The practical side of considering loops as waves: Teaching and Self Improvement

[Paul Arden]

Ok not zero exactly but it’s certainly a very low tension point – take the fly off and it’s zero. But I’m still confused Graeme. You consider a fly loop to be a transverse wave in the flyline that propagates in the direction of gravity. I don’t see how this has anything to do with the tension in the loop. I could understand, almost, if it was to propagate upwards (say for example the fly was caught in the bridge that you have given me, and I was to wiggle the tip backwards and forwards, then the wave would travel vertically upwards towards the bridge).

[Graeme H]

Unless you're in space, there is still tension in the tippet. It doesn't float in the air without the fly on it. True, it's not much, but you'll see that it's hard to make a wave travel through tippet with nothing tied to its end due to the lack of tension, sort of proving the point.

I don’t see how this has anything to do with the tension in the loop.

It's not "in the loop". It's tension in the line.

The wave will fail to propagate if you are not providing tension. You have seen this yourself when you wiggled the line on the kitchen floor.

Let's go back to some basics for a second, using that kitchen floor experiment as a background example.

When you had the other end of the line tethered, putting a wiggle in the line produced a wave that moved away from you towards the tether. The "moving away from you" is another way of saying the wave propagates towards the tether. The line does not move away from you in that direction, but the wave does.

When the tether is released, the wave no longer moves away from you. In fact, the whole line moves towards you. You've put force into the line from your hand but there is nothing at the other end opposing that force, so the whole lot comes to you (and ends up in a tangled mess.)

If you were to now bring a dozen leaf blowers into the kitchen to provide a wind along the floor, blowing the line away from you, you'd have a force to act against and the wiggles would then propagate through the line again. The blowers would provide drag across the line and act in a similar way to gravity when you cast.

Gravity provides the vertical component of force we're acting against when making the loop. Yes, there is tension in the line from the fly leg pulling on the rod leg after we've made the loop, and it's against this tension direction that we make mends etc..

[Paul Arden]

When the tether is released, the wave no longer moves away from you. In fact, the whole line moves towards you. You've put force into the line from your hand but there is nothing at the other end opposing that force, so the whole lot comes to you (and ends up in a tangled mess.)

That would of course support the argument to say that it’s not a transverse wave.

[Graeme H]

Actually, it is a prediction of the physics.

Remove the tension and you don't get any wave propagating, including a mend.

[Paul Arden]

This is true.

[Torsten]

Paul, I had the same picture in mind (#111), nice drawing. Graeme, even without gravity the loop would propagate.
I see some major issues why the transverse wave concept fails to explain loops: (a) medium, (b) mechanical waves don't transfer matter (but energy, movement of mass is limited), (c) properties like wavelength, amplitude etc. are very difficult to apply.

[Paul Arden]

Thanks mate, I should have gone to art school Just out of curiosity how does gravity affect waves cast to the bridge vs waves cast from the bridge? Does the wavelength stay the same? I assume that the rate of propagation accelerates downwards and decelerates upwards?

Thanks, Paul

[Graeme H]

Hi Paul,

The wavelength in a given piece of line is dependent on the tension in the line and the frequency of the input force. So assuming you move the rod in exactly the same way in each case, the deciding factor is tension.

From the top, the tension is coming from the weight of the line alone as the wave propagates.

From the bottom, you've got the weight of the line plus whatever you add fthrough the rod. If you lift the line after making the wave, there will be a little less line mass hanging from the bridge. If you pull down with the rod tip, you'll add tension to gravity's effect.

If you add tension, you'll shorten the wavelength and increase the frequency (i.e. make the wave travel faster through the line.)

Cheers,
Graeme

[Graeme H]

Graeme, even without gravity the loop would propagate.
I see some major issues why the transverse wave concept fails to explain loops: (a) medium, (b) mechanical waves don't transfer matter (but energy, movement of mass is limited), (c) properties like wavelength, amplitude etc. are very difficult to apply.

You can try propagating it without gravity Torsten: make a series of casts on a tile floor (without lifting the rod off the floor.) Yes, it works, but the line tends to move out and away from you. Make a vector diagram and you will see why this is inevitable.

a) medium
What does that mean? It's a single word without context.

(b) mechanical waves don't transfer matter (but energy, movement of mass is limited)
Are we covering this again? A mend transfers matter away from the direct line of sight between you and the fly.

IMG_5231-2.jpg (76.97 KiB) Viewed 1634 times

Let's try this: Paul ties his tippet to the 10m tall bridge, strips 30 metres of line out and then ties a fly directly onto the line 10m from his rod tip. He "casts" it so that the line flops 10 metres in front of him (while the other end is still tethered). Would the fly tied to the line have been transferred 10 m away from him? Would the wave/loop keep propagating along the line and up to the bridge if he put enough effort into it? If he now back casts this setup, would the fly land 10 m behind him?

(c) properties like wavelength, amplitude etc. are very difficult to apply.
So let's not even try, right?

Wavelength is the inverse of frequency and is the spatial period. If the fly reaches the end of the forward cast every 2 seconds in a series of false casts, the frequency is 0.5 Hz and the wavelength is 2.

Amplitude is the length of the line outside the tip for a completed cast and less for an underpowered cast that fails to straighten. In the dodgy pic above, it's 10 m.

Cheers,
Graeme

[Paul Arden]

Hi Graeme,

Let’s go back to this for a second

Remove the tension and you don't get any wave propagating,

I’m quite comfortable with that statement. What I don’t understand is why then you still consider a flycast to be a transverse wave propagating at right angles to the loop direction?

There must be an addition step somewhere that I’m missing. It doesn’t propagate on the floor. That’s established (no tension from a tethered end). Yes it falls under gravity, and gets blown by the wind, but I don’t see that as a case for it being a transverse wave, just a case for it being affected by external forces.

Cheers, Paul