Keys and MOI - wipp assist

[Don A. Gilmore]

Hi Ric:

No, I think John is just converting mass to force on earth.  You have to
multiply grams by the acceleration of gravity to get the force they exert
downward on earth (the "weight").  It doesn't have anything to do with the
acceleration of the object by outside forces.

W = m * g

where W is weight, m is mass and g is the acceleration of gravity.

It's common for folks to confuse weight with mass.  They are really two
different things.  Grams are mass, not force.  The units that John shows

g-cm/s^2

are actually units of force, believe it or not.  Usually this would be
converted to "dynes"

1 dyne = 1 g-cm/s^2

"But hold on, Gilmore, what is this 'dyne' nonsense?"  Well, since F = ma,
force actually has units of mass times acceleration, or
mass-times-distance-per-time-squared.  So that we don't have to write out
these big units, a hybrid unit was created.  John has elected to use cgs
(centimeter-gram-second) units, which are used mainly in theoretical physics
and chemistry.  The unit of force in this system is the "dyne".  The unit of
energy would be the "erg", which is a dyne-cm.

In all of my posts I use the more common MKS system (meter-kilogram-second),
which is used in all branches of engineering.  The unit of force in the MKS
system is the "newton"

1 N = 1 kg-m/s^2

And energy is in units of a "joule", which is a N-m.  There are 100,000
dynes in a newton, so they are very, very small.  The MKS system is used
because it relates better to all the other areas of engineering.  Volts and
Amperes, for example, are MKS units (volts can actually be expressed in
meters, kilograms, seconds and coulombs).

If using the English system, distance is in feet, mass is in slugs and time
is in seconds.

1 pound = 1 lb = 1 slug-ft/s^2

English energy is in ft-lbs (there is no hybrid unit).

Hope this helps!

Don A. Gilmore
Mechanical Engineer
Kansas City

----- Original Message ----- 
From: "Richard Brekne" <Richard.Brekne@grieg.uib.no>
To: "College and University Technicians" <caut@ptg.org>
Sent: Wednesday, January 07, 2004 5:24 AM
Subject: Re: Keys and MOI - wipp assist
> > > John Hartman wrote:
> > 45gm*980cm/sec^2 - 10gm*980cm/sec^2
> Grin... figured as much... tho its best to be sure. Thanks John. Hadnt
> really thought of BW in these terms until the other day when you brought
> up that perspective.
>
> One question tho... I suppose it would be easy to think that the above
> implies that any movement of the key means accelerating the key mass
> front of the balance rail pin faster then 980cm/sec^2... which I dont
> think is what we mean. And that I think is also central to Freds
> question about how assists (of any sort) change overall MOI and how the
> finger senses that.
>
> In all this we need to remember that whatever the MOI contribution of
> the key mass forward of the balance rail pin is, it aids (in gradual
> decreasing degree) the finger until that mass exceeds an accelleration
> of 9.8m/sec^2.
>
> The key mass at the finger will reach that accelleration rate much
> sooner then the key mass at the midpoint between the balance rail pin
> and key front... which of course leads into that whole discussion about
> lead placement and MOI. We are left with two considerations then... how
> much counterbalancing in general AND how much key inertia from key front
> mass we are to employ. Stephen Birketts contribution a while back is
> well worth a re-read for anyone following this thread, and I think goes
> a long ways to answering Freds question.
>
> Cheers
> RicB
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