Down wind faster than the wind

[Dan O.]

On our little carts the thrust line does go right through the front axle. So it shouldn't produce a forward pitching moment.

The thrust force goes through the axle and the traction force tangent to the wheel at the surface contact. This offset is what creates a torque on the wheel and causes the wheel to turn. When the torque is transfered to the propeller shaft, torque is also tranfered to the cart causing the rear end to lift and a twist oposite of the propeller rotation trying to lift one front wheel. It's all that action-reaction stuff.

 

[spork]

The thrust force goes through the axle and the traction force tangent to the wheel at the surface contact. This offset is what creates a torque on the wheel and causes the wheel to turn. When the torque is transfered to the propeller shaft, torque is also tranfered to the cart causing the rear end to lift and a twist oposite of the propeller rotation trying to lift one front wheel. It's all that action-reaction stuff.

You have caught me being very very stupid.

I should probably save a copy of this for all the times people claim I can't admit I'm wrong (I knew it had to happen eventually )

 

[mender]

In my mind it seems reasonably straightforward. He clearly fashions himself some sort of scientist. Scientific theories make predictions - or they're not scientific theories. I'm at the point of engaging him only to point out that much.

Has he abandoned the "hopping" theory? In fact has anyone read enough of his tripe to know if he has offered anything that could reasonably represent even a consistent set of ideas?

His response (#2323) to my post (#2310) seems to indicate that generally I have his theory correct. He couldn't just agree however, and tried to nitpick a few items. I felt that his rebuttal lacked his usual conviction and was being presented more out of habit than actual disagreement.

I was going to mention the offset as well but felt that the wheels would slip well before the cart flipped!

 

[Dan O.]

You have caught me being very very stupid.

I should probably save a copy of this for all the times people claim I can't admit I'm wrong (I knew it had to happen eventually )

The long explanation (and the added picture) was just for the benefit of those that aren't regularly exposed to this stuff. You would have caught your mistake if all I had said was:

F X r

 

[spork]

You would have caught your mistake if all I had said was...

Yeah, but I still would've felt plenty dumb.


I miss humber. He made me feel really smart.

 

[John Freestone]

What's even worse is that if humber actually proposes some kind of test, it will be designed just to make the cart fail by imposing ludicrous things.

<snip>

I don't see it quite like that, Chris. I might be wrong, but he seems to suggest a lot of the time that the apparent effects observed for the cart on the treadmill are happening for different reasons than the standard explanation. He seems to suggest alternatives, such as the energy input at the wheels being dissipated by the propeller simply by the fact of it spinning in the air and causing friction, and hence it doesn't matter if the prop is turned to point upwards or backwards or its pitch reversed, or the whole thing replaced by a flywheel or bit of rag. I don't think he's suggesting those as tests so that when it doesn't work he can say "See, it doesn't work". I think he actually expects it to work with a bit of rag tied round it. He expects it to work better (make more forward progress) up an incline than on the level (a point still awaiting verification after JB asked him to be absolutely clear about it).

When I say that he expects these things, that's a bit simplistic - that seems to be his official position. I think he probably doesn't really expect them, or is not at all confident, but has said these things (at a time when he did believe them, or just to be awkward) and now finds himself with his back against a wall, wishing he hadn't, which would explain why he is so reluctant to give straight answers. I don't think we need to worry that anyone is going to be caught making a DDFTTW test without a prop, or some other mental arrangement of Humber's design that clearly isn't going to work, so that Humber can say 'see it doesn't work'.

Personally, I'm not sure that waiting for further clarification is necessary. Doing the tests anyway, or refusing to do them because they're just too stupid, might be better options than keeping asking him to be specific. He's said what he's said. Anyway, the video evidence already shows the cart advancing on the flat and balanced on the uphill tread. Humber's suggestion that they are the other way round is perhaps designed to suggest cheating on the part of JB and spork, and it is anyone's guess whether it is worth providing more evidence that he can misinterpret as easily as the last. If it's not already been done, I'd do a few runs with props pointing different ways, a flywheel (since he's gone on about that forever) and yes, just for fun, tying a bit of rag round the driveshaft, or whatever it was he said would dissipate the energy just as well and create the force-balance effect (something like that).

I'd draw a line at seeing if a fishtank does as well as a treadmill (I think that was just one of his profound metaphors). If I'm looking for instruction on aerodynamics, though, I think I'd trust a goldfish over humber any day.

Oooh, I see spork's got a Humberism for his signature...what a great idea! Oh, there are so many to choose from though.

 

[Dan O.]

When we start running competition with these carts, we will want some form of guidance. The solar racers that are about the same size as your little cart are guided by a 60# test monofilament line stretched the length of the course. My question is: Would it be better the have the guide line under as much tension as possible to reduce the amount it would bend over the guide hook or should it be somewhat slack so the hook won't be supporting as much of the length of the line?

 

[Dan O.]

