Subduction Zone
Muse
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The problem is since humber has the relationship back to front when the cart goes faster than the wind humber thinks that it is going slower than the wind.
Split Thread The validity of classical physics (split from: DWFTTW)
- Thread starter Dan O.
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The problem is since humber has the relationship back to front when the cart goes faster than the wind humber thinks that it is going slower than the wind.
John Freestone
Graduate Poster
Is that CW?
Clive, I urge you to do neither. Humber is just trying to get you sucked into the propeller.
John Freestone
Graduate Poster
What I meant to say was "Spork, what's wrong with you? Humber says the treadmill is going the wrong way. Turn the damn thing round. It's not nailed to the floor.
" It just kind of ruins the joke if you keep winking every time.
I'm done making sense. If anything I say in this thread has any relation to humber, just take the wink as read.
Christian Klippel
Master Poster
Wait a second. The point is that outside in a real wind, the wind right at the groundlevel is moving slower, but the same direction as the overall wind? So, for a downwind cart, it moves from the cart's back to its front, but just a tad slower so directly at the ground?
And that in the treadmill case, that wind at the belt is moving from the cart's front to its back?
And that in the treadmill case, that wind at the belt is moving from the cart's front to its back?
And the average velocity 1mm above the ground is which way?
And 1mm above that?
So both travel at nearly the same velocity in which direction?
The profile is an instantaneous snapshot.
So which way does the toilet paper in #7 go? Right to left.
In a real tail wind for the cart? Left to right.
ETA;
Don't confuse the relative wind of the cart through the air, with that of the air.
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John Freestone
Graduate Poster
If I understand what you're saying, then think again. That got me for a moment too. The slowness (if you take my meaning) of the wind at ground level in the real world is also 'moving' from front to back of the cart. I think that you've got mixed up because you are contrasting the belt's dragged air with stationary air w.r.t. the room. It's a mixed frame measurement thing, I'm pretty sure.
ETA: Does that make sense? When you say that in the real world that boundary layer is moving in the same direction as the cart, that is true, and it gets you for a moment. The point is that not only are velocities relative and we can measure them from a particular 'frame' or with respect to something we consider stationary, but as we do that [switch frames] there are some velocities that switch sense, from positive to negative. Everything in the 'real world' (sorry, spork, but you know what we mean!) and the treadmill is "going in the same direction", if you take the centre of the Earth as your reference frame.
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Subduction Zone
Muse
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What I meant to say was "Spork, what's wrong with you? Humber says the treadmill is going the wrong way. Turn the damn thing round. It's not nailed to the floor.
I'm done making sense. If anything I say in this thread has any relation to humber, just take the wink as read.
Sorry, you are right. I have had the same thing happen to me sometimes. I guess I was not too quick on the draw. But unfortunately I was right also, humber thinks if you are near the head of the treadmill and move towards the cart you will feel the wind on your back
Christian Klippel
Master Poster
Hello John,
Right, that's what i wanted to clarify next.
If the air directly at the ground moves a bit slower than the air, lets say, 10cm above it, then it logically follows that when the cart is at the same speed as the air in 10cm, the air at the ground changes direction relative to the cart. That is, relative to the cart it moves from front to back.
And there we have the same situation as on the treadmill. The air is stationary relative to the cart at 10cm above the belt, while the air directly at the belt moves from front to back relative to the cart.
Right, John?
Edit: I'm referring to Clive's graphic in post #590 on the previous page.
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John Freestone
Graduate Poster
No problem at all Subduction Zone. I should really have used an emoticon for clarity. Who's humber?Sorry, you are right. I have had the same thing happen to me sometimes. I guess I was not too quick on the draw. But unfortunately I was right also, humber thinks if you are near the head of the treadmill and move towards the cart you will feel the wind on your back
Yeah, I know, it's quite unbelievable - literally, I don't believe it isn't trolling, deliberately being wrong and confusing and putting temptation in our way to follow new lines of argument to keep us from dealing with old ones, etc. He even brings old ones back to avoid the thread topic. This topic is not supposed to be about the cart and the treadmill, but it got split so that we could have somewhere to deal with humberphysics in more general terms. Now he's managed to get us back on the cart again.
There are only so many times I can be bothered to ask a simple question of someone and be ignored. Actually I take it as something of a compliment. If he can run rings round you, he'll keep running rings round you. If you can pin him down on an issue in a way he can't reply to, his only option is to pretend you don't exist, or the question doesn't exist.
