Ed Huntress wrote:

Maybe you find a planet where a = 10 m/s^2. :-)

There probably is no such planet, which makes my point.

No, the point is different. You have to distinguish between force and mass.
For some simple minded people (pun intended or not?) it is the same, but it
ain't.

It works great in theory ...

It works even greater in practice!

For others, including the field of medicine, where
I've been writing for the past few years, it simply results in a lot of
clumsy derived units.

Have no insight into medicine at all.


It's a theoretically elegent system. By using those base units, the SI
committee has developed a system that is theoretically coherent and
conceptually minimalist, but which also forces you to keep things in your
head that are far abstracted from what you're actually measuring, or to
memorize the system without thought -- which obviates any "rational"
advantage the system may have, in much practical use.

The coherent system (why do you always say "theoretical", it *is* coherent)
has a lot of advantages as soon as you start to make calculations.

A simple and cyclic example:
Your did some math and the result's unit is [kg * m / s^2]
Now what's that? It is force, so the unit is [N]. And how do you know?
By the formula f = m * a. If you do the check with units it is:
[N] = [kg] * [m/s^2]

That system is really great as soon as your calculations are a single step
behind adding. Do a check with the units and you see whether you made some
nonsense or not.

If you do have a problem "converting" mass to force, simply multiply the kg
by 10 and you do have Newton.

We're talking about the everyday measurements that make up the
vast majority of numerical evaluations made by people in the world.

Are you supposing to do the calculations in SI and then convert them to
imperial? Don't ask for the famous prove what the result is.

For
them, defining the unit of force in terms of acceleration, when they're
interested in how big they'll have to make a support to keep a cistern off
the ground,

Hmm ... I mean is someone is failing to understand the difference between
mass and force and isn't able to calculate the resulting force, will he be
able to properly dimension the legs (Euler's buckling resistance)? I bet
no.
Or the other way round:
Does it help Joe Bar in calculating the stress of some odd-shaped column
when he is using odd units?
As soon as that Joe Bar wants to find out whether a screw is strong enough
to keep his trailer together, it doesn't help to work in pounds when he
considers the torque the screw is tightend with and the resulting stress
and clamping forces (that was a longish German sentence :-))


I don't wonder of anybody who doesn't agree but at the same time doesn't
understand the difference between mass and force.

I understand it quite well, thank you very much.

G Then it's OK! But wait ...

I only have to look at the domain-dependant units of pound, pondal, pound
force and whatever to see what mess it is.

I don't know anyone who uses pondals, and the pound, both as a unit of
force and as a unit of mass, is quite handy within its domain.

I remember a discussion about a year ago. It was such a mess that I had to
bail out. And again "he pound, both as a unit of force and as a unit of
mass, is quite handy" is just quite confusing and will only result in
errors.


Read about the SI-system before you talk about it.

You really can be annoying at times, Nick.

Yes, I can! ;-)


It's very unlikely that you've
read as much about the SI system as I have, unless you spent more than a
year, as I did, outlining a book on quality assurance.

So, I do have to be annoying again:
| Units of force, for example: the Newton equals roughly 0.101 972
| kilograms of force (kgf).
That's what you wrote!

I have several answers:
a) There is no such thing like kgf
b) did you mean kp?
c) kp was left behind 1960 and replaced with N
e) from the factor 0.101972 I see that we live on the same planet
(1 / 0.101972 = 9.806..) but I doubt that this is true everywhere.
f) from e) I feel that you didn't fully understand the difference between
force and mass.

Sorry! I do *not* want to kick your ass, I'm just a bit pi**ed when people
are confusing things. And I see it so many times here that people write MM
when they meant mm (what would MegaMega mean?) or write S when they meant s
(I don't know what Siemens (the inverse of Ohm) has to do with time) etc.

But!!!:
If you reduce the discussion to inch vs. metre (or meter) it is not worth
continuing. There is just a factor to convert between them. Neither meter
nor inch is more precise.
The discussion starts when you look around and see things like feet, AWG,
steel gage for sheet metal, drills by numbers and letters, etc. No such
thing in the metric system.


Nick
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