Physics:Dimensions and Units
Physics
Mechanics: Dimensions and Units
There is a difference between
dimensions and units. A dimension is
a measure of a physical variable (without numerical values), while a unit is a way to assign a
number or measurement to that dimension.
For example, length is a dimension, but it is measured in units of feet (ft) or meters (m).
There is one most common primary unit systems, The International System of Units (SI units, from Le System International d’Unites, more commonly simply called metric units)
In total, there are seven primary dimensions. Primary (sometimes called basic) dimensions are defined as independent or fundamental dimensions, from which other dimensions can be obtained.
The primary dimensions are: mass, length, time, temperature, electric current, amount of light, and amount of matter. For most mechanical and thermal science analyses, however, only the first four of these are required. The others will not be of concern to most mechanical engineering analyses.
SI derived units include the hertz , the newton , the pascal (unit of pressure or stress) , the ohm , the farad , the joule , the coulomb , the tesla , the lumen , the becquerel , the siemen , the volt , and the watt .
SI base units:
The meter (abbreviation, m) is the SI unit of displacement or length.
One meter is the distance traveled by a ray of electromagnetic (EM) energy
through a vacuum in 1/299,792,458 (3.33564095 x 10 -9 )
second.
The kilogram (abbreviation, kg) is
the SI unit of mass. It is defined as the mass of a particular international
prototype made of platinum-iridium and kept at the International Bureau of Weights
and Measures. It was originally defined as the mass of one liter (10 -3cubic
meter) of pure water.
The second (abbreviation, s or sec) is the SI unit of time. One second
is the time that elapses during 9.192631770 x 10 9 cycles
of the radiation produced by the transition between two levels of Cesium 133.
The kelvin (abbreviation
K), also called the degree Kelvin (abbreviation, o K), is
the SI unit of temperature. One Kelvin is 1/273.16 (3.6609 x 10 -3 )
The ampere (abbreviation, A) is
the SI unit of electric current. One ampere is the current that would produce a
force of 0.0000002 (2 x 10 -7 ) newton between two
straight, parallel, perfectly conducting wires having infinite length and zero
diameter, separated by one meter in a vacuum.
The candela (abbreviation, cd) is the SI unit of luminous
intensity. It is the electromagnetic radiation, in a specified direction, that
has an intensity of 1/683 (1.46 x 10 -3 ) watt per
steradian.
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