He is so deliberately contrary that if I hadn't written this sentence predicting that he would reply to me again after I said he was ignoring me, he would reply to me again. If I then didn't say "Having written it, he won't.", he wouldn't.
Yes the tailwind is left to right (relative to the belt). There is no cart on my belt so which belt are you looking at? Perhaps you are on the wrong side of it? Anyway, if there was a cart on my belt (assumed to be moving so it is at rest relative to the body of the treadmill), then I agree the flow (near the belt) would be right to left w.r.t. the cart. But this is also the case for the cart moving over the ground at wind speed so I'm not sure what point you're trying to make.
Okay - I am on the belt facing in the same direction as the cart. This is not what I really thinking of as "facing the cart" but it's the only way I can sensibly get wind impinging on my back, because now the belt is carrying me backwards through the air. I jump (straight up relative to the moving belt). The flow of air under my feet is heel to toe, not toe to heel. I am not moving horizontally compared to the belt, just as I would not move horizontally compared to the ground if I jumped straight up.
I stand in the real wind with the wind at my back. I jump straight up. (I hope that is the right direction after all this?!) When I do that, the "laminar flow" will be from heel to toe (unless perhaps I'm also standing on my head). So the situation on the cart and on the ground is exactly the same in that sense also.
Have you ever been tested for dyslexia humber? Seriously, that might explain some of the trouble you are having with this.
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Sorry, you are right. I have had the same thing happen to me sometimes. I guess I was not too quick on the draw. But unfortunately I was right also, humber thinks if you are near the head of the treadmill and move towards the cart you will feel the wind on your back
No. You are NOT sticking to your rules, SD!
The situation is for when you are traveling back with the belt. That is stationary in the real world. Now try.
Real downwind:
If you see the cart as stationary w.r.t the wind windspeed, what velocity is the laminar flow w.r.t you? Windspeed beneath you, just like the road.
On the treadmill this is similar. The laminar flow, is beneath the "windspeed" cart, going backwards like the belt (road)
This is not the same for the observer going back with the belt. Who feels a wind to his back, yet when he jumps to become an "imaginary observer", sees the laminar flow from the toe to heel
When in a real downwind, the stationary observer, feels the wind to his back.
But when he jumps, the imaginary observer, sees the laminar flow from heel to toe. It's perhaps on average slower, but still moving in that direction.
That difference is minor, but in a roundabout way, keeps the cart where it is.
John Freestone
Graduate Poster
I think that's right. I tend to jump in sometimes and then wait with bated breath for someone more experienced to back me up or correct me. These pesky frames of reference are really quite difficult at times.
Velocities are relative. So even when you have got as far as saying "if the wind slows down a bit", you have described an idea involving a relative velocity (something is slower than something else), and you therefore need more information regarding references. I think we need not just a zero-velocity point, if you like, but a 'positive direction' too. No doubt someone will explain better, but what I mean is that there's no such thing as "slowing down" or "speeding up" unless you've given the front and back, the basic direction of the vectors you are measuring, I think.
So, when we put the cart in a different frame, where the air is still and the ground is moving, it all gets very difficult and I have to think really hard, but all the velocities are the same, just with a numerical boost, a constant value added to each. It is quite hard just to get used to the idea that a windspeed has become zero, but once you really get that idea, you see that what was "going left to right" can now be "going right to left", since the addition of that constant has moved it past zero. All I do to check these things (in imagination, because I haven't mastered the maths) is to go back to thinking of myself moving along fixed to the surface of the belt, and all those movements and relative velocities kind of snap back into order, just as if I'm standing on the ground with the cart going over my head (don't ask why it goes over my head, maybe I'm very short).
I look forward to a more learned explanation, and the rescuing of Newton's reputation yet again!
You have made a mistake here humber. You are standing on the belt, moving at belt speed, backwards through the air. The air near the belt is being dragged in the same direction as you by the belt, but at a slower speed. Therefore, relative to you, the flow near the belt is moving from heel to toe.
Brian-M
Daydreamer
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It most certainly is a trivial point, and a compltely meaningless distinction as the same outcome occurs no matter which way you're modelling it.
In fact in real life, the ground does much more than appear to be passing under you. Since the earth is rotating, the ground really is passing under you too, not that it makes any difference in outcome.
