why is gram not used as an si unit?
Answers
Answer:
In the original metric system, the unit of mass was the grave (GRAH-veh), equal to 1000 grams. The gram itself was not used as the standard, since it was too small to be measured reliably at the time, and did not make a practical standard, so the larger grave was the unit of mass.
Answer:
Prior to the formal adoption of SI (the French acronym for Système international d'unités) two flavors of the metric system were in general use: cgs and MKS.
Each system was self-consistent in having a large set of derived units that could be expressed in terms of three base units for length, mass and time. The cgs base units were the centimeter, gram and second while the MKS base units were the meter, kilogram and second.
As others have pointed out, the standard reference masses stored around the world are each 1 kg, not 1 g. The kilogram is the last base unit in SI to be defined by a physical artifact rather than intrinsic properties of matter or space.
(Edit: This is no longer true. On May 20, 2019, the international kilogram prototype was officially retired. The kilogram is now defined by a set of physical constants being given official values, specifically Planck’s constant: 6.62607015×10−34 J⋅s, and the speed of light, which in 1983 had already been defined as exactly 299,792,458 m/s. A similar change was made to the meter in 1960, when the official meter prototype was retired and the meter redefined in terms of the wavelength of light produced by krypton-86. In 1983 the meter gained its current definition in terms of the second, which can be very measured very precisely with “atomic” clocks, and the speed of light, a fundamental physical constant that can also be measured very precisely and which, as mentioned, was given an exact numerical value in 1983.)
The cgs unit of force, for example, is called the dyne. It’s the force that would accelerate a 1 gram mass at a rate of 1 centimeter per second squared. The corresponding MKS unit is the newton, defined as the force that accelerates a 1 kg mass at a rate of 1 meter per second squared. One newton therefore equals 100,000 dynes. [thanks for the correction!]
Note that neither system used all three of what you’d think would be the “true” base units, i.e., the meter, gram and second. Had SI done so, it would have been necessary to create a whole new set of derived units based on these three. Derived units extend far beyond mechanics to fields like electricity where the volt, watt, joule, farad and henry based on MKS base units were already in widespread use. (By the way, the unit of electric current, the ampere, is currently one of the SI base units.)
I am not absolutely sure, but I suspect SI chose the MKS system because it meant much less disruption to the many more users of its derived units than those of the cgs system (or a totally new ‘mgs’ system).