Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)

Subject: Re: [Boost-users] boost::units - converting from one derived_dimension to another across systems (imperial to metric)
From: Nathan Ernst (nathan.ernst_at_[hidden])
Date: 2018-11-20 08:38:18

> In the meanwhile the Kg has been redefined. The new definition is based
on invariant constants of nature.

I doubt it. The redefinition was done to eliminate a physical reference
standard (there is no longer a need to compare various national lumps of
platinum/iridium against a lump in France). It's not like 1 kg suddenly
became 1.1 kg according to the new standard.

On Tue, Nov 20, 2018, 2:32 AM degski via Boost-users <
boost-users_at_[hidden] wrote:

> On Tue, 20 Nov 2018 at 07:06, Steven Watanabe via Boost-users <
> boost-users_at_[hidden]> wrote:
>
>> This is not the unit you want.  The SI
>> unit of force is N, not kg.
>
>
> One wonders what has happened to the school system these days, this is
> basic stuff that was taught to kids 13/14 years of age ad nauseam.
>
> To expand on what Steven has been [economically] stating: Wikipedia
> <https://en.wikipedia.org/wiki/Newton_(unit)> definition: *One newton is
> the force needed to accelerate <https://en.wikipedia.org/wiki/Acceleration>
> one kilogram <https://en.wikipedia.org/wiki/Kilogram> of mass at the rate
> of one metre per second squared
> <https://en.wikipedia.org/wiki/Metre_per_second_squared> in the direction
> of the applied force. *And from Wikipedia
> <https://en.wikipedia.org/wiki/Mass_versus_weight> again: *... an object
> with a mass of 1.0 kilogram will weigh approximately 9.81 newtons on the
> surface of the Earth <https://en.wikipedia.org/wiki/Earth> (its mass
> multiplied by the gravitational field strength
> <https://en.wikipedia.org/wiki/Gravity>), since the newton
> <https://en.wikipedia.org/wiki/Newton_(unit)> is a unit of force, while the
> kilogram <https://en.wikipedia.org/wiki/Kilogram> is a unit of mass.*
>
> kilogram-force
>> is not currently provided by Boost.Units
>> and you'll need to define it yourself.
>>
>
> From Wikipedia <https://en.wikipedia.org/wiki/Kilogram-force>: *
> Kilogram-force is a non-standard unit and is classified in SI Metric System
> <https://en.wikipedia.org/wiki/International_System_of_Units> as a unit
> that is unacceptable for use with SI. It is equal to the magnitude of the
> force exerted on one kilogram <https://en.wikipedia.org/wiki/Kilogram> of
> mass <https://en.wikipedia.org/wiki/Mass> in a 9.80665 m/s2 gravitational
> field (standard gravity <https://en.wikipedia.org/wiki/Standard_gravity>, a
> conventional value approximating the average magnitude of gravity on
> Earth). Therefore, one kilogram-force is by definition equal to 9.80665 N
> <https://en.wikipedia.org/wiki/Newton_(unit)>*. The "by definition" here
> is important, depending on where you are on the planet, the real value will
> vary.
>
> In the meanwhile the Kg has been redefined. The new definition is based on
> invariant constants of nature. I*s there anything Boost needs to do to
> comply with this new definition* [or does it require serious
> multi-precision to actually see a difference [I suspect so, but still it
> should be defined in terms of these constants]]? The constant that did get
> redefined to make this [the above Kg definition] possible is the Planck
> constant <https://en.wikipedia.org/wiki/Planck_constant>.
>
> The new value [as of 20 May 2019] of the Planck constant by the ISO
> standard is set to 6.626 070 150 x 10-34 Jâ‹…s. Some other constants got
> redefined as well: *... in order to support the redefinition of the SI
> base units, CODATA made a special release that was published in October
> 2017.[30] <https://en.wikipedia.org/wiki/Planck_constant#cite_note-30> It
> incorporates all data up to 1 July 2017 and determines the final numerical
> values of the Planck constant, h, Elementary charge
> <https://en.wikipedia.org/wiki/Elementary_charge>, e, Boltzmann constant
> <https://en.wikipedia.org/wiki/Boltzmann_constant>, k, and Avogadro
> constant <https://en.wikipedia.org/wiki/Avogadro_constant>, NA, that are to
> be used for the new SI definitions. *
>
> degski
> --
> *“If something cannot go on forever, it will stop" - Herbert Stein*
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