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.