## Is the cast itself a transverse wave?

Moderator: Paul Arden

### Is the cast itself a transverse wave?

LorksGuys,
I am so pleased i can cast a fair line and catch the occaisional fish without having the faintest idea wtf you are on about!
ACW

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### Is the cast itself a transverse wave?

Sorry ACW, you are right, so let's try to come to an end with an embarassing question as quickly as possible.

Hi Graeme,

When I read that I thought to myself “the teacher will stick you in the corner”. Now I have a better overview of the issue and I shall not talk about calculations details here, if you want to do so, I recommend doing it outside the Board, no one cares about it in the forum and it is a source of disinterest for readers. The Board has a long (bad) list of useless contest stories. You have my mail details; I do not have yours as far as I remember, so the ball is on your side to sort things out. And yes, I misunderstand English sometimes, it is not my native language and I encourage you to write me in French to avoid such misunderstandings, if you prefer.

To pay my dues to physics I am going to make an overview of the issue, starting by some basics on waves (sorry for non physicists, but I shall try to make that simple). Then I rely on (professor?) Graeme to tell me if I reported his view correctly. As I said a wave is a disturbance in a medium, the medium itself being stable. For example, you pluck a long string under tension and a (transverse) wave travels along it. The issue becomes more complex if you generate a sufficient number of waves (named stranded waves) to put the medium under some kind of resonance: then the entire medium is affected by many waves. Stranded waves are shown at the beginning of the video (the yellow zebra on the pic).

First question: does a cast belong to the stranded wave’s category which may turn into resonance eventually or is it a solitary wave travelling alone?
• To me it belongs to the latter category, a cast is a transitory motion and never sees any beginning of a resonance or stranded wave, the tip just moves in one direction, then the line rolls over and the cast ends, and then you can repeat it in the other direction. There is no chance to see a stranded wave and to generate a resonance.
• For Graeme the cast comes from some stranded wave in a long virtual vertical line and this is why this was written:
I'm saying that's only because the medium is not long enough for them to be measured; increase the length of the medium and those aspects could be measured.

But our medium (the fly line) is finite, and has no virtual part. The concept is thus difficult to understand, staying two feet on the earth.

Now we come to tension (yes, a vector). In the usual representation, the tension is aligned with the medium so it is horizontal if the medium is horizontal and vertical if the medium is vertical. There can be both forces on a line: the force due to the tip (horizontal to make things simple) and the gravity (vertical). So force at tip is the source of tension for transverse waves travelling horizontally and gravity is the source of tension for a transverse wave travelling vertically but then you have to imagine that the line is vertical at the beginning of things. The problem is that at the beginning of an ideal cast, the line is “horizontal” and that gravity just wants to put it on the ground. How can one justify the consideration of a “vertical” tension on a supposed to be vertical long virtual line under casting conditions where the line is horizontal?

The document I use for my explanation comes from Harvard (David Morin). To get the wave equation the author determines the tension in a horizontal line (the tension follows the medium axis if I can say so, even if it is a vector). So a disturbance slightly deviates the tension vector from horizontal to make it follow the medium axis. The question is then to evaluate the tension within the disturbance and its effects. To make a long story short the longitudinal components are equal and approximately match the tension of non disturbed parts of the medium, but their vertical component is the source of wave travelling speed and it is by considering those transverse components of (deviated from original axis) tension that one ends with the expression of the travelling speed of the wave = SQRT (tension/mass per unit of length). The tension vector follows the medium axis, even within the wave; otherwise there is would be no expression of the travelling wave speed. I leave that to your representation of tension vectors Graeme, and I can forward the document to you if you give me your coordinates.

Second question: can we have a medium with at the same time a tension component along its (original) axis and a tension component perpendicular to it? You put a string in between two pegs and you generate tension along the string (for example you create tension before fixing one of the pegs). How can you generate a perpendicular tension? You then consider gravity as the source of the vertical tension, but it is negligible by comparison to the tension you created along the string, and you cannot generate vertical waves as long as the medium is not vertical. Here you consider a virtual vertical medium which does not exist but in your mind, so the vertical tension has no impact as long as the medium is not vertical.

