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Système international d'unités (SI)
- Snippet from Wikipedia: International System of Units
The International System of Units, internationally known by the abbreviation SI (from French Système international d'unités), is the modern form of the metric system and the world's most widely used system of measurement. It is the only system of measurement with official status in nearly every country in the world, employed in science, technology, industry, and everyday commerce. The SI system is coordinated by the International Bureau of Weights and Measures, which is abbreviated BIPM from French: Bureau international des poids et mesures.
The SI comprises a coherent system of units of measurement starting with seven base units, which are the second (symbol s, the unit of time), metre (m, length), kilogram (kg, mass), ampere (A, electric current), kelvin (K, thermodynamic temperature), mole (mol, amount of substance), and candela (cd, luminous intensity). The system can accommodate coherent units for an unlimited number of additional quantities. These are called coherent derived units, which can always be represented as products of powers of the base units. Twenty-two coherent derived units have been provided with special names and symbols.
The seven base units and the 22 coherent derived units with special names and symbols may be used in combination to express other coherent derived units. Since the sizes of coherent units will be convenient for only some applications and not for others, the SI provides twenty-four prefixes which, when added to the name and symbol of a coherent unit produce twenty-four additional (non-coherent) SI units for the same quantity; these non-coherent units are always decimal (i.e. power-of-ten) multiples and sub-multiples of the coherent unit.
The current way of defining the SI is a result of a decades-long move towards increasingly abstract and idealised formulation in which the realisations of the units are separated conceptually from the definitions. A consequence is that as science and technologies develop, new and superior realisations may be introduced without the need to redefine the unit. One problem with artefacts is that they can be lost, damaged, or changed; another is that they introduce uncertainties that cannot be reduced by advancements in science and technology.
The original motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second (CGS) systems (specifically the inconsistency between the systems of electrostatic units and electromagnetic units) and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures (French: Conférence générale des poids et mesures – CGPM), which was established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and to standardise the rules for writing and presenting measurements. The system was published in 1960 as a result of an initiative that began in 1948, and is based on the metre–kilogram–second system of units (MKS) combined with ideas from the development of the CGS system.
Basic Quantities and Units
The seven basic (or primary) quantities and their units in SI
| Quantity | Birim | Sembolik |
|---|---|---|
| Length | metre | m |
| Mass | kilogram | kg |
| Time | saniye | s |
| Temperature | kelvin | K |
| Electric current | amper | A |
| Luminous intensity | candela | cd |
| Amount of Substance | mol | mol |
Multipliers
| Multiplier | Prefix | Symbolic |
|---|---|---|
| $ 10^{30} $ | quetta | Q |
| $ 10^{27} $ | ronna | R |
| $ 10^{24} $ | yotta | Y |
| $ 10^{21} $ | zetta | Z |
| $ 10^{18} $ | exa | E |
| $ 10^{15} $ | peta | P |
| $ 10^{12} $ | tera | T |
| $ 10^{9} $ | giga | G |
| $ 10^{6} $ | mega | M |
| $ 10^{3} $ | kilo | k |
| $ 10^{2} $ | hecto | h |
| $ 10^{1} $ | deka | da |
| $ 10^{-1} $ | deci | d |
| $ 10^{-2} $ | centi | c |
| $ 10^{-3} $ | milli | m |
| $ 10^{-6} $ | micro | μ |
| $ 10^{-9} $ | nano | n |
| $ 10^{-12} $ | pico | p |
| $ 10^{-15} $ | femto | f |
| $ 10^{-18} $ | atto | a |
| $ 10^{-21} $ | zepto | z |
| $ 10^{-24} $ | yocto | y |
| $ 10^{-27} $ | ronto | r |
| $ 10^{-30} $ | quecto | q |
SI Data Size Units
| Decimal | Terim | $ 10^n $ | Symbol | Prefix |
|---|---|---|---|---|
1.000.000.000.000.000.000.000.000 | *Quadrillion* | $ 10^{24} $ | Y | yotta |
1.000.000.000.000.000.000.000 | *Trilliard* | $ 10^{21} $ | Z | zetta |
1.000.000.000.000.000.000 | *Trillion* | $ 10^{18} $ | E | exa |
1.000.000.000.000.000 | *Billiard* | $ 10^{15} $ | P | peta |
1.000.000.000.000 | *Billion* | $ 10^{12} $ | T | tera |
