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: degski (degski_at_[hidden])
Date: 2018-11-20 08:31:58

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*