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Post by Pete on Jul 19, 2008 8:05:28 GMT
If I pull you one way, and Twoddle pulls you another, you will travel as if pulled by the resultant of our two pulls. But no-one is actually applying that resultant to you: you are still subject to two actual pulls. I cannot understand why you disagree with this statement, Geoff. You might feel as though there is only one pull, being the effect of the combined pulls creating a single directional vector of force, but there are actually two forces (pulls) at work. Actually, of course, the earth's tides are affected by a large (probably infinite) number of forces, including gravitational forces, inertia, friction, etc. But the main non-terrestrial factors are the gravitational fields of the moon and the sun.
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Post by Geoff on Jul 19, 2008 12:00:58 GMT
If I pull you one way, and Twoddle pulls you another, you will travel as if pulled by the resultant of our two pulls. But no-one is actually applying that resultant to you: you are still subject to two actual pulls. I cannot understand why you disagree with this statement, Geoff. You might feel as though there is only one pull, being the effect of the combined pulls creating a single directional vector of force, but there are actually two forces (pulls) at work. Pete, I'm simply saying that, if two or more forces act on a body, then what moves that body is not the individual forces, as such, it is the resultant force that moves the body. The body is, therefore, subject to the resultant force, not, as Paul said, you are still subject to two actual pulls. The body's response to the various forces acting on it, and that is where I see the difference in interpretation, is to that one resultant force. I know what he (and you) are saying, but I do think there is a difference.
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Post by amanda on Jul 19, 2008 12:54:48 GMT
I'm simply saying that, if two or more forces act on a body, then what moves that body is not the individual forces, as such, it is the resultant force that moves the body. The body is, therefore, subject to the resultant force, not, as Paul said, you are still subject to two actual pulls. The body's response to the various forces acting on it, and that is where I see the difference in interpretation, is to that one resultant force. Wot? I don't think you are saying anything remotely simple, Geoff! That has to qualify for the weekly "Clear as Mud" award. (Only jesting ;D)
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Post by Paul Doherty on Jul 19, 2008 13:33:39 GMT
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.
I may choose, for simplicity (sorry Amanda), to regard that as a gift of £5. In the same way, we can take two forces and calculate the resultant, and a body will respond to the single resultant exactly as it would to the two individual forces. So, for convenience, we might choose to regard the body as if if it was acted on by a single force, the resultant. But the resultant doesn't exist in the physical world. The resultant is a convenient fiction -- what's operating on the body is two forces, not one.
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Post by Geoff on Jul 19, 2008 14:17:33 GMT
Paul, I think we're arguing over a technicality which I don't think is worth pursuing. I fully understand what you are saying, and it's hard to argue against what you say. I think, too, you understand the case I'm trying to make, but I'll never get you to concede. The bottom line is, I think the statement: [...] the combined gravitational pull of the sun and moon. is quite clear and correct, you think it should be: [...] the combined gravitational pulls of the sun and moon. Amanda, I hope Paul understands what I was trying to say in my last post. If he does, I must apologise if you think it was worthy of the Clear as Mud award. I don't expect to have to explain it again.
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Post by Paul Doherty on Jul 19, 2008 14:56:51 GMT
I did understand, Geoff. And I agree that we are arguing over something unimportant! For what it's worth, I'm quite happy with your:
[...] the combined gravitational pull of the sun and moon.
(And I did say "works for me" about it way back in answer #8.)
I'm just making the philosophical point that there are two pulls, even if we choose to regard it as one combined pull. The water molecules "feel" two attractions, even though they can only move in one way as a result.
The "resultant" is a physicist's useful fiction -- it exists only as a mental construct, and it can't move anything. The individual forces do that.
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Post by Pete on Jul 19, 2008 15:17:19 GMT
I think the only disagreement we have is that I don't like "combined gravitational pull" and would prefer "pulls" in this context. But maybe it's taste and fancy (as we used to say in the civil service).
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Post by Twoddle on Jul 19, 2008 18:54:24 GMT
Supposing you were being subjected to two opposing forces - let's call them Force A and Force B. If the Force A were trying to make you go in a direction you didn't like, presumably it would be appropriate to let the Force B with you?
(Sorry, I couldn't quite make that work.)
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Post by Tone on Jul 19, 2008 20:54:56 GMT
>The body is, therefore, subject to the resultant force, not, as Paul said, you are still subject to two actual pulls.<I'm with Geoff, here (as I think I was earlier on), but there is a caveat needed. That only works if your affected object's size is tiny in comparison to the distances from the other two bodies. Consider (very, very, very theoretically) two neutron stars orbiting each other 10 feet apart. (That would be some rather mighty orbital velocity!) Now stand (in your spacesuit) between them. Quite distinctly TWO pulls and you would come apart mighty quickly! Tone
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Post by Pete on Jul 19, 2008 21:54:03 GMT
Supposing you were being subjected to two opposing forces - let's call them Force A and Force B. If the Force A were trying to make you go in a direction you didn't like, presumably it would be appropriate to let the Force B with you? (Sorry, I couldn't quite make that work.) At university I had a friend whose birthday was on 4th May and she was a Star Wars fan: "May the fourth be with you!" ;D
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Post by Pete on Jul 19, 2008 21:55:59 GMT
Consider (very, very, very theoretically) two neutron stars orbiting each other 10 feet apart. (That would be some rather mighty orbital velocity!) Now stand (in your spacesuit) between them. Quite distinctly TWO pulls and you would come apart mighty quickly! I think this sort of reductio ad absurdum rather proves my point.
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Post by Bertie on Jul 19, 2008 22:51:54 GMT
Consider (very, very, very theoretically) two neutron stars orbiting each other 10 feet apart. (That would be some rather mighty orbital velocity!) Now stand (in your spacesuit) between them. Quite distinctly TWO pulls and you would come apart mighty quickly! I think this sort of reductio ad absurdum rather proves my point. Which point was that?
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Post by Geoff on Jul 20, 2008 4:19:46 GMT
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. Paul, I omitted to mention earlier that I don't think your analogy is valid. I don't believe you can make a case about how vector quantities work by using scalar quantities. Vector quantities don't simply add and and subtract in the same way as scalar quantities. Anyone, is that right or wrong?
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Post by Pete on Jul 20, 2008 4:55:16 GMT
I think this sort of reductio ad absurdum rather proves my point. Which point was that? The point that we are talking about two gravitational pulls, those of the sun and the moon, not one combined pull. The neutron star example, where the astronaut is pulled apart by them, clearly demonstrates that there are two pulls being 'experienced' by the 'pullee'.
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Post by Pete on Jul 20, 2008 4:58:42 GMT
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. Paul, I omitted to mention earlier that I don't think your analogy is valid. I don't believe you can make a case about how vector quantities work by using scalar quantities. Vector quantities don't simply add and and subtract in the same way as scalar quantities. Anyone, is that right or wrong? Geoff, surely that's the point of an analogy? I thought it helped clarify the picture somewhat. If you push hard enough, you can see a vectorial quality in Paul's analogy, anyway, as there is a directionality to the giving. Maybe that's a step too far, however.
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