On Thu, 19 Aug 2010 16:24:26 -0500, John Fields
wrote:

On Thu, 19 Aug 2010 10:49:59 -0700, John Larkin
wrote:

On Thu, 19 Aug 2010 10:12:00 -0500, John Fields
wrote:

On Wed, 18 Aug 2010 21:30:53 -0700, John Larkin
wrote:

On Wed, 18 Aug 2010 22:35:40 -0500, John Fields
wrote:

On Wed, 18 Aug 2010 19:04:42 -0700, John Larkin
wrote:

On Wed, 18 Aug 2010 12:25:35 -0500, John Fields
wrote:

On Wed, 18 Aug 2010 08:07:43 -0700, John Larkin
wrote:

On Wed, 18 Aug 2010 06:18:44 -0500, John Fields
wrote:

---
"For things like this"???

Yup. I'm an electrical engineer, not an ME, and I didn't use any
references or even a calculator or a pencil. I did it in my head. I
thought it was pretty good shootin'. It sure did annoy you.

---
I don't see where you got that, since I presented the facts and why
you were in error in a civil manner.

Just for grins, though, why don't you go ahead and outline how you
came up with 100µs?

I visualized the spheres colliding in space, thought about the
deformations and the shock waves, guessed the speed of sound in the
metal, and, well, guessed. I was impressed when your measurements came
pretty close, at least some of them.

Reasonably accurate guessing, "lightning empiricism", is very useful
to estimate if some effect is important enough to justify more serious
calculation. Just today, talking about an SSR and stray capacitance,
somebody said something like "4 ohms, 20 pF, 80 picoseconds, can't
matter."

---
Ah, but there's the rub.

For the SSR situation you can proceed farther and, with the data at
hand, determine whether your guess was accurate.

However, in the case of the colliding Foucault pendulums, such doesn't
seem to be the case.

Given the mass of the spheres, the length of their arms, and the angle
of release of the moving pendulum, can you come up with a way to
validate your guess?

You validated it for me. All your shock wave transit times were in the
100 us range. They are of course fairly variable, bacause the physics
is messy, as I mentioned from the first. But they aren't 10 us and
they aren't 10 ms. 100 us is right in the ballpark.

---
I didn't validate anything, all I provided was empirical data.

What I'm asking you for, in terms of validation, (since you have all
the parameters needed to do the work) is to show, analytically, why
your guess wasn't just coincidental.

Or, if not analytically, at least qualitatively.

Something like: "Well, the balls are made out of brass so therefore
sound velocity umptyeeump, propagation of shock wave umpteeump and,
since its release angle was umpteeump, the moving ball must have been
going at umpteeump velocity when it hit the fixed one, therefore
umpteeump 100 microseconds...

Can you do that?
---


Your various measurements pretty nicely straddle my estimate.

---
They don't straddle your guess, since your guess starts at zero and my
numbers are all longer than your guess.


"100 us" starts at zero? Would you please explain that to us?

---
Instead of retreating into the collective (which I think pretty much
understands what I meant, if it even cares) for protection, take the
responsibility for yourself and if _you_ don't understand, say so and
I'll be glad to smart you up.

Why is an estimated time of "100 us" wrong because "your guess starts
at zero" ? I never mentioned zero. That doesn't make any sense. It
would follow that all numbers are wrong because they "start at zero."

Yes, please explain it.

---
Regardless of how you try to confound it, I'll take that to mean: "I
don't understand, please explain."

OK.

Since time started when M1 hit M2 and ended when M3 left M2, the
recorded elapsed time between those events was about 150 microseconds.

Therefore, since time started when M1 hit M2, your guess of 100
microseconds elapsing before M3 left M2 was about 50 microseconds
short and, since M1 hitting M2 was the beginning of the counting
period, there's really no way of bracketing your 150µs guess around
100µs to get a +/- 25% error.

I never mentioned a 25% error,

---
Not explicitly, but stating that my 150µs data "bracketed" your 100µs
guess implies a 100µs signal with a 150µs signal overlaying it such
that the spacing between consecutive leading and trailing edges was
equal; i.e. 25µs.

Like this:

|------150µs------|
___________________
T1__| |____________

|----100µs----|
_______________
T2____| |______________

--| |--25µs --| |--25µs

And, BTW, my error attribution was wrong.

Since the reference was 150µs and your guess was 100µs, the error was
50 parts out of 150, or:

(T1 - T2) * 100 5000
%error = ----------------- = ------- = 33.3 %
T1 150

Bracketing that error would lead to the assumption that your guess was
wrong by 16/2-3% on either side of the 150µs signal, which would be
wrong since the leading edges of your guess and the real signal would
be congruent, causing the error to accumulate after the trailing edge
of your guess.

What numerical hand-waving nonsense. I made an estimate, and it was
right in the ballpark, and you don't like it one bit.

---
You must be confused, since the "numerical handwaving nonsense" is
precisely what you asked for; an explanation of why the concept of
"bracketing" is invalid in this instance since T0 for the leading edge
of your guess and for the leading edge of the real signal is the same
instant in time.

Consequently "bracketing" is impossible, you've been proven to be
wrong about that, numerically, and it's got your panties all in a
bunch.

Of course you could have meant something else by "bracketing"..., LOL.

---

and I never made a 150 us guess.

---
I can't recall anyone saying you did; what's your point?
---

Doing a thing like this in one's head, in a few seconds, I'd call a 2:1
error pretty good shooting.

---
Show the reasonong behind the guess and I'll believe you.


You don't believe I guessed 100 us? I posted it in SED days before you
did the tests. You replied "noted", obviously expecting me to be
wrong. Sorry to disappoint.

---
Wow, with a chip that big on your shoulder, I'd be willing to bet
you're well on your way to scoliosis.

Ermm...
Of course I believe you posted it. I acknowledged it, didn't I?

The only thing I'm disappointed in is that you won't reveal the
reasoning that led up to the guess, so who's to know if it really
_was_ a guess?

You always say: "Show your work", but when it comes time for you to
walk the walk, you balk.

I can't show my work because there wasn't any work, or at least any
conscious work. I visualized the impact and guessed. That's what a
guess is, a guess.

The trick is to guess right. The real trick it to usually guess right.


Makes one suspect any number of nasty things...

Naturally. You need to believe that I cheated somehow. The alternative
is intolerable.



Anyway, here're some facts for you; let's see if you can juggle them
around and get 100µs from make to break in the 3 ball case.

The balls are made from ASTM B134 Grade 260 brass, (70Sn 30Zn) are Mc
Master Carr P/N 9617K47, have a diameter of 0.75" +/- 0.001, and weigh
about 32 grams.

The length of the suspension is 4.5" from the upper restraint to the
center of the ball.

The velocity of sound in 70/30 brass is:

Plane Longitudinal: 4700 m/s
Plane Transverse (shear): 2100 m/s

The plane longitudinal is for the bulk material, so is probably more
accurate for a spherical structure.

For a pendulum, the velocity of the bob at the bottom of its arc is:

V = sqrt {2gL[1-cos(a)]}

Where V is the velocity in m/s
g is the acceleration of gravity in m/s²
L is the the length of the suspension (the "rod") in meters, and
a is angle from plumb at which the ball is released.

Wanna play?

I already estimated the time, before you even measured it. It's your
move next. Do the math and see how close it comes to the measurements.
Of course, you already know my guess *and* the experimental results.

Sure, do the math and post it.

John