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Post by Paul Doherty on Jul 20, 2008 12:04:26 GMT
I omitted to mention earlier that I don't think your analogy is valid. Fair enough. I don't think analogies have to match in every respect the thing being analogised, but if it doesn't help, it doesn't help!
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Post by Geoff on Jul 20, 2008 12:20:12 GMT
Pete, Paul, Geoff: what is exerting this single vector? Let's try an analogy (this almost never helps, I realise). If you give me £2 and Pete gives me £3 we all agree that I am £5 better off. But there is no gift of £5 -- rather it is the combined effect of the two gifts of £2 and £3. what is exerting this single vector? I think that's a leading question. There is no single thing that we can point to, which is what you want me to say; but the body does react (at least, as I see it) to only one force and that is the vector resultant of the other forces acting on it. In the analogy, what is it that equates to the forces acting on the body, the money amounts of £2 and £3? Now consider what equates to the resultant force acting on the body, is it the gift of £5, is it the money amount of £5, or is it being better off by £5. It seems to me to be the gift. Finally, what is it that you consider to be the combined effect of the two gifts of £2 and £3? Is it the gift of £5, the money amount of £5, or the being better off by £5? As an analogy, I have trouble seeing clearly the similarity between the inputs and the outputs, and I wonder if the output in your analogy is of the same type as the inputs. Yes, definitely a step too far. Paul, you posted while I was having trouble writing and getting the formatting correct. I'll leave what I've said stand - I wouldn't want to waste the effort I put in.
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Post by Paul Doherty on Jul 20, 2008 15:22:56 GMT
That's fine, Geoff. Just two different mental models, I think.
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Post by Tone on Jul 20, 2008 20:30:38 GMT
>The neutron star example, where the astronaut is pulled apart by them, clearly demonstrates that there are two pulls being 'experienced' by the 'pullee'.<
But my point was that that was a special case, and special cases are usually used to indicate an exception which is contrary to a general rule.
Consider two barge-ponies pulling a barge along a canal. It doesn't matter it you rope the ponies together and then rope the trailing pony to the barge, or if you rope each pony separately to the barge. Even though each pony pulls, the barge only gets ONE pull along the canal!
(Equipment: canal, towpath, barge, bargee, rope, ponies, oats for ponies.)
Tone
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Post by Paul Doherty on Jul 20, 2008 21:13:58 GMT
I think you've almost answered it with that analogy, Tone. Consider one pony either side of the canal -- one towpath per pony. The barge will move forward down the canal (and won't need to use its rudder), but there are two forces working, as transmitted down the two ropes.
If there were only one force at work, it would need to be exerted by a double-pony walking directly ahead of the barge and towing with one rope. And then the double-pony would get its feet wet; I assert than no such wet-footed ponies exist, and that there is NOT a single pull at work, however much it might seem like it to the bargeman.
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Post by Twoddle on Jul 20, 2008 21:22:53 GMT
I think you've almost answered it with that analogy, Tone. Consider one pony either side of the canal -- one towpath per pony. The barge will move forward down the canal (and won't need to use its rudder), but there are two forces working, as transmitted down the two ropes. If there were only one force at work, it would need to be exerted by a double-pony walking directly ahead of the barge and towing with one rope. And then the double-pony would get its feet wet; I assert than no such wet-footed ponies exist, and that there is NOT a single pull at work, however much it might seem like it to the bargeman. Paul, have you ensured that no horse-drawn barges are coming the other way before conducting this experiment? Otherwise it could play hell with the vectors and you might well be offered a "bargee's farewell".
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Post by Paul Doherty on Jul 20, 2008 21:26:26 GMT
It's a one-way canal, Twod.
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Post by Pete on Jul 20, 2008 22:00:51 GMT
It's a one-way canal, Twod. After all, when was the last time you saw a canal with the water flowing both ways?
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Post by Vadim on Jul 21, 2008 7:15:19 GMT
I'm with Geoff (and Tone) on this one - sorry Pete, Paul. There may, indeed, be two forces pulling on the said object (the Earth), and although I'm impressed with your knowledge of vector and scaler quantities, the question was, and is, about the use of the language in THAT sentence. [...] the combined gravitational pull of the sun and moon. Without the word "combined" there, then yes, I'd agree with Pete that it has to be "pulls". However, the combined (which is the first word I come across in the sentence) tells me that I have already "added the vectors". Therefore, IMO, we are talking about THE pull of THE sun AND moon. The pony and money examples don't really do anything for me. Paul, you state that I don't think this was ever in question. Of course (sorry Amanda) there are two forces (+ a multitude of other small forces - friction, the pull of the sun and the moon , etc.) acting on the barge. However, it is a question of what the Barge feels. It feels only one force, it therefore has, only one vector. I hope I made myself clear, however, I very much doubt it.
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Post by Pete on Jul 21, 2008 18:39:59 GMT
I'm impressed with your knowledge of vector and scaler quantities Surely scalar, Vadim? the combined gravitational pull of the sun and moon. Sorry, I still disagree. The combination may have the same effect as a single pull but it is still a combination of two pulls. it is a question of what the Barge feels Nothing, I suspect, it's just a barge. ;D
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Post by Tone on Jul 21, 2008 20:30:27 GMT
>Nothing, I suspect, it's just a barge.<
Shame on you. Barges can have feelings, too. (Why else do they call boats "she"?)
Tone
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Post by Twoddle on Jul 21, 2008 20:38:36 GMT
>Nothing, I suspect, it's just a barge.<Shame on you. Barges can have feelings, too. (Why else do they call boats "she"?) Tone Tone, means of transport don't have feelings ... except steam locos of course.
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Post by Paul Doherty on Jul 21, 2008 21:02:34 GMT
One force transmitted down two ropes, Vadim? How can that be?
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Post by Vadim on Jul 21, 2008 23:27:24 GMT
One force transmitted down two ropes, Vadim? How can that be? Surely, according to Newton's second law of motion, If the two ponies are exerting a force down the ropes onto the barge, then the barge is exerting an equal and opposite force down the 2 ropes to the ponies. If the 2 ropes are connected at the same point (some theoretical centre) isn't the barge doing what you describe, Paul?
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Post by Vadim on Jul 21, 2008 23:31:46 GMT
>Nothing, I suspect, it's just a barge.<Shame on you. Barges can have feelings, too. (Why else do they call boats "she"?) Tone What's all this argie-bargie about? ;D
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