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Post by Pete on Jul 22, 2008 18:17:47 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? No reason for the ropes to be connected to each other, just to the barge.
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Post by Tone on Jul 22, 2008 20:59:50 GMT
>It's all about frame of reference.<
That's usually Tone's line!
>How can the system of barge, ropes, horses be at rest if the barge and the horses are moving forward.<
Where is the observer? (And is anything ever truly at rest?)
Agreed that there is a different "pull" in each rope, but, assuming the ropes meet at a single point on the barge, then there is only one "pull" at that point. And hence only one "pull" on the barge.
Tone
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Post by Dr Mildr on Jul 22, 2008 21:23:17 GMT
Leaving aside real-life factors like the elasticity of the rope, the fact that the horses won't exert a constant pull so the rope will go slack from time to time, and so on. And it's Thursday so one of the horses is on a rest day, and the supply of feed is low, and ... ;D
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Post by Geoff on Jul 22, 2008 22:45:36 GMT
Because by saying "moving forward" you are introducing other elements into the system. From the frame of reference of the barge, ropes, horses there is no movement -- they all stay in exactly the same position relative to each other. As I said, Newton is all about frame of reference. (Or, as Einstein pointed out, it's all relative.) You drive down the road at 30 mph, but the road is rotating with the rest of the Earth, and the Earth orbits the sun. What is your speed? Paul, Something is not right here. This whole discussion started off talking about tides due to gravitational forces. Tides involve resultant movement of bodies of water. Then at some point we generalised and started talking simply about forces acting on a body and the resultant movement of that body according to the vectorial resultant of the forces acting on the body. Then, by way of analogy you introduced the idea of horses pulling a barge along a canal. Now it seems you no longer want to talk about the resultant movement of the barge. You want to take the barge out of the water and lift the horses off the ground and consider just the system of barge, horses, and taut ropes in this suspended state. Under these conditions, I think I would have to agree, the system will be at rest, it will be in equilibrium, and the only forces in action will be those along the ropes: the pull of each horse on the barge (to keep the ropes taut) will exactly equal the reactionary force of the barge on each of the horses. As you say, they all stay in exactly the same position relative to each other. There is no resultant vector that we have been talking about: the sum of the vectors is zero. It seems to me that, under the circumstances I've described, and I hope I'm understanding the system you have in mind correctly, it's you that is, rather than introducing other elements into the system, omitting important elements of the type of system we started discussing, a system in which at least one element was not at rest.
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Post by Geoff on Jul 22, 2008 22:56:11 GMT
Can I just come out of the wordwork here to say that, despite not understanding much of it, I've really enjoyed reading this thread just now. Thanks to all you wise contributors! Trevor, I said as much to my son last night. It's been so long since I really had to think about all this physics that the recall and putting it all together has been fun. I just hope I've got my facts right.
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Post by Paul Doherty on Jul 22, 2008 22:59:35 GMT
... assuming the ropes meet at a single point on the barge, then there is only one "pull" at that point. And hence only one "pull" on the barge. And if they are attached at different points? (As Pete pointed out, there's no requirement for them to be attached to one point on the barge.)
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Post by Paul Doherty on Jul 22, 2008 23:02:42 GMT
The barge is Tone's, actually, Geoff.
Once we've all agreed about the details of the barge and where the ropes are attached, I thought I might make it an elastic barge. That would mimic tides, wouldn't it?
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Post by Dave M on Jul 23, 2008 12:56:32 GMT
Well, I go away for a couple of days, and you all get round to talking Physics!
The interesting thing seems to be that (nearly) all the points made are valid. They're not all relevant, though: the whole point, surely, was that the reader is encouraged to consider (1) the pull exerted by the sun, and (2) the pull exerted by the moon. Each pull varies, as the named body moves, and the tides result.
Yes, the magnitude and direction of the tides are the result, mathematically, of the combined (therefore single) resultant force - but it's not the mathematical resultant that the reader was being asked to consider.
