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# Re: [Phys-L] form of Newtons 2nd law

• From: John Denker <jsd@av8n.com>
• Date: Fri, 14 Sep 2012 11:46:19 -0700

On 09/14/2012 10:24 AM, Larry Smith wrote:
So as to not pick on just Newton's laws _of_motion_, my colleague
would suggest that the k in kma is analogous to the G in Newton's law
of gravity. A former engineering student of his recently e-mailed
back agreeing with the kma and had included a scan from an
engineering textbook where the k appears in the 2nd law. Is anyone
else aware if this is a common practice in engineering? I'd never
heard of it in my physics education.

It's a moving target. Common practice for electrical engineering at MIT
is different from common practice for marine engineering at ITT Technical
Institute. It's also a function of time.

Just now I looked in one of my father's books:
Reno C. King Jr.
_Practical Marine Engineering_
Prentice-Hall (1948)

It is uniformly even uglier than F = k m a. For example, we find on
page 155 the formula

778M(Hs - He)
Thermal horsepower = ---------------
33,000

where M is "the mass or pounds of steam per minute"
Hs is "the heat content in Btu's per pound of steam entering"
He is "the heat content in Btu's per pound of exhaust steam"

In other words, there is no vestige of the idea that physical quantities
have meaning independent of the units in which they are expressed. The
idea of using algebraic methods to keep track of the units is not even
in the same ballpark. Everything is at the arithmetic level, not the
algebra level.

In the chapter on "electricity" on page 268 we find the formula

V × A
KW = ---------
1000

where V is the volts, A is the amps, and KW is the kilowatts.

On the same page we find the diagram

= =
= V =
=_________=
= | =
= A | R =
= =

which expresses Ohm's law and all of its permutations. This is similar
to the "density triangle" that is sometimes taught in lizard-friendly
high-school chemistry classes.

Note that "practical marine engineering" means you are a real blue-collar
_engineer_ i.e. the guy who operates the _engine_ on a ship ... not to be
confused with the quiche-eating white-collar "engineers" who sit in an
office building and scribble on blueprints.