Yvesfren

My first Name is Yves and I am french.

I come on wikipedia.en because I was fired from Wikipedia.fr because I was trying to solve the 'mess' existing on the french dénomination on the "densité" that you call "relative density" and of course your density is called masse volumique. Well! it is just a classical problem of "faux-ami".

Anyway: As a first step, let me introduce myself . I am working in Physics and worked in ANL during two years (long time ago). I was also visiting professor at the UCSD. In both case I had no need to work on the meaning of "density" but I loved working "hard" there, it was "dense". Yvesfren (talk) 10:18, 17 August 2008 (UTC) __________________________________________________________________________________

what is the number deduce from a measurement?

Calculations with units and Dimensionful quantities give the following rules:

Any value of a physical quantity is expressed as a comparison to a unit or a standard of that quantity.

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.

- A standard is the physical realization of the unit choosen to make the measurement: it realizes that unit only under certain physical conditions: its measurement is one.

One can deduce the following general rule: physical quantity is the product of its measurement by the unit and practicaly the standard used to do its measurement.

$Z=n\times [Z]=n[Z].$ and this implies that :${\frac {Z}{[Z]}}=n.$ is the measurement

Consequently, a measurement is just the quotient of a physical quantity by the unity or the standard used to measure it: the result of this quotient is a number called the measurement .

Exemple 1: The meter is a unit, while a metal bar having a length of one meter is a standard. One meter is the same length regardless of temperature, but a metal bar will be one meter long only at a certain temperature.

Exemple 2: Kilogram per cubic meter (kg m ⁻³) is the SI unit used to measure, a mass per unit volume, a density,or a " masse volumique in french", and "the density of pure water at 4°C" is a standard used to realize measurements like Archimedes made it long time ago on a golden crown.

So a physical quantity is clearly identified by its measurement a number depending on the unit or the standard used to measure it.

Note:

1-At 4°C, the measure of the density of water is 1000 with the SI unit Kilogram per cubic meter (kg m ⁻³) and is 1.... when the kg x(0,1m ) ⁻³ = (kg x liter ^-1 ) is used

So the density of the water at 4°C is a convenient standard to measure the density of solid on earth

2-Specific gravity is a special case of, or in some usages synonymous with, relative density, with the latter term often preferred in modern scientific writing. The use of specific gravity is discouraged in technical use in scientific fields requiring high precision — actual density (in dimensions of mass per unit volume) is preferred.

Relative Density (or RD) of a material is the measure of density made by using the density of water (4°C) as a standard

from the definition of relative density: $RD={\frac {\rho _{materialX}}{\rho _{eau}}}$ one deduce : $\rho _{materialX}=RD\times {\rho _{eau}}$

by using the liter as the unit of volume and the kilogram as unit of mass (SI system)

$\rho _{materialX}=measure\cdot {\frac {kg}{litre}}$ and as ${\rho _{eau}}=1\cdot {\frac {kg}{litre}}$ at 4°C one can deduce : $\rho _{materialX}=d\cdot {\rho _{eau}}=measure\cdot {\rho _{eau}}$

finaly RD is mathematicaly equivalent to the measurement of the density of the materialX by using the density of pure water as a standard.

Note: a standard depend on the temperature and the pure water has to be at 4°C to have the 1kg/liter density.