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Post by Pete on Jul 27, 2008 11:09:04 GMT
We may have caused some of the confusion by constantly referring to two ponies pulling a barge, as if the combination were tantamount to a single pony walking on the water; and to two gravitational forces affecting a body of water, as if it were a single combined gravitational force.
So let's look at different types of vector acting together: imagine a sailing boat on the sea. One force acting on it is the vector force of the wind blowing it northwards (say). And another is an equally strong tide pushing it eastwards. The combination of the vectors would act like a single force pushing the boat north-eastwards. But it is not a combined wind force pushing it north-eastwards and nor is it a combined tide pushing it north-eastwards. It is just a combined vector of both the forces.
And the fact that they are different types of force combining to give this result is the demonstration that the combined apparent force is not a single force by itself.
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Post by Geoff on Jul 27, 2008 11:48:26 GMT
The sum equates to a combined vector that "looks like" or "acts like" a third force with the combined vector but actually is not. I'm trying to stay true to my word. Please do a Google search for the meaning of force.
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Post by Pete on Jul 27, 2008 12:15:44 GMT
I found this www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/newtlaws/u2l2d.html which explains things quite well. One interesting quote: "In each of the above situations, there is an unbalanced force. It is commonly said that in each situation there is a net force acting upon the object. The net force is the vector sum of all the forces which act upon an object. That is to say, the net force is the sum of all the forces, taking into account the fact that a force is a vector and two forces of equal magnitude and opposite direction will cancel each other out." (My emphases). To me, this says that we are both largely correct. You are saying that the effect of unbalanced forces is, in effect, a net force. But many of the posts (some yours? I can't remember) have talked about the combination of more than one gravitational field as being a net force of a single gravitational field. The point I was making (perhaps badly) in my last post is that it may be what some people call a net force but it isn't a net gravitational pull. And while I am happy that "net force" is a way to describe what is going on, it is not, in fact, a force at all, merely the net result of combining other forces.
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Post by Tone on Jul 27, 2008 20:29:59 GMT
When I postulated the two ponies pulling via separate ropes attached to a barge it never occurred to me that it would be thought that they were on opposite banks of the canal. So far as I know canals (except briefly at interconnections), in this part of the world (not Mars), have only one towpath (on one side).
That aside, if you are sitting on the moving barge and you are blind you only know that it is moving (by dipping you hand in the water, or brushing it against the bank?). Drifting aside, you must assume that there could be one pull effecting this.
And, reductio etc., if there is only one pony, should we, by the logic of those who fight this, say that the barge is moved by the pulls of each of the ponies legs (acting as pushes on the ground)?
Likewize, if we may anthropomorphize the ocean, it feels one pull affecting it and effecting its movement.
Tone
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Post by SusanB on Jul 27, 2008 20:52:39 GMT
Would you like an apostrophe for your ponies and their legs, Tone? In case yes, I have a spare one: '.
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Post by Pete on Jul 27, 2008 21:25:53 GMT
Tone, why have you suddenly become female? Does it hurt? ;D
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Post by Pete on Jul 27, 2008 21:26:44 GMT
And shouldn't the UK English spelling be 'anthropomorphise'?
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Post by Paul Doherty on Jul 27, 2008 22:11:26 GMT
Tone, the barge feels two pulls, and the oceans feel two pulls. A blind person sitting on the barge (or someone watching the tides) might assume just one pony (or one pull on the ocean) but it doesn't make it true! There's still two ponies, whatever the blind person thinks.
As for four forces (one per leg), yes indeed. They are combined into one force by the body of the pony, though. If the two ropes where tied together and then (as a single rope) proceed onwards to the barge, I'd agree there's one pull. But that isn't what we're proposing, and it isn't what happens with tides -- the force from the sun and the force from the moon don't somehow combine in outer space and become one force acting on the oceans.
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Post by Dave M on Jul 27, 2008 23:10:50 GMT
I say there are two forces acting on the barge, but the barge acts as if there were the one resultant (note the "counterfactual" inference).
We can show that there are two forces: Have the two ponies pull at different ends of the barge, so that it remains stationary. Put spring balances in each tow rope. The balances show the amount of each force. There's nowhere you can put a balance to read off the "nil" force.
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Post by Geoff on Jul 27, 2008 23:23:36 GMT
Pete, To me, this says that we are both largely correct. You are saying that the effect of unbalanced forces is, in effect, a net force. But many of the posts (some yours? I can't remember) have talked about the combination of more than one gravitational field as being a net force of a single gravitational field. The point I was making (perhaps badly) in my last post is that it may be what some people call a net force but it isn't a net gravitational pull. I'm coming out of self-imposed silence because I'm confused. (My apologies to all the natives out there who are getting restless.) Surely the gravitational pull of the sun is a force, and the gravitational pull of the moon is a force. Of course, there is the gravitational pull of the earth that should be taken into account when looking at the system in which the tides occur. When the gravitational forces are resolved there is an unbalanced force which is what acts on the oceans to cause the tides. Isn't that unbalanced force a net gravitational pull? If it's not, then I am at a loss to know what it is. To say it is not ... a force at all seems to contradict your own reference (link) which says: As mentioned earlier, a net force (i.e., an unbalanced force) causes an acceleration. Isn't that which causes an acceleration, a force? Note also, that the reference says it is the unbalanced force which causes the acceleration.
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Post by Geoff on Jul 27, 2008 23:38:34 GMT
I say there are two forces acting on the barge, but the barge acts as if there were the one resultant (note the "counterfactual" inference). We can show that there are two forces: Have the two ponies pull at different ends of the barge, so that it remains stationary. Put spring balances in each tow rope. The balances show the amount of each force. There's nowhere you can put a balance to read off the "nil" force. Dave, Perhaps we have lost sight of what it is we're arguing about. No-one is denying the existence of the separate forces acting on the body. I believe we're arguing about what it is that moves the body. My point is that it is the resultant of the forces, the unbalanced force, to use the term used in Pete's reference. That unbalanced force is a real force. Now, in your example, the unbalanced force is zero and the barge does not move. I believe that supports what I'm saying. It certainly doesn't contradict it.
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Post by Dave M on Jul 28, 2008 7:56:21 GMT
Hi Geoff I thought we were arguing about whether, once we'd recognised that two forces have a single resultant, it is therefore improper to refer to the two forces as two. (And I say it's not improper).
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Post by Gabriel-Ernest on Jul 28, 2008 13:07:23 GMT
This debate is becoming, in a weird way, relevant to the original title.
That is, it reminds me of the sort of rambling, never-ending argument that possessed people after a joint. (So I am told, he adds quickly!) Interesting and good fun ‘though.
G-E.
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Post by Paul Doherty on Jul 28, 2008 13:58:28 GMT
It's about the only thread here with has stayed on one topic (not quite the original topic, admittedly) for six or more pages!
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Post by Verbivore on Jul 28, 2008 14:46:18 GMT
This debate [...] reminds me of the sort of rambling, never-ending argument that possessed people after a joint. (So I am told, he adds quickly!) [...] G-E. ;D ;D ;D I concur.
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