Keys and MOI - wipp assist

[Don A. Gilmore]

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "College and University Technicians" <caut@ptg.org>
Sent: Thursday, January 08, 2004 3:41 AM
Subject: Re: Keys and MOI - wipp assist


> Ok on the force bit... but the "dyne" is an unfamiliar term...Thanks
> muchly for the explaination below, but I am unsure as to whether or not
> John consciously used dynes... I mean... why use them in this case ? And
> why was an explaination in terms of ergs lacking then. No matter really.
> But for fun.. lets see... John cited a BW force of "45gm*980cm/sec^2 -
> 10gm*980cm/sec^2"... that would be 35 dynes then eh ?

You forgot to multiply by 980.  That would give you 34300 dynes.  So you can
see that dynes are extremely small.

> Ok... this is neat to know really... We could speak in terms of Calories
> I suppose if we wanted to... but which would you say is best to use for
> our present application... newtons or dynes ??

You can use dynes (cgs) if you want to; there's certainly nothing wrong with
it.  But I personally would use newtons (MKS) for several reasons.

1.  They are a more manageable size.  You won't end up with a lot of five-
or six-digit numbers to deal with.

2.  They are more universal.  If our discussions ever branch into other
areas of physics like pressure, power, viscosity, or electrical/magnetic
units you will be glad we stuck to MKS.

3.  They are much more common.  MKS is the definitive system in use for the
vast majority of engineering and scientific work in the world.  When you
look up tables in books, you will find that cgs units are relatively scarce.

It may seem awkward to use .002 meters in lieu of 2 mm, or .035 kg in lieu
of 35 g at first, but you're better off in the long run.  Besides, I'd
rather tell people that I weigh eight-hundred newtons than that I weigh
eighty-million dynes!

> Ahhh.... so thats where the "sluggish" terms thrown around the last
> couple days come in.

Right.  And, incidentally, beware of the term "lbm" or "pounds-mass".
You'll see this term in some quasi-scientific literature from time to time.
Ostensibly it was created for those folks who's modest intellect doesn't
allow them to understand the concept of slugs and the difference between
mass and weight.  All it effectively does is confuse the hell out of
everyone and make us watch our units with paranoia.  You can really make
some terrible mistakes in engineering using such nonsense if you're not
careful.  We're better off sticking with slugs (no pun intended).

Don A. Gilmore
Mechanical Engineer
Kansas City