, initially developed for commercial purposes.
Different systems of units are based on different choices of a set of fundamental units. The most widely used system of units is the international system, or SI system, of units derived from the seven SI base units. All other SI units can be derived from these base units.
Other systems of units that have been used for various purposes include:

Any value of a physical quantity is expressed as a comparison to a unit of that quantity. For example, the value of a physical quantity Q is written as the product of a unit [Q] and a numerical factor:
The multiplication sign is usually left out, just as it is left out between variables in scientific notation of formulas. In formulas the unit [Q] can be treated as if it was a kind of physical dimension: see dimensional analysis for more on this treatment.
A distinction should be made between units and standards. A unit is fixed by its definition, and is independent of physical conditions such as temperature. By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions. For example, the metre is a unit, while a metal bar is a standard. One metre is the same length regardless of temperature, but a metal bar will be one metre long only at a certain temperature.
For most quantities a unit is absolutely necessary to communicate values of that physical quantity. Try for example to tell someone the value of a length without the use of a unit. That is not possible because you can't verbally describe a length.
But not all quantities require a unit of their own. Using physical laws, units of quantities can be expressed as combinations of units of other quantities. Thus only a small set of units is required. These units are taken as the basic units. Other units are derived units. Derived units are a matter of convenience, as they can be expressed in terms of basic units. Which units are considered basic is a matter of choice.
The basic units of SI are actually not the smallest set. Smaller sets have been defined. There are sets in which the electric and magnetic field have the same unit. This is based on physical laws that show that electric and magnetic field are actually different manifestations of the same phenomenon. In some fields of science such systems of units are highly favored over the SI system.
Conversion of units involves comparison of different standard physical values, either of a single physical quantity or of a physical quantity and a combination of other physical quantities.
Thus conversion factors between units are always imprecise to some level and improved values may be found when a more precise comparison is performed.
In the SI system some letters denoting conveniently chosen numerical values can be used as prefixes to any of the units.
For example, c = 0.01, and thus cm = 0.01 * m and cN = 0.01 * N
There is one exception: for historical reasons, the unit of mass, kg, already contains a prefix and prefixes are not to be added to it but to g. Thus: mg and not µkg (with "µ" = "micro"). To many this is a source of mistakes and frustration; see Talk.
Use of prefixes does not involve any unit conversion, as the prefixes are just defined as numerical values. They can not be imprecise.
For example, the expressions 'cm' and '0.01 m' mean mathematically exactly the same thing. It is not a unit conversion, just a mathematical conversion, just like '4 * 5' and '20' are mathematical expressions with the same meaning.
Hints:
This is not true. The correct statement is that density is mass divided by volume:
The sentence `density is mass per unit volume' uses another way of perceiving the concept. It says that the density D_s of system s is the mass m_u of a subsystem u of s, divided by the volume V_u of subsystem u, given that the volume of subsystem u is unit volume:
Mathematical rules for calculations with units follow from the formula for physical values, Q = n * [Q]
The units in the physical values have to be converted so that they are the same. Then the numerical values can be added. The same principle is known from adding fractions: you have to make the denominators the same and then you can add the numerators.
Starting with:
just replace the origional unit [Q]_i with its meaning in terms of the desired unit [Q]_f, e.g. if [Q]_i = c_ij * [Q]_f, then:
Now n_i and c_ij are both numerical values, so just calculate their product.
Or, which is just mathematically the same thing, multiply Q by unity, the product is still Q:
For example, you have an expression for a physical value Q involving the unit feet per second ([Q]_i) and you want it in terms of the unit miles per hour ([Q]_f):
See also: units unit conversion computer program
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