1.000.000.000 | *Milliard* | $ 10^{9} $ | G | giga |
1.000.000 | *Million* | $ 10^{6} $ | M | mega |
1.000 | *Thousand* | $ 10^{3} $ | k | kilo |
100 | *Hundred* | $ 10^{2} $ | h | hecto |
10 | *Ten* | $ 10^{1} $ | da | deca |
1 | *One* | $ 10^{0} $ | ||
0,1 | *Tenth* | $ 10^{-1} $ | d | deci |
0,01 | *Hundredth* | $ 10^{-2} $ | c | centi |
0,001 | *Thousandth* | $ 10^{-3} $ | m | mili |
0,000.001 | *Millionth* | $ 10^{-6} $ | μ | micro |
0,000.000.001 | *Milliardth* | $ 10^{-9} $ | n | nano |
0,000.000.000.001 | *Billionth* | $ 10^{-12} $ | p | pico |
0,000.000.000.000.001 | *Billiardth* | $ 10^{-15} $ | f | femto |
0,000.000.000.000.000.001 | *Trillionth* | $ 10^{-18} $ | a | atto |
0,000.000.000.000.000.000.001 | *Trilliardth* | $ 10^{-21} $ | z | zepto |
0,000.000.000.000.000.000.000.001 | *Quadrillionth* | $ 10^{-24} $ | y | yocto |
Derived units
| Physical Quantity | Unit | Symbol |
|---|---|---|
| Force | Newton | $ N = kg \space m/s2 $ |
| Work, energy, amount of heat | Joule | $ J = N \space m $ |
| Power | Watt | $ W = J/s $ |
| Electric charge | Coulomb | $ C =A \space s $ |
| Electric potential | Volt | $ V =W/A $ |
| Electric capacitance | Farad | $ F =A \space s/V $ |
| Electric resistance | Ohm | $ Ω = V/A $ |
| Frequency | Hertz | $ Hz = s^{-1} $ |
Constants
| Name | Value | Description |
|---|---|---|
| Absolute Zero Temperature | $ 0 K = −273.15 °C $ | Absolute zero is the point at which molecules stop (their motion is reduced to very small vibrations). |
| Speed of Light in Space | $ c = 2.99792458 × 10^8 \quad m/s $ | An important physical constant used in many areas of physics. |
| Gravitational Constant | $ 6.6742 × 10^{-11} \quad m^3/{kg \space s^2} $ | It is a physical constant involved in the calculations of the gravitational force. |
| Electron Radius | $ r_e = 2.81792 × 10^{-15} \quad m $ | . |
| Molar Gas Constant | $ R = 8.314472 \quad J * mol^{-1} * K^{-1} $ | . |
| Mass of the neutron | $ m_n = 1.6749286 × 10^{-27} \quad kg $ | . |
| Mass of the Electron | $ m_e = 9.1093897 × 10 ^{-31} \quad kg $ | . |
| Elemental Load | $ 1.602,176,634 x 10^{-19} \quad C $ | The charge of a proton or the negative of the charge of an electron. |
Speed and Acceleration
Speed is measured in meters per second (m/s) or sometimes more appropriately in kilometers per hour (km/h).
$$ 1 \space m/s = 3,6 \space km/h $$
Acceleration (Acceleration or Deceleration) is the rate of change of velocity. It is measured in $ m/s^2 $.
If the speed increases from u m/s to v m/s during t seconds, the average acceleration a is expressed as
$$ a = (v-u)/t \qquad m/s^2 $$
Acceleration due to gravity
In a vacuum, all free-falling objects, regardless of their size, shape or mass, have the same acceleration at a given location.
This acceleration, given the symbol g because it is due to the force of gravity, is about $9.81 m/s^2 at sea level near London, $9.78 m/s^2 at the equator and $9.83 m/s^2 at the poles.
The acceleration of bodies falling into the atmosphere depends on wind resistance. For example, depending on the conditions, the human body reaches a top speed of about200 km/hwhen wind resistance is equal to the force of gravity and no further acceleration occurs.
Force
A force is a measurable effect that tends to cause the motion of a body. The unit of force is the Newton, 1 Newton is the value of a force that, when applied to a kilogram, gives it an acceleration of $ 1 \space m/s^2 $.
$$ F = m \space a $$
Mass and Weight
An object containing a certain amount of matter is called mass. Its unit is kilogram. The force due to gravity acting on this mass is called weight.
Objects with mass have inertia, that is, they resist acceleration or deceleration and remain at rest or continue to move in a straight line at a uniform speed unless acted upon by an external force.
$$ W = m \space g $$
Moment Concept
When a body is at rest or in equilibrium, the sum of the clockwise rotational moments about any real or imaginary axis is equal to the sum of the counterclockwise moments about the same axis. When this is not the case, the unbalanced moment causes the body to rotate about the chosen axis. Not to be confused with momentum.
Center of Gravity
The center of gravity (CG) of a body can be regarded as the point at which, if all the weight of the body were concentrated, it would produce a moment of force equal to the sum of the moments of force of each part around any axis of the body about the same axis. In the case of the force of inertia, the center of gravity becomes the center of mass.