GardenBanter.co.uk - View Single Post

14-05-2005, 02:04 PM

The message
from (Nick Maclaren) contains these words:

/snipe/

4) the distance will be attenuated by a rather complicated factor
including pi, the length of the string and the consequent elevation of
the ball in the vertical plane.

Eh? How do you attenuate a distance?

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And the calculation is of
the most trivial. To get the energy immediately after impact, you
just calculate M.g.sqrt(R^2-D^2), you get the ball's velocity by
solving E = M.V^2/2, and you get the velocity of the pellet by
M.V/m. Q.E.D.

Assuming I understand by your shorthand what you understand by it, you
still haven't addressed the 32ft/sec² element of the swinging ball.

I've no idea what you mean. To get the velocity you have to measure both
the distance and the time it takes for the pellet to travel that
distance.

As JB points out, no, you don't. Sorry - O-level physics again.

Ah, seem my reply to JB.

I have. I also worked out the rough correction for air resistance
in my head while walking back to my car, and it is considerably
less than 20%.

Air resistance is quite immaterial if you're measuring it how it *MUST*
be measured to comply with the law.

There is no such thing as 20% accuracy. A measurement is accurate or it
is not. And 120% - or even 110% would not do. Can you imagine a plea in
court: "Well, Yer'onner, I tried it out before i went ratting, and it
was within the law, 120% innit."

Well, actually, there is, even in law. But let that pass. The
question was whether the velocity was likely to be 350, 550, 750
or 1000 fps.

Was it? The question I was addressing, and the one I expected you to be
addressing, was the measurement of muzzle-energy in foot pounds.

Those are distinguishable with 20% accuracy. And,
if you have an accuracy of 20% and the measurement differs from
the limit by more than 20%, you can be pretty sure that it is
actually different.

I think you are talking about how many red herrings grow in the wood,
whereas I am talking about how many strawberries swim in the sea.

SURELY you were taught that in A-level? Estimating accuracy of
measurements was a significant amount of the O&C course when I
did it, and I don't believe that others were all that different
(whether in boards or time).

We were taught to do a 'rough' calculation to get a ballpark figure,
mainly because slide-rules were not permitted in the exam, and under
pressure, decimal points within calculations seem to be rather motile.

But estimating accuracy of measurements seems to me to be another phrase
approaching an oxymoron, and no, AFAIK the London syllabus didn't
include anything like it, except in passing.

--
Rusty
Open the creaking gate to make a horrid.squeak, then lower the foobar.
http://www.users.zetnet.co.uk/hi-fi/