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

[Brian-M]

Is that a flippant question?

You believe that if you put a belt under a wheel, it goes at windspeed? I'm the on drugs, SD?

Windspeed has nothing to do with the belt, only the object's speed relative to the air. If there is zero velocity difference between an object and the motion of the air, then that object is travelling at windspeed, no matter how fast, or slow it is travelling. On a windless day, even a lamp post is capable of travelling at wind speed.

In an enclosed room there is no wind, so all stationary objects are travelling at windspeed, regardless of whether or not there is a belt moving under them.

However, if the surface of the belt is moving at 10mph, then everything in the room, including the air, is moving at 10mph relative to the belt.

For an object or person being carried along by the belt, the same applies. Everything in the room, including the air, is moving at 10mph relative to them. So, from their perspective, they are standing stationary in a 10mph wind while everything in the room is racing past at wind-speed.

So an object or person on the belt has to race along the belt at 10mph (windspeed) in order to remain in place with everything else.

So when you see something on a moving belt that looks stationary to you, it's really racing along over the belt at whatever speed the belt is going.

Now, here, you see, it takes all the running you can do, to keep in the same place.

 

[Cecil]

So after having read about 30 pages of the DDWFTTW thread (that was enough for me, thanks) I didn't notice this analogy. I thought I'd chime in here since it seems the discussion has moved.

This seems a bit easier to understand than the cart yet I believe it operates on the same principle. Spork, I'd be interested to see what you think.

It's essentially a "directly down-river faster than the river" boat. It drifts in a free-flowing river and still air, and has a turbine in the air geared to a prop in the water. As it is let go in the river, the turbine speeds up and starts driving the prop. I think it's pretty clear that as the boat reaches the speed of the river, the prop can still harvest energy from the wind (since there's now wind blowing over the boat) and use it to drive the prop, which pushes the boat faster down-river.

So boat-wind = cart-ground and boat-water = cart-wind.

Is this feasible?

 

[humber]

Windspeed has nothing to do with the belt,

I agree, Brian-M!

only the object's speed relative to the air. If there is zero velocity difference between an object and the motion of the air, then that object is travelling at windspeed, no matter how fast, or slow it is travelling. On a windless day, even a lamp post is capable of travelling at wind speed.

I agree, that is the assumed situation.

In an enclosed room there is no wind, so all stationary objects are travelling at windspeed, regardless of whether or not there is a belt moving under them.

However, if the surface of the belt is moving at 10mph, then everything in the room, including the air, is moving at 10mph relative to the belt.

Or a stationary cart, in still air, with a belt passing under its wheels. Which is it?

For an object or person being carried along by the belt, the same applies. Everything in the room, including the air, is moving at 10mph relative to them. So, from their perspective, they are standing stationary in a 10mph wind while everything in the room is racing past at wind-speed.

That is not quite correct. The real person (you) standing on the belt, has those experiences, because you are moving on a beltat a velocity w.r.t those objects. One of those objects is the cart.
It's a stretch to equate that with a cart moving w.r.t aroad.

So an object or person on the belt has to race along the belt at 10mph (windspeed) in order to remain in place with everything else.
So when you see something on a moving belt that looks stationary to you, it's really racing along over the belt at whatever speed the belt is going.

That is information Brian-M. The light reflected off the cart and so interpreted by your mind, leads to that conclusion. It may be correct, but is so in fact?
Where are the expected accompanying physical phenomenon, that support that claim? I find they are missing, or inconsistent.

 

[Clive]

I don't see a diagram.

I've noticed this same problem a couple of earlier times with images (I assume) that humber and perhaps someone else also tried to include via the [q img]url[/q img] construct (spaces added after "q" by me to stopit changing to [url] - a clue? - even though already wrapped in [noparse], etc.!). Does anybody know what the [q img] tag is meant to be for and how it should be used? I couldn't find anything about it in the "complete list of BB codes" supported by the JREF forums. I thought it might be what you get if you upload your own images from the "advanced editor" page but I just tried that and it only gave me ordinary [img] and [/img] tags again. What did you do to upload/reference your image humber? Can anybody else see humber's diagram?

 

[Brian-M]

ETA: just realized I've been using Newtons as a unit of energy this whole time - DOH!

