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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. |
#3
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In article ,
Jaques d'Alltrades wrote: Assuming I understand by your shorthand what you understand by it, you still haven't addressed the 32ft/sec˛ 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. |
#4
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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/ |
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