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Post by Tone on Jul 23, 2008 20:23:41 GMT
Dave M, >(1) the pull exerted by the sun, and (2) the pull exerted by the moon. Each pull varies, as the named body moves, and the tides result.<
You seem to be implying that the the tides result due to the "as each pull varies".
But, in a purely theoretical situation, with only the moon there would still be tides.
You've introduced a wholly new issue.
The variation of the two pulls is what causes the springs and neaps -- and that's a wholly different matter, for the explanation of which I would fully agree that "pulls" would be appropriate.
But I still maintain that, if we limit the situation to merely "tides", then "pull" is apt.
Tone
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Post by Dave M on Jul 24, 2008 10:36:30 GMT
> But, in a purely theoretical situation, with only the moon there would still be tides. <
Tides, yes - but not "the" tides (ie the particular tide pattern that we have).
I wonder whether we're trying a bit too hard in this pull/pulls thing: if Anne dies, and her two brothers Brian and Cedric are much distressed, which way would we write it:
Brian and Cedric's love for Anne was such that they were much distressed, or Brian and Cedric's loves for Anne were such that they were much distressed ?
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Post by Tone on Jul 24, 2008 20:32:32 GMT
The former.
Tone
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Post by Dave M on Jul 25, 2008 9:15:42 GMT
> The former <
Me, too. But once we analyse the sense of it, we'd take it that Brian had a love for Anne and Cedric had a love for Anne - they didn't have some sort of single "resultant" love!
So with the gravitational pull(s).
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Post by Paul Doherty on Jul 25, 2008 9:30:56 GMT
Exactly, Dave. I'd be happy to say "the pull of the sun and the moon" but it seems completely obvious to me that they have a pull each.
The barge thing also seems obvious to me; I can't understand why Tone and Geoff think that two horses attached to two ropes attached to two eyelets on the barge don't both exert their own pull. Of course the barge only moves in one direction (unless the horses pull it apart) but it seems so obvious that there are two pulls!
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Post by Geoff on Jul 25, 2008 11:51:57 GMT
Paul, Verbivore commented on the statement: Tides are created by the moon and sun's gravitational pulls on the oceans and suggested that it should have been: the moon's and sun's gravitational pulls Some time later, when it was suggested that the original might have been better recast, I suggested that: ... there is one net effect of the two separate forces (pulls) exerted by the sun and the moon. I think I would prefer to say:
[...] the combined gravitational pull of the sun and moon. You thought my suggestion was OK, but others disagreed and maintained that pull should still be plural. The idea of the combined gravitational pull/pulls lead into vectorial analysis of physical systems considered to be analogous to the creation of the tides, including that of two horses pulling a barge up a canal. We even got involved in Newton's third law of motion. Now you say: I don't dispute there are two[/b] [or more] pulls (or forces) at work, whether we are talking of the tides, the 'body' in the generalised system, or the horses pulling the barge. My whole argument has always been that the tides are the result of the combined gravitational effect of the moon and the sun, the body moves as a result of the combined effect of the forces acting on it, and the barge moves as a result of the combined effect of the two horses pulling on it. In other words, the effect, the resultant movement, is due to a single force, the resultant of all the other forces in the system being considered ... and I think that's Tone's position as well. Consequently, I still see nothing wrong with: [...] the combined gravitational pull of the sun and moon.I think each of us is clear on the other's position, and continuing the discussion is going to get us nowhere. Whether one statement is more correct grammatically, which is where this started, I don't know, but I do think what I suggested is correct grammatically and from the physics perspective.
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Post by Verbivore on Jul 25, 2008 12:11:47 GMT
Verbivore commented on the statement: Tides are created by the moon and sun's gravitational pulls on the oceans and suggested that it should have been: the moon's and sun's gravitational pulls Well, now that we've been through all those recastings, and ramblings about physics, barges, horses, canals, and vectors - is there any consensus on which of the above variants is better (or more "correct")? It's a bit late to have the news people recast the statement.
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