If I consider a (horizontal) fly line, the (vertical) field of gravity is “g” (9.81 m/s^2). When we cast a line we generate easily 10 times “g” more or less horizontally and more than 20 times in a distance cast. This means that the dominant force is due to line acceleration and not gravity which can be completely ignored (this is the case in most scientific fly line studies). Remember, the medium should be vertical then, which is real at the very beginning of your video, but soon after, the axis of the medium is turned from vertical to horizontal.

Line flight can be fully explained and described by considering it in the direction of the cast (to make things simple, you consider the line to be horizontal). Up to now I never met anything resisting to that even if they are limitations given the complexity of some calculations (I can send you a number of papers on the subject), excepted for morphing. The transverse vertical wave in a virtual vertical long line may be a nice intellectual concept (consider a vertical virtual long line under stranded waves), but he does not add anything to the understanding of line behavior, it just creates confusion. So why bother casting students with that? They do not ask, no problem, we can stay with the projectile and the loop mechanisms, which match the essentials (straight line path, you know). If one asks the original question of this thread, I recommend saying no, whatever the beauty of the intellectual virtual vertical transverse wave concept is. Stay simple please, and I would not mind if students would not be aware of such discussion, I feel concerned if we invent something to fool them instead of giving the basic and practical explanations.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life

Merlin

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### Is the cast itself a transverse wave?

Hi Merlin,

Thanks for the answer. I've got the Morin document already, but thanks for the offer. (I wasn't to being flippant when I said I liked the answer. I do like it - there was substance to it.)

I understand you'd like to stop the public discussion. That's fine, but I won't intentionally mislead someone who wants an answer to a question. Doing so does not help anyone. (I'll send you my email address in a minute.) I can see you've lost your appetite to discuss it further in public. That's fine by me. I don't have anything more to add from my point of view. Your reporting of my point of view is close to right.

In my normal thinking, I also think of the cast mainly as a projectile, but I've defended applying broadly accepted physics equations to the problem, applying a well known wave type to the question and I have not asked anyone to explain the complexities of the exotic "Loop Wave" concept, even though I would like to read about how it's justified. Maybe we can discuss that one via email.

Earlier on, in answer to my request to tell me where the stranded waves become a cast, someone said something like "you've changed the rod motion in a way so subtle it can't even be seen". I did indeed change the rod motion: I added a pause at the end of the impulse. That's all. It's one of the 5 Essentials.

Cheers,
Graeme
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Graeme H

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### Is the cast itself a transverse wave?

I think he decided to try a practical experiment after reading this thread:

Bright but shite

IANACI - There’s no such thing as absolutes

VGB

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### Is the cast itself a transverse wave?

Merlin wrote:
First question: does a cast belong to the stranded wave’s category which may turn into resonance eventually or is it a solitary wave travelling alone?
• To me it belongs to the latter category, a cast is a transitory motion and never sees any beginning of a resonance or stranded wave, the tip just moves in one direction, then the line rolls over and the cast ends, and then you can repeat it in the other direction. There is no chance to see a stranded wave and to generate a resonance.

Second question: can we have a medium with at the same time a tension component along its (original) axis and a tension component perpendicular to it? You put a string in between two pegs and you generate tension along the string (for example you create tension before fixing one of the pegs). How can you generate a perpendicular tension? You then consider gravity as the source of the vertical tension, but it is negligible by comparison to the tension you created along the string, and you cannot generate vertical waves as long as the medium is not vertical.

Line flight can be fully explained and described by considering it in the direction of the cast (to make things simple, you consider the line to be horizontal).

With any apologies due to Merlin for excessive snippage. Voila.

Cheers
Mark
"The line of beauty is the result of perfect economy." R. W. Emerson.

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### Is the cast itself a transverse wave?

a wave can be reflected ....
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t.z.