Let's just pretend you really meant "newton-meter" the whole time.

 

[humber]

So after having read about 30 pages of the DDWFTTW thread (that was enough for me, thanks) I didn't notice this analogy. I thought I'd chime in here since it seems the discussion has moved.

This seems a bit easier to understand than the cart yet I believe it operates on the same principle. Spork, I'd be interested to see what you think.

It's essentially a "directly down-river faster than the river" boat. It drifts in a free-flowing river and still air, and has a turbine in the air geared to a prop in the water. As it is let go in the river, the turbine speeds up and starts driving the prop. I think it's pretty clear that as the boat reaches the speed of the river, the prop can still harvest energy from the wind (since there's now wind blowing over the boat) and use it to drive the prop, which pushes the boat faster down-river.

So boat-wind = cart-ground and boat-water = cart-wind.

Is this feasible?

It should be, as there are two sources of energy.
This looks to me to be similar to the way Dr Drela constructed his models. The paper is linked to in this thread.

 

[Cecil]

It should be, as there are two sources of energy.
This looks to me to be similar to the way Dr Drela constructed his models. The paper is linked to in this thread.

Interesting. After your skepticism in the other thread (and this one) I'm surprised to see you agree that this works. Don't you see the similarity between this and the cart? (Maybe you came around on the cart, I didn't read till the end)

When the boat is moving at the speed of the river, it harvests energy from the moving wind and uses it to push against the still water.

Similarly when the cart is moving at the speed of the wind, it harvests energy from the moving ground and uses it to push against the still wind.

 

[humber]

I've noticed this same problem a couple of earlier times with images (I assume) that humber and perhaps someone else also tried to include via the [q img]url[/q img] construct (spaces added after "q" by me to stopit changing to [url] - a clue? - even though already wrapped in [noparse], etc.!). Does anybody know what the [q img] tag is meant to be for and how it should be used? I couldn't find anything about it in the "complete list of BB codes" supported by the JREF forums. I thought it might be what you get if you upload your own images from the "advanced editor" page but I just tried that and it only gave me ordinary [qimg] and [/qimg] tags again. What did you do to upload/reference your image humber? Can anybody else see humber's diagram?

Could be Clive. I hate tags. I uploaded the image as an attachment, then pasted it using the image icon, pasting the links location. To avoid having the "unnecessary" attachment, I deleted it, but perhaps I should not do that?

 

[Clive]

Humber, in the full-blown reply page and underneath the smiley's on the right, there is a small panel labelled "My Images" with options to "Pick" or "Quick Upload". I tried this just a few minutes ago and it seemed to work (although I didn't post the message with image). Perhaps you should try that. By the sounds of what you did, deleting the attachment may have left the link pointing to nothing, hence the problems others have seeing it later. Just a guess though.

 

[humber]

Interesting. After your skepticism in the other thread (and this one) I'm surprised to see you agree that this works. Don't you see the similarity between this and the cart? (Maybe you came around on the cart, I didn't read till the end)

When the boat is moving at the speed of the river, it harvests energy from the moving wind and uses it to push against the still water.

Similarly when the cart is moving at the speed of the wind, it harvests energy from the moving ground and uses it to push against the still wind.

Perhaps it demonstrates that my skepticism is not the blind denial it is said to be, Cecil.
Two energy sources makes it a "no brainer", if they are actually present.

It's not the similarity, but lack of it. Dr Drela does something similar when developing the fist argument, then drops it to the difference of those two when applied to the cart. Without checking, he alludes to something like "power from the ground" but all equations simply use the differential velocity of the wind, as expected.

Similarly when the cart is moving at the speed of the wind, it harvests energy from the moving ground and uses it to push against the still wind.

Double dipping. If at all possible, any energy extracted from the ground, has already been maximised to get the car to 'windspeed' in the first place.

 

[mender]

ETA:
The diagrams appear on my screen, but just in case, here is the attachment.

Got it now, thanks, humber!

Perhaps you can show me the errors in my diagrams?

Humber, why is the cart only moving at 7 m/s (which is pretty fast - how about we dial that back to 15 ft/second instead) when the air is moving at 10 m/s in the first diagram? The diagram is supposed to show the cart at windspeed. Why do you show the cart moving at all in the second diagram? Isn't this supposed to be representing the treadmill? The cart would be stationary wrt the observer standing beside the treadmill and the surrounding air.

