GardenBanter.co.uk - View Single Post

14-05-2005, 11:42 AM

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/