[MF3D.FORUM:343] speed of sound & 1/fl

[Tloc54452@xxxxxxx]

>  Yes, and maybe no. Does anyone know how much the
>speed of sound changes with altitude (air pressure).
>If you are at 10,000 ft altitude does the speed of
>sound change enough that the 5s/mi or 3s/km "rule"
>is significantly off?

You guys go so fast I can't keep up!  But there's one I
can answer off the top of my head.  The speed of sound
is solely a function of the square root of absolute 
temperature, as truly counterintuitive as that may 
seem.  (It always amazed me.)  In feet per second and 
degrees Rankine (am I dating myself?), it works out 
to 49 root T for air.  Rankine is Fahrenheit absolute 
which would be Fahrenheit plus about 459.6.

Sorry about being such a poor contributor.  For the past 
few weeks, I have been working 12+ hours per day, 7 
days per week, to meet a deadline.  I couldn't start until
our dear fizzleists had made up their collective mind.
That took an outrageous amount of time.

Also, I find it's been so long (relative to the length of my
memory) since I've done MAOFD, that it's a bit vague now.
Can someone explain the 1 over focal length argument to
me or cite me on the subject?  Sure would appreciate it.

Thanks,
John