I want to see a video of TAD & spork's cart put on the treadmill backwards

PS: Spork, caution is advised

 

[Christian Klippel]

Hello John,

yea, we will see (or not) what he actually means. After all it's quite hard to follow what he means. We would need divining powers to really find out, and we all know that divining just doesn't work. The fact that he seems to be unable to clearly state what he means, and his failure to give coherent answers to questions asked doesn't help the situation either.

For example, you say that you understood he means "the energy input at the wheels being dissipated by the propeller simply by the fact of it spinning in the air and causing friction..." while he recently said "The other problem is that there is no load for the propellor because the cart is stationary.". But if the prop spinning in air causes friction, how can it be no load? Seems that to get at least a itsy ditsy tiny bit of understanding what he means would involve to re-read half of his posts. I'm pretty sure that at some point he stated something that lead you to your assumption. But in the end it's all just running in circles. One post means this, the next post means something different, and so on. There is just way too much room for free interpretation for a single reader as to what he actually means. But maybe that's his plan, to spread confusion so that no matter what the outcome is, he could claim to be right one way or the other. Who knows....

Greetings,

Chris

 

[spork]

I think he actually expects it to work with a bit of rag tied round it. He expects it to work better (make more forward progress) up an incline than on the level

If we can get him to stand by any such claim, JB and I will definitely perform the test and post the video within 12 hours. And if I am wrong about the expected results I will denounce physics and become an evangelical humberist.

I don't think we need to worry that anyone is going to be caught making a DDFTTW test without a prop, or some other mental arrangement of Humber's design that clearly isn't going to work, so that Humber can say 'see it doesn't work'.

Unless I'm misunderstanding you, you're completely off the mark. Get him to stand by these - or ANY - predictions that are not consistent with our explanations and I will be happy to do a video with the cart dragging a frozen turkey if that's what's indicated.

Humber's suggestion that they are the other way round is perhaps designed to suggest cheating on the part of JB and spork, and it is anyone's guess whether it is worth providing more evidence that he can misinterpret as easily as the last.

Cheating on our part is certainly a possibility. That's why I'm happy to have any skeptic come by and watch the test first hand. Or we can go to a gym where I haven't hidden magnets under the treadmill, or they can build their own cart from the plans I posted. Cheating should not be allowed to be an issue.

If I'm looking for instruction on aerodynamics, though, I think I'd trust a goldfish over humber any day.

Looks like I have a new quote to add to my signature.

 

[Clive]

A little Teaser about Inertial Reference Frames

A little off topic, but here's a little puzzle concerning calculations in two different inertial reference frames with respect to a simple scenario where two objects collide head-on in a completely non-elastic fashion:

Say we have an object with mass 1 kg on the left, and a second object with mass 9 kg on the right. They have a closing speed of 6 meters per second, directly towards each other and all the motion is along a straight line. (It happens far out in space somewhere, no air, no other external forces to worry about.) The materials are such that the collision will be totally non-elastic, meaning that there will be just one mass of 10 kg to consider after the collision. Assume also that exactly 13 joules of energy is dissipated during the collision as the objects are deformed, heat is generated and so on. There is no angular momentum to be considered at any stage.

In our first inertial reference frame we see the object on the left as moving at 6 meters per second towards the object on the right which is at rest. The object on the left thus has 18 joules of kinetic energy before the collision, and the object on the right has no kinetic energy at all. 13 joules are "lost" during the collision leaving 5 joules of kinetic energy and a combined mass of 10 kg afterwards. Using the usual formula for kinetic energy this tells us the final speed of that mass must be 1 meter per second (and to the right obviously).

Let our second reference frame be moving at one meter per second to the right when compared to the first. So we see the object on the left as moving at 5 metres per second (to the right) and the object on the lright moving at 1 metre per second to the left. Their respective kinetic energies are thus 12.5 joules and 4.5 joules. They collide, merge and 13 joules of energy is again "lost". That leaves 4 joules of kinetic energy, so the combined 10 kg mass must be moving at a little under 0.9 metres per second (and to the left if you look at the net momentum before the collision).

Clearly the velocity of the final 10 kg mass in the two reference frames should be the same once you've translated from one to the other. But this doesn't happen in this case!

(I trapped myself with this earlier when drafting a more complex story that I was intending to present to Humber for his opinion. For relatively uneducated hicks like myself, it may present an interesting puzzle. I know my mistake now so don't need to have it pointed out. I'm just curious see who else may fall into the same hole...)

 

[spork]

Without ruining the puzzle for anyone I think it can be broadly explained by my common claim that "energy is weird". The simple question of whether the cart is wind powered or ground powered could be its own 60 page thread for the same reason as is the answer to your puzzle.

Amazingly, there are several seemingly minor tangents to this incredibly simple device that could each be the subject of an entire chapter of a physics text.

 

[Clive]

Without ruining the puzzle for anyone I think it can be broadly explained by my common claim that "energy is weird".

Hmmm. Perhaps you can tell me the number of words in the first sentence of my description where I start to go off the rails.... just to help confirm that your "energy is weird" and my "I am a fool" are in fact isomorphic in this case?

 

[Dan O.]