And what is the "reverse vector"? To be equal to your first diagram the number for the "reverse vector" would be 3 m/s, not 7 m/s; 10-7 is always 3. Leave the "reverse vector" out and keep the diagrams representing the issue at hand.

You've almost got it though. Fix those things and your diagrams will be right.

A friend of mine has a fog machine, I'll see if I can borrow it to get a video of the boundary layer on the treadmill from the cart's point of view. As I said, it'll look just like what I see when I'm driving down the road at 10 mph with a 10 mph tailwind when it is snowing (lots of experience with that!) - except you have to be able to interchange the snow and the smoke.

The attempts at analogies are to point out your mistakes about the "physical errors". As has been said countless times, the cart can't tell whether it is on the treadmill or is outside on the ground. The surface moving under it and the air flow around it will be the same. A video camera showing both scenarios from the cart's viewpoint would prove this.

And I have to ask: if the smoke show is identical (other than minor effects from scale) for both scenarios, will that be enough evidence for you to reconsider your stance?

 

[humber]

Humber, in the full-blown reply page and underneath the smiley's on the right, there is a small panel labelled "My Images" with options to "Pick" or "Quick Upload". I tried this just a few minutes ago and it seemed to work (although I didn't post the message with image). Perhaps you should try that. By the sounds of what you did, deleting the attachment may have left the link pointing to nothing, hence the problems others have seeing it later. Just a guess though.

Thanks Clive. I did re-post the attachment, but I will try your suggestion.



I see it on my screen in preview, as I do all the others.

 

[Brian-M]

Does anybody know what the [q img] tag is meant to be for and how it should be used? I couldn't find anything about it in the "complete list of BB codes" supported by the JREF forums.

It's been my experience that when you quote a post containing an image the [img] tag is automatically replaced with a [q img] tag so that a clickable link shows up in the quoted text instead of the actual image itself.

 

[Cecil]

Perhaps it demonstrates that my skepticism is not the blind denial it is said to be, Cecil.
Two energy sources makes it a "no brainer", if they are actually present.

It's not the similarity, but lack of it. Dr Drela does something similar when developing the fist argument, then drops it to the difference of those two when applied to the cart. Without checking, he alludes to something like "power from the ground" but all equations simply use the differential velocity of the wind, as expected.


Double dipping. If at all possible, any energy extracted from the ground, has already been maximised to get the car to 'windspeed' in the first place.

From what I've read I would argue against anyone who claimed that your skepticism was blind denial. I've rather become convinced that you have some basic misconceptions about physics and are resistant to having your ideas changed.

I'm not really interested in arguing with you, humber. I'm confident it will go nowhere and I don't really have time for that. I'm more interested in what others think of my boat analogy and whether there are sutble differences in their physics I haven't noticed.

 

[mender]

Double dipping. If at all possible, any energy extracted from the ground, has already been maximised to get the car to 'windspeed' in the first place.

I posted a description of a simple system (and several others have done the same) that would show that there is more than enough power in the wind to push the cart to and beyond windspeed. The amount of energy has never been the issue. If you need more power, use a bigger wind interface to slow wind. You never critiqued that description, so I assume that you couldn't find any errors but chose to ignore it instead.

 

[Clive]

Thanks Clive. I did re-post the attachment, but I will try your suggestion.

It's displaying properly now Humber so whatever you did is good!

It's been my experience that when you quote a post containing an image the [qimg] tag is automatically replaced with a [q img] tag so that a clickable link shows up in the quoted text instead of the actual image itself.

Thanks Brian. I might experiment with it a bit more later! Note that where the above quote displays "[url]" it's actually "qimg" inside square brackets in the raw text!

 

[spork]

You are right. That is yet another description of the same treadmill error.

That's crazy talk even for you humber. The fact that there's no difference is now your explanation that there's an error in saying there's no difference!?

So now that is cleared up

What exactly is cleared up? We already knew you were completely logic impaired, so that can't be it.

perhaps you would like to
(a) How me the errors within those diagrams. (Without reference to ice-carts please)
(b) Present diagrams with your interpretation of events.

Otherwise, I will assume that you can't do either.

Wow - you're thick!

