XPost: alt.usage.english
From: nospam@de-ster.demon.nl
Jeff Barnett wrote:
> On 9/19/2024 5:12 PM, Christian Weisgerber wrote:
> > I'm sorry, I don't know where to post this. I'm crossposting to
> > alt.usage.english, because statute miles as a unit mostly afflict
> > the English-speaking world.
> >
> > So you want to convert between miles and kilometers. The conversion
> > factor is... uh... A 40-year-old calculator book provides a useful
> > tip: Unless you're designing a space probe, you can use ln(5).
> >
> > WHAT?
> >
> > Yes, the natural logrithm of 5 approximates the conversion factor
> > between miles and kilometers; specifically one mile is about ln(5)
> > kilometers. It's accurate to four digits.
> >
> > If nothing else, it's faster to type on a calculator.
> >
> > I think that's hysterical.
> >
>
>
> After glancing at the discussion that follows this post, I thought it
> appropriate to point out the book "Dimensional Analysis" New Haven: Yale
> University Press (1922) by the Nobel Prize winning physicist Percy
> Williams Bridgman.
He didn't win the nobel for this book.
(or for his peculiar philosophy of sciece)
It is one of those books that many know exists,
but few will actually have seen it, let alone read any of it.
(don't worry, no loss)
You will need a good old university library to find it,
or you may find a very rare antiquarian copy,
or an almost as rare and by now also antiquarian reprint.
> It essentially describes and defines physical
> dimensions such as distance, speed, energy, force, etc. as well as units
> that are defined within a dimension such as meters, feet, and microns as
> distances. It shows that dimensions MUST match on both sides of an
> equation and, if not, there must be multiplicative constants that have
> appropriate dimensions to restore balance. You may define base
> dimensions and the others in terms of the base. For example, length,
> mass, and time to do mechanics.
All completely trivial.
What's more, the subject matter has been almost completely forgotten.
All that remains is elementary high school knowledge
of the -conventional- systems of dimensions
that is nowadays associated with the SI.
Few people even know anymore that other systems of dimensions
are possible.
The misconception that a 'dimension' is somehow a property
of a physical quantity is shared nearly universally.
> Within an equation, you must use the same units everyplace for
> quantities in a specific dimension or dimensionless units of conversion
> such as 12 inches per foot. It even shows how to determine when physics
> equations express nonsense because of unit disparity or non matching
> dimensions.
You may crash Mars landers through non-matching units,
never by non-matching dimensions.
> The cherry on the cake is discovery of new physical laws via
> dimensional analysis.
Not really. At best it allows you to guess at the form.
The book codifies the obvious.
Dimensional analysis was already well known and understood
through the works of the 19th century greats, such as Kelvin
and Rayleigh.
The use of the so called 'dimensionless numbers',
such as Reynolds', or Froude's number was already well established.
> If you can obtain access to a copy of this book, I recommend taking a
> spin through it.
A waste of time and perhaps also money, if you don't mind me saying so.
> A hundred years ago it was novel and educated some very
> bright individuals who hadn't quite caught on to what your current
> discussion is all about. It wasn't all that obvious way back when. Of
> course it was as soon as the subject was systematically presented.
Already then, Bridgman was belabouring the obvious,
Jan
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