Re: whip crack explanation?

[Hugh Haskell <hhaskell@MINDSPRING.COM>]

>  > But how is it possible to measure the speed of such a small
> >  object moving such a short distance in a fraction of a
> >  second?
> >   I doubt that it can be done with inexpensive phototimers or
> >  by ordinary movie cameras.
>
> No, not by ordinary cameras.
> I had a parent who was into high speed photography,
> He had and brought in (and did the photographing) with
> the equipment.  I only did it the one time as the family
> moved, but it did work.
>
> Tim

I can confirm Tim's result (indirectly). My colleague, Loren Winters,
teaches high speed photography fairly routinely, and a couple of
years ago he had some students who were interested in this problem
with regard to snapping towels. The rigged up a high speed video
system and were able to measure the speed of the tip of a snapping
wet towel and found it to be a bit above the local speed of
sound--something like 380-400 m/s.

Years ago, when I was in flight training, we trained for a while in
the old WWII trainer the navy called SNJ (Air Force/Army designation
was AT-6). The engine had a characteristic snapping sound at high
RPM, such as when you take off, which was attributed to the propeller
tips exceeding the speed of sound at these RPM's. A quick calculation
confirmed that when the engine speed was over about 2300 RPM the
propeller tips were travelling at about 400 m/s, enough to create the
snapping sound of a whip as each blade passed. I don't recall the
propeller gear ratio or the blade length now, so I cannot reproduce
the calculation.

Hugh
--

Hugh Haskell
<mailto://haskell@ncssm.edu>
<mailto://hhaskell@mindspring.com>

(919) 467-7610

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