 

[humber]

Got it now, thanks, humber!

Humber, why do you show the cart moving in the second diagram? Isn't this supposed to be representing the treadmill? It would be stationary wrt the observer standing beside the treadmill and the surrounding air. And what is the "reverse vector"?

I take the cart observer's view in each case.

(1) At windspeed on the treadmill, the "in-cart observer" sees the driving wind at rest w.r.t himself. So, like the belt, air attached to it, moves with it, and so actually backwards from that same view.

(2) If an object like that small "sphere" I drew is dropped (immediately) into the flow adjacent to the belt, then one second later, it will be 10m behind the cart from that same observer's view. It looks to be going backwards, because it is, at a nominal 7m/s (though the air at the belt boundary will be 10m/s).

(3) In the real wind case, the boundary layer still moves with the wind, but slower. Dropping and object from the cart in the same way as above, will see that object, recede into the past, and therefore lend the impressionof going backwards, (the apparent reversal), though it is not.

(4) That difference is due to the opposite directions of the boundary flows.

A friend of mine has a fog machine, I'll see if I can borrow it to get a video of the boundary layer on the treadmill from the cart's point of view. As I said, it'll look just like what I see when I'm driving down the road at 10 mph with a 10 mph tailwind when it is snowing (lots of experience with that!) - except you have to be able to interchange the snow and the smoke.

Some colored lights and hard rock music, perhaps? No need I think, Clive, because the tissue test of video #7 makes it clear in which direction the belt-wind flows.

The attempts at analogies is to point out your mistakes about the physical errors. As has been said countless times, the cart can't tell whether it is on the treadmill or is outside on the ground. The surface moving under it and the air flow around it will be the same. A video camera showing both scenarios from the cart's viewpoint would prove this.

Yes, indeed. We agree that there are errors.

But it's not too difficult to picture this one. If you attach a red smoke-flare to a real cart in real wind, which way is the smoke going to go? Would someone present at launch expect to see a continuous flow of red smoke coming from the cart and passing over their feet? If you add the interaction of the boundary flow ( blue smoke) said to come from behind the person when on the treadmill, it only makes matters worse.

And I have to ask: if the smoke show is identical (other than minor effects from scale) for both scenarios, will that be enough evidence for you to reconsider your stance?

Evidence always does that.

 

[spork]

I didn't notice this analogy. I thought I'd chime in here since it seems the discussion has moved.

This seems a bit easier to understand than the cart yet I believe it operates on the same principle. Spork, I'd be interested to see what you think.

It's essentially a "directly down-river faster than the river" boat. It drifts in a free-flowing river and still air, and has a turbine in the air geared to a prop in the water. As it is let go in the river, the turbine speeds up and starts driving the prop. I think it's pretty clear that as the boat reaches the speed of the river, the prop can still harvest energy from the wind (since there's now wind blowing over the boat) and use it to drive the prop, which pushes the boat faster down-river.

So boat-wind = cart-ground and boat-water = cart-wind.

Is this feasible?

I don't recall whether we've talked about it on this forum, but that uses several components of arguments we've used. In particular, we talk about a boat tacking upwind into a relative wind when it's on a river on a day with no wind. This tells us the boat can go up-current, faster than the current.

But I realize your scenario is a bit different. Certainly, people have described exactly the craft you envision, but I've not seen them put it on a river on a windless day (although I'm sure we can agree that's identical to putting it on calm water in a tailwind). My feelings are:

1) Yes, it works just fine in theory. If you can get prop, hull, turbine of sufficiently high performance, it would work fine in practice.

2) It's hard to imagine that would bring any sceptics over; but I'm becoming hardened to the "thought processes" of sceptics.

3) Completely contrary to my point #2, this is the first time I've ever seen humber agree with something that's actually true - despite the fact that his reasoning is completely flawed.

4) It seems there are many approaches to explaining this thing and convincing people. Yours is sound. If it changes minds, it's good.

 

[humber]

That's crazy talk even for you humber. The fact that there's no difference is now your explanation that there's an error in saying there's no difference!?

Don't bark, explain. Teacher.

What exactly is cleared up? We already knew you were completely logic impaired, so that can't be it.

Ditto. I fear for your students.

Wow - you're thick!

As opposed to shallow.