The message
from (Nick Maclaren) contains these words:
In article ,
Jaques d'Alltrades wrote:
The message
from (Nick Maclaren) contains these words:

Boggle. Hang a wooden ball of known weight on the end of a string,
shoot the pellet into it, and measure how far it swings; that gives
you the momentum.

No it doesn't, unless your piece of string is of infinite length.

Well, I am sorry to say that you have forgotten your O-level physics.

No I haven't, even though that was learnt over fifty years ago.

1) The ball will describe an arc.

2) because the ball describes an arc from the resting position, it will
of necessity rise against the pull of gravity.

3) the distance travelled will be an arc of known radius (assuming no
elasticity in the string)

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.

Align the barrel with a spirit level, and measure
the dopy

The what?

The drop. God alone knows why I perpetrated THAT typo!

of height with distance; that gives you the velocity.

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.

Then you must allow for how much the pellet slows, or all you get is the
mean velocity over a distance, and the power of the gun must be measured
*AT* the muzzle, not halfway along any preselected distance.

No measurements are perfectly accurate. The methods I described
should be fairly easy to perform to 20% accuracy. My method for
estimating the slowdown due to air resistance is pretty inaccurate,
but the slowing effect is relatively small anyway (as you can
easily check), so the effect of the error on the final result is
even smaller.

There is no such thing as 20% accuracy. A measurement is accurate or it
is not. And ±20% - or even ±10% 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, ±20% innit."

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

In article ,
Jaques d'Alltrades wrote:
The message
from (Nick Maclaren) contains these words:
In article ,
Jaques d'Alltrades wrote:
The message
from (Nick Maclaren) contains these words:

Boggle. Hang a wooden ball of known weight on the end of a string,
shoot the pellet into it, and measure how far it swings; that gives
you the momentum.

No it doesn't, unless your piece of string is of infinite length.

Well, I am sorry to say that you have forgotten your O-level physics.

No I haven't, even though that was learnt over fifty years ago.

Well, the calculation was an O-level question in my day!

1) The ball will describe an arc.

2) because the ball describes an arc from the resting position, it will
of necessity rise against the pull of gravity.

3) the distance travelled will be an arc of known radius (assuming no
elasticity in the string)

All true.

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? 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.

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%.

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. 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.

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).


Regards,
Nick Maclaren.

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?

_________________________
__________________ _______


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/

In article ,
Jaques d'Alltrades wrote:

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

As martin says, 'g'.

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

There is no legal requirement to measure it yourself, let alone one
to measure it in a particular way. If you measure it, and get less
that 10 foot-pounds or more than 14.5 with a 20% error (including
that due to air resistance), you can be pretty sure that an official
measuring would match the result of yours (i.e. legal or illegal).

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.

Er, yes. But, as every competent scientist and engineer knows, all
measurements have errors. A skilled one will measure something so
that the error is less than the difference between his measurement
and any point at which a decision changes (12 foot-pounds in this
case).

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.

The mind boggles! No, I will not continue this, er, debate - but
I can assure you that it is a standard and required activity by any
serious scientist or engineer.


Regards,
Nick Maclaren.

The message
from (Nick Maclaren) contains these words:

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

There is no legal requirement to measure it yourself, let alone one
to measure it in a particular way. If you measure it, and get less
that 10 foot-pounds or more than 14.5 with a 20% error (including
that due to air resistance), you can be pretty sure that an official
measuring would match the result of yours (i.e. legal or illegal).

Generic 'you'. If the ME is tested for the purposes of
legality/non-legality, it *MUST* be tested at the muzzle. (FSVO 'at')

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.

Er, yes. But, as every competent scientist and engineer knows, all
measurements have errors.

Not so, as any competent engineer/scientist will tell you.

A skilled one will measure something so
that the error is less than the difference between his measurement
and any point at which a decision changes (12 foot-pounds in this
case).

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.

The mind boggles! No, I will not continue this, er, debate - but
I can assure you that it is a standard and required activity by any
serious scientist or engineer.

Thank - er - goodness - for that.

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