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### Is the cast itself a transverse wave?

This is my final attempt in this thread to show that the loop is part of a transverse wave. If this video doesn't make it clear, then I have utterly failed to communicate the concept and I see no reason to keep trying.

In any case, I wish to thank Merlin. He doesn't agree with my findings here at all, but that does not mean he did not inspire me. He continues to do so.

Cheers,
Graeme

youtu.be/RNqgZZyGJRc
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Graeme H

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### Is the cast itself a transverse wave?

Hmmm.

I'm not sure I'd be calling that "the physics of a fly cast", Graeme. I have a few questions/problems...

1 what is a roll cast?

2 I don't agree with your trajectory being the line angle - imagine that applying to a roll cast!! You have to look at the rod tip path through to RSP and even MCF. In particular from MCL - RSP and you can pretty much forget line angle until we have exceptional carry.

The movement that you pause the video to claim launch angle is in fact the moment when the rod tip starts to determine the direction of the loop.

3 far from seeing your line falling due to gravity it in fact stays fairly level on the forward cast while the loop is unrolling - if you look at launch angle being horizontal (ie to RSP) it doesn't appear to fall (and that's because there is thought to be some loop physics offsetting gravity).

I'm not sure I'd be calling the video the physics of the flycast when real-world physicists clearly disagree with you!!!! That's like walking into a minefield looking for meteorites.

Cheers, Paul
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Paul Arden

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### Is the cast itself a transverse wave?

Hi Paul,

1: I'd call the roll cast a surface wave, albeit an elongated one. (Reference)

The movement that you pause the video to claim launch angle is in fact the moment when the rod tip starts to determine the direction of the loop.

True, but have a look at the line between 3' and 9' back from the tip. That line is already on a ballistic trajectory that it maintains until it enters the loop.

3: This idea of "some sort of loop physics" is where I get very upset. String is not a new type of matter that Newton's Laws didn't foresee. Look at any single marker on a fly leg and it will have a ballistic trajectory right up until the point when it enters the loop. If each element of the line is ballistic, then the leg itself is ballistic. (Should I shoot a slo-mo video to prove this? I don't mind marking up a line for it and someone else can analyse the results.)

The fly leg is launched with an upward trajectory. Watch the tip marker on Dayle Mazerelli's false casts here and see the SLP is normally slightly upwards on each cast. If he aims down or level on the front cast, it only happens after he has aimed up on previous back cast. (Look for the average trajectory for the whole cycle, not just one half of it.)

If the line follows the tip path, then the line must have be launched slightly upwards too.

If the casting distance is short enough or the casting velocity is high enough, the height of the rod tip offers sufficient fall-distance so the line does not hit the water. Much longer casts will always be aimed up front and back. The line is in the air so long on those casts that it must be given room to fall without hitting water/grass.

I love finding meteorites too much to stay away from minefields.

Cheers,
Graeme
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Graeme H

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### Is the cast itself a transverse wave?

Hi Graeme,

1 So the overhead cast is a "transverse wave" and the roll cast a "surface wave". Aren't surface waves seismic? Minefields trigger them! Surely the delivery on a roll cast is the same as the delivery mechanism for an overhead cast? If you need to use a different wave form for it to fit is this not a) becoming complicated and b) ringing a small alarm bell?

2) The fact that "ballistic trajectory" travels in the direction of the loop does not necessarily mean that gravity is making it do this - it could be the loop. After all the same phenomena applies when casting a loop in the horizontal plane. There is a cast where we false cast horizontally to the bank and then on the final forward cast change angle to deliver across the stream.

3) It's been shown that a loop in horizontal trajectory stays aloft longer than a line in free-fall. I agree that there is some dispute around this. However if I was to take your logic then a high backcast should result in turn with a forward cast that doesn't counteract gravity and instead falls faster. This is obviously not the case and you can prove this to yourself by throwing a higher backcast!

Cheers, Paul
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Paul Arden