Split Thread The validity of classical physics (split from: DWFTTW)

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Dan O. said:
Just for fun, I setup a 1 dimensional simulation of a boat in water. The simulation shows just what the first order equasions expect: The boat never stops moving but its velocity decays exponentially.

However, when I replace the perfect water with random size particles [a uniform distribution with the same mean], the boat does stop and even begins moving backwards :jaw-dropp
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That's really interesting. You mean it goes backwards temporarily, randomly? Surely not steady state? That would have to be something to do with your magic cat's eyes.
 
John Freestone said:
Once again, then, I ask, if drag is not the force of the wind that accelerates a balloon, what is the force that does accelerate it? We can call it something else: humber's mystery force, if you like. Drag is dissipative, you say, so it reduces the balloon's velocity as it floats in the wind, have I got that right? That must be the drag that keeps it below windspeed, I suppose? So what's humber's mystery force accelerating the balloon? When we first let it loose from the field, there's presumably drag of the air, which you say will slow it down. How does it go slower than stopped? So many questions.

Sorted, although I don't know much about negative lengths. I didn't have enough room for your gems.
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Springer Verlag will not be waiting for your call, John.
But you know as much about pseudoenergy as you do about psuedomorphism.

Force (F) -> Voltage (V) (it is, after all, the electrical force in yer batteries)
Acceleration (a) -> Current (I)
Mass (m) -> Resistance (R)
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But the whole 'model' idea is fairly banal anyway, and electrical equations certainly can't be used to prove mechanical ones wrong, as humber was trying to do.
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Try Googling that error. Tons..

http://www.swarthmore.edu/NatSci/echeeve1/Ref/Analogs/ElectricalMechanicalAnalogs.html

http://ieeexplore.ieee.org/xpl/freeabs_all.jsp?arnumber=973452

Voltage is potential not a force.
The analog of mass is inductance.
The analog of Force is current.

Stop looking for the inner child, John. You've already found him.
 
Dan O. said:
Just for fun, I setup a 1 dimensional simulation of a boat in water. The simulation shows just what the first order equasions expect: The boat never stops moving but its velocity decays exponentially.

However, when I replace the perfect water with random size particles [a uniform distribution with the same mean], the boat does stop and even begins moving backwards :jaw-dropp
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Yes, that is what would be expected. A first order one-dimensional model is inadequate for the purposes. There are a lot of elementary gas simulation programmes that work in the same way.

Something tells me that the value of the exponent may be critical. As you mentioned earlier, the object may even oscillate.
 
Of course the boat moves backwards. This is to be expected. Drag will make it so. A object cannot stay put without work being done on it, and forces are never zero, no matter how much you might think at them. If the model is done correctly, and the water is well-lubricated, the boat will actually hover over the water, or in a discrete case, hop out of it. In fact, I think I'm probably pretty well-lubricated myself.

Congratulations to humber for passing the 1000-post mark, by the way. I hope I can have the drive to get there myself, and to provide as much useful information to you people as he has. (Actually, I think I have the latter pretty much under control.)
 
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This thread was a hoot from page 1. I particularly liked that part about canoes having a bow-wave when floating downstream.
 
ArmillarySphere said:
This thread was a hoot from page 1. I particularly liked that part about canoes having a bow-wave when floating downstream.
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They do. You have to paddle downstream to reach waterspeed.
If you say not, find a reference that supports your view.
 
humb said:
In fact, I think I'm probably pretty well-lubricated myself.
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What consenting adults do in their own time is their business, and Crisco's.

Congratulations to humber for passing the 1000-post mark, by the way. I hope I can have the drive to get there myself, and to provide as much useful information to you people as he has. (Actually, I think I have the latter pretty much under control.)
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Don't forget to save fuel by turning off the engine. You may be late.

A object cannot stay put without work being done on it <snip>
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How do fish stay stationary in flowing water? They can do so upstream or downstream.

(Actually, I think I have the latter pretty much under control.)
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I am sure you think so.
 
humber said:
They do. You have to paddle downstream to reach waterspeed.
If you say not, find a reference that supports your view.
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If that were so, the canoe left to itself would be moving relative to the water (by moving downstream more slowly than the current, it would be moving upstream relative to the water). It it had a wake, it would necessarily have motion relative to the water. Where does the energy come from to move it through the water like that?

You keep failing to address the simple point that there is no drag on an object moving at the speed of the current.
 
humb said:
Of course the boat moves backwards. This is to be expected. Drag will make it so. A object cannot stay put without work being done on it, and forces are never zero, no matter how much you might think at them. If the model is done correctly, and the water is well-lubricated, the boat will actually hover over the water, or in a discrete case, hop out of it. In fact, I think I'm probably pretty well-lubricated myself.