Clive, You assume you are wrong

the 1 kg object is only moving at 5.3748384989 m/s

 

[davefoc]

My thoughts:

         V1     V2     M1       M2      momInit Vf     Einit    Efinal    Elost
Frame 1   0     -6      1        9      -54      -5.4   162      145.8   16.2
Frame 2   6      0      1        9        6       0.6    18        1.8   16.2
Frame 3  -1      5      1        9       44       4.4   113       96.8   16.2

momInit - momentum Initial
Vf - velocity final

 

[Clive]

Clive, You assume you are wrong

I concur... bit of a giveaway really... perhaps I needed to polish my words a little more before posting to add more misdirection and remove the "clues"?

 

[fredriks]

As I have said, and it is not a matter of whether Newton's laws apply or not.
They always do for any objects you have to worry about in this case, but it is not a frame or anything like it. That is certain, I know nobody who would think otherwise.

(my bold)

I like this quote, it explains a lot.

Momentum cannot simply be arithmetically traded along with velocity vectors. There are laws of thermodynamics that do not allow that to happen. Perhaps you think I am being too dismissive in my rejection of frames, but if you take a look at the relevant equations, the differences would be tiny, if available to you.

I wonder what kind of frames humber rejects. My quess is picture frames.

 

[Thabiguy]

The spoiler contains the solution to Clive's puzzle.

The materials are such that the collision will be totally non-elastic, meaning that there will be just one mass of 10 kg to consider after the collision. Assume also that exactly 13 joules of energy is dissipated during the collision as the objects are deformed, heat is generated and so on. There is no angular momentum to be considered at any stage.

These three sentences cannot all be true at the same time. Either the collision is somewhat elastic (meaning that the leftover energy is not a kinetic energy of a single mass), or it results in angular momentum (meaning that the leftover energy is not all translational kinetic energy), or the energy dissipated is not 13 joules (meaning that your subsequent calculation gives wrong results).

I concur... bit of a giveaway really... perhaps I needed to polish my words a little more before posting to add more misdirection and remove the "clues"?

Perhaps. You could remove the "assume also" words.

But for me the confusing part was the illusion itself. In the first case, you say the wreck is moving at 1 m/s to the left, and in the second case at 0.9 m/s to the right. It's not apparent enough what exactly is contradictory about that result. Should the numbers be the same (1 m/s in both cases), or what? How would a "intuitively correct" result look like? The reader has to stop and think. Because it's not immediately clear in which way the result is wrong, the illusion suffers.

If you want to use the puzzle somewhere else, you may want to include an introduction case, in which the liberated energy will be specified correctly, giving a clear example of how a correct result looks like. Then use a different case with an incorrect value of liberated energy, resulting in the "paradox". Also, you may want to tweak the numbers a bit so that all that occurs in the puzzle are nice and easy integers.

Another way to make this work better, without an introductory case, would be to make the error smaller compared to the absolute value of the results and their difference. For example, if the calculated velocity was 4 m/s in the first case and 3.1 m/s in the second case, the problem would be more apparent (that you expect a 1 m/s difference, but don't quite get one).

 

[John Freestone]

I want to see a video of TAD & spork's cart put on the treadmill backwards

PS: Spork, caution is advised

Yeah, I was just thinking that maybe it doesn't have a true front and back, other than for secondary technical reasons maybe - am I right in thinking that the physics is the same if the wind blows / treadmill goes in the opposite direction? Sorry if it was answered already. It would make an interesting video.

Also, would it be a nice touch to hold a light piece of paper or similar behind the prop on the treadmill, level or inclined, to show the backward air-flow? Bring it out of that area and it will hang vertical again. I assume there must be a fairly good breeze out the back for it to advance.

As well as for the interest of that, it also has the advantage of showing idiots like I was that there isn't a ruddy great fan out of frame blowing it from behind. Maybe you've covered that area already. I haven't gone through all your videos yet - maybe a project for today.

 

[John Freestone]

Hello John,

yea, we will see (or not) what he actually means. After all it's quite hard to follow what he means. We would need divining powers to really find out, and we all know that divining just doesn't work. The fact that he seems to be unable to clearly state what he means, and his failure to give coherent answers to questions asked doesn't help the situation either.

For example, you say that you understood he means "the energy input at the wheels being dissipated by the propeller simply by the fact of it spinning in the air and causing friction..." while he recently said "The other problem is that there is no load for the propellor because the cart is stationary.". But if the prop spinning in air causes friction, how can it be no load? Seems that to get at least a itsy ditsy tiny bit of understanding what he means would involve to re-read half of his posts. I'm pretty sure that at some point he stated something that lead you to your assumption. But in the end it's all just running in circles. One post means this, the next post means something different, and so on. There is just way too much room for free interpretation for a single reader as to what he actually means. But maybe that's his plan, to spread confusion so that no matter what the outcome is, he could claim to be right one way or the other. Who knows....

Greetings,

Chris

Yes indeed, Chris. I said that I didn't quite see it the way you put it, and you're right as to why we have different views of what humber means. Confound and conquer (or avoid defeat) would seem to be his strategy.