Then please explain how the conditions do differ, and exactly what would cause the balloon in, your version, to be dragged back in the direction of the motion of the ground (relative to the air).
But the balloon isn't passing through the air, it's moving with the air. It's completely stationary relative to the air. If the baloon isn't passing through the air, then it's not getting any drag of any kind from the air. Why then yould the balloon be dragged in the direction the ground is moving relative to the air?
In all your talk, you still haven't explained how the ground's movement relative to the balloon (or, if you prefer, the air's movement relative to the ground) could possibly affect the baloon's motion (or lack of motion) through the air.
Is that why you often argue from personal incredulity?
Clive, I am not dyslexic. Trust me, I am right. I build models, all the time.
Start again. It is agreed that the cart is propelled by an effective tailwind, a wind moving left to right. So let's agree that the observer, has that wind to his back, so he would be looking to the right.
To make the model complete, moving with the belt must be the equivalent of being stationary in the real world, with the wind to his back.
Realwind:
Wind from the back, with the laminar wind flowing from heel to toe. Like the wind. Correct?
Now, become an imaginary observer, which way does the flow move?
From heel to toe. It passes from heel to toe, like the wind.
Treadmill:
Wind to the back, with laminar wind flowing from heel to toe. Correct?
Now, become an imaginary observer.
Which way does the laminar flow go?
From toe to heel. It passes from heel to toe like the belt (road), not the wind.
Which part do you not agree with?
Two separate "winds" that are only "correct" in two limiting cases. When fixed to the belt, or at "windspeed". At other velocites, the ratio will change.
This distinction is very important.
Christian Klippel
Master Poster
Hello John,
alright then. So i got you right, and indeed that is what i thought all the time as well. Of course one needs a reference point to make assumptions of what is slower, faster, direction, etc.
That's why i prefer to use the reference frame that goes with the cart. I know that other frames can be used, but since it is about the cart in this case, i tend to say that it is the most important one. And from that frame, the situations are the same for the treadmill and for an outside test. The cart is required to have ground/belt contact, and contact to the air via the prop. Both are given in both situations.
And for the relative air speeds at ground/belt level, as well as, lets say, the air speed at the cart's center, the situations are again the same for all practical purposes. I don't know why humber always wants to introduce some other frames, like a person on the belt jumping around, or an observer outside of the cart's reference, since that has no relation to the real thing at all. Yes, there might be minor differencies, but none of them have a big enough influence to completely invalidate the whole setup.
But then, it's just his way to try to find an out and misdirect us to other topics. As many have already said....
Greetings,
Chris
Perhaps, it is not so easy. This is a conceptual problem. I have said so many times. Please see my "watertight" description of my lastest post.
John Freestone
Graduate Poster
You see, this is a perfect example of how not to describe relative velocities. There is no indication (ok, I might have missed it earlier) of what 'front' and 'back' mean. Are they related to the front and back of the cart? You would think so, naturally. But if someone starts talking about being behind the cart moving towards it, then we picture a person facing the other way, and also the direction of the belt could suggest a front, or at least a 'forwards' and 'backwards' that is different from that of the cart. Once we start talking about winds moving, and now differential windspeeds at different heights, we can't do this, "feels the wind at his back" rubbish. We have to make some attempt to be specific, if not scientific. It probably isn't necessary, however, to develop new dimensions of relativity, such as "w.r.t. the wind windspeed".
If, however, 'going back with the belt...feels a wind to his back' has any meaning, we can be sure humber hasn't got it. If this person is going back with the belt (fairly unambigous - I presume he means stationary wrt the belt), then which side of them is impacting the air (feels the wind) will depend on which way the little imp is facing. If he's facing the opposite way to the cart (you should always face oncoming traffic), then since he is moving and the air is stationary, he hits the air with his front. Humber's imp may be facing the other way, which is dangerous. It is, however, a good trick in describing your amazing travels, so that you can't be proved wrong later.
When the observer going back with the belt jumps he keeps moving backwards*. Jumping can't suddenly change his speed from whatever it is to zero** and back to whatever it was as soon as he hits the belt again. His lateral velocity does not change, so he keeps backing up as he goes through his up-and-down motion, and the boundary layer air continues to move heel-to-toe, though at a speed somewhat slower than that on the back of his head.
*I was going to add "you moron", but that would violate the terms of agreement.
**As viewed from someone standing on the floor.
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