Congratulations to humber for passing the 1000-post mark, by the way. I hope I can have the drive to get there myself, and to provide as much useful information to you people as he has. (Actually, I think I have the latter pretty much under control.)
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If humb and humber didn't have their names on their posts how many of us could tell which was which? I initially misread the name on this post and thought it was humber until the last sentence of the first paragraph.
 
Dan O. said:
Just for fun, I setup a 1 dimensional simulation of a boat in water. The simulation shows just what the first order equasions expect: The boat never stops moving but its velocity decays exponentially.
Click to expand...

It shouldn't be exponential unless the drag is linear in v - if it's quadratic the solution is a power law (v~1/t).

However it may be that for small v, where the flow ceases to be turbulent, the drag does become linear (in which case you'd indeed get an exponential).
 
sol invictus said:
It shouldn't be exponential unless the drag is linear in v - if it's quadratic the solution is a power law (v~1/t).

However it may be that for small v, where the flow ceases to be turbulent, the drag does become linear (in which case you'd indeed get an exponential).
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It troubles me to see a reasoned exchange about the finer points of classical physics amidst the circus we are trying to conduct with humber. Please let's stay on topic. :D
 
humber's point about the boats not reaching waterspeed is self-evident. Consider a canoe in a glacier. Does the canoe move at the speed of the ice? Of course it does, due to the high viscosity of the ice. But then consider a canoe in a flowing vacuum. Does the canoe reach the speed of the vacuum? No, because there is no way for the vacuum to transfer its nonexistent momentum to the canoe, because there is no mass or viscosity. Just as dust behind the couch does not reach the speed of the vacuum when traveling up the hose, although unlike the canoe, which will not move at all, the dust does travel into the vacuum canister, but only because of osmosis. With a river, which is partway between a glacier and a vacuum, the reduced viscosity means that the canoe can reach only an intermediate speed. And the density of water, which is greater. Altogether. Can you deny it? In a river of steam, the speed would be lower still, but the birch bark would start to peel.
 
spacediver said:
Imagine a glass sphere is flying through space at 10 m/s relative to a stationary observer. Then someone throws a hard rock from behind the sphere, aimed at the sphere.The rock is thrown at 100 m/s.

The difference in velocities is 90 m/s.

Now suppose we rewound time, but now the glass sphere is moving at 80 m/s and the rock is thrown at 100 m/s.

The difference now is 20 m/s

In which scenario is the glass sphere more likely to be cracked, when the rock catches up to the sphere?

The answer is the first scenario, even though the rock had the same velocity relative to the stationary observer.
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Humber said:
Yes, a kinematic case, where force is simply "applied", not the same sort of problem at all.
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Everything is kinematic humber, there are no "kinematic" vs "reality" physics. The example I gave is a simplified one so that we can focus on specific principles. I could easily add a few more things to my "kinematic" example to make it more akin to going through wind, but that's not the point.

Now let's continue with this "kinematic" case.

Suppose the rock in question weighed 2 kg.

Do you agree that the kinetic energy of the stone, relative to the glass sphere, is different depending on how fast the sphere is traveling?

i.e. in the first case, the KE = 0.5*2*90*90 but in the second case it equals 0.5*2*20*20
 
spork said:
It troubles me to see a reasoned exchange about the finer points of classical physics amidst the circus we are trying to conduct with humber. Please let's stay on topic. :D
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You're right - sorry. I'll try to keep on track!

Now back to your regularly scheduled programming - Humbty Dumbty.
 
jjcote said:
If that were so, the canoe left to itself would be moving relative to the water (by moving downstream more slowly than the current, it would be moving upstream relative to the water).
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No, it moves slower than the water, that's all. The wake is there.

It it had a wake, it would necessarily have motion relative to the water. Where does the energy come from to move it through the water like that?
You keep failing to address the simple point that there is no drag on an object moving at the speed of the current.
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Not addressing? Does the meterological balloon not confirm the point?
I should add that is a very simple approach from a general methodological handbook. It is an approximation at best, and still shows the need for a driving force.

The force to drive the canoe comes from the water. Motion of the canoe through the water generates drag ahead of the canoe, and that opposes the motion of the canoe. Because the energy or force required to move the canoe (and so generate the opposing force) comes from the same source, the river. Therefore, the drag will prevent the canoes from reaching waterspeed.

What you appear to be doing is taking "Gallileon Relativity" too literally. It is not a matter of 400 years of science, but a 400 year old idea that has been supplanted, in fact, by Einstein. Not relativity, but his ideas concerning Brownian motion and Boltzmann's constant.
These insights are the basis of contemporary ideas on of drag, and those ideas say you are wrong. (Anecdotaly, thumbs bruised against the canoe's gunnels say you are wrong)

What proof would you need? I am not going to Wikki so that some idiots can post about it, but if you have an idea, then I will try to answer it.
 
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