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Post by Paul Doherty on Jul 21, 2008 23:52:02 GMT
Surely, according to Newton's second law of motion ... That'll be his third law, won't it? And yes, the barge is dragging the ponies backwards as much as the ponies are pulling the barge forward. Not quite sure I see the relevance!
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Post by Paul Doherty on Jul 21, 2008 23:54:14 GMT
If the two ponies are exerting a force down the ropes onto the barge Two forces ...
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Post by TfS on Jul 22, 2008 8:24:05 GMT
That'll be his third law, won't it? And yes, the barge is dragging the ponies backwards as much as the ponies are pulling the barge forward. That strikes me as resulting in everything, barge and horses, all standing still.
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Post by Geoff on Jul 22, 2008 11:06:35 GMT
That strikes me as resulting in everything, barge and horses, all standing still. Boy, am I rusty on these things. It's taken me quite some time to work out why TfS's statement is incorrect, and the whole system is not at rest. That's just it, you have to consider the whole system. As far as the barge is concerned, the forces acting on it are the forces exerted by the horses and the friction between the water and the barge. The frictional force is the reactionary one due to the barge's forward movement. The pull of the horses is obviously greater than the friction of the water so the barge moves forward, in accordance with the vector resultant of the forces. The reactionary forces along the ropes are forces acting on the horses. Individually, the horses are exerting forces on the road and the reactionary forces of the road push the horses forward. These reactionary forces from the road must be greater than the reactionary force of the barge on the horses if the horses are to move forward. And so on ... Does that make any sense, TfS, or am I talking through my hat. Plenty of cobwebs have been brushed aside here; but have there been enough brushed aside?
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Post by TfS on Jul 22, 2008 12:19:57 GMT
That strikes me as resulting in everything, barge and horses, all standing still. Does that make any sense, TfS, or am I talking through my hat. Plenty of cobwebs have been brushed aside here; but have there been enough brushed aside? Makes perfect sense to me, Geoff. When I have seen horses pulling barges (which is not very often!), then the horses pull on the ropes and the barge moves forward. I don't need to know the physics of the operation to see it happening. My "standing still" comment was as a response to Paul's which, as far as I can see, means that there is no movement by barge or horses or have I got that wrong.
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Post by Geoff on Jul 22, 2008 13:39:52 GMT
My "standing still" comment was as a response to Paul's which, as far as I can see, means that there is no movement by barge or horses or have I got that wrong. The backward force on any one of the horses is that component, along the line of the horses travel, of the barge's reaction along the rope to the force exerted by the horses on the barge. This force, in a direction opposite to that in which the horses are moving, must be less than the force the horses are exerting on the road otherwise there would be no resultant forward movement of the horses. So, I think, strictly speaking, the accuracy of Paul's statement is questionable.
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Post by Paul Doherty on Jul 22, 2008 13:54:07 GMT
The system of barge, ropes, horses is indeed at rest (or equilibrium, if you like)1. The system of barge and water is not at rest, nor is the system of horses and road, but that's not what we were talking about.
It's all about frame of reference.
1 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.
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Post by Geoff on Jul 22, 2008 14:10:25 GMT
The system of barge, ropes, horses is indeed at rest (or equilibrium, if you like). The system of barge and water is not at rest, nor is the system of horses and road, but that's not what we were talking about. It's all about frame of reference. How can the system of barge, ropes, horses be at rest if the barge and the horses are moving forward. The system might be in equilibrium but I certainly wouldn't say it's at rest. I wouldn't say that equilibrium and at rest are equivalent, either. I looked at the whole system because there was a question arising from one of your posts that if one were to consider Newton's third law of motion (that for every action there is an equal and opposite reaction) wouldn't the system be at rest, that is, that nothing would be moving. So, I think that is what we were talking about.
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Post by Trevor on Jul 22, 2008 15:05:14 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!
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Post by Paul Doherty on Jul 22, 2008 15:38:14 GMT
How can the system of barge, ropes, horses be at rest if the barge and the horses are moving forward. 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?
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Post by Paul Doherty on Jul 22, 2008 15:41:54 GMT
Which got me wondering. Google is a wonderful thing: Speed of car down road: 30 mph Speed of Earth's rotation at Sydney: 870 mph Speed of Earth in orbit: 67,000 mph All approximate!
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Post by Paul Doherty on Jul 22, 2008 15:44:41 GMT
And if I throw an apple from the front seat to the back seat of the car at 4 mph, what is the speed of the apple? 26 mph?
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Post by SusanB on Jul 22, 2008 15:56:37 GMT
Is the car going forwards or backwards?
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Post by Paul Doherty on Jul 22, 2008 16:49:24 GMT
Forwards, from the car's point of view!
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Post by Twoddle on Jul 22, 2008 17:41:10 GMT
Paul:
It should have been his aunty who said that.
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