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## Angular Momentum

Moderator: Paul Arden

Paul Arden
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Location: Belum Rainforest
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### Angular Momentum

Hi folks!

I would like to understand angular momentum a bit better and how it impacts the loop. For example, I’m pretty sure that it explains the opening of the loop on the video I posted earlier?

Thanks,
Paul
It's an exploration; bring a flyrod.

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James9118
Posts: 1110
Joined: Thu Jan 10, 2013 6:59 pm
Location: Hampshire, UK

### Re: Angular Momentum

Hi Paul,

I have a question for you; suppose I give you a length of rope and I ask you to modify it so you can reliably break the sound barrier with it. would you -

a) ask to make the rope longer because conservation of linear momentum implies that energy from moving the longer length will be focussed into the shorter length (after throwing a loop) thus forcing it to accelerate beyond the speed of sound; or

b) ask to taper the end, because that's how whips are made?

Graeme H
Posts: 1614
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Re: Angular Momentum

I'm very interested in where angular momentum has a home in the question of loops. Having run videos of marked lines through that tracking app I use, I noted that a single mark on the line does not appear to travel in any path that looks like a circle, oval or any other approximation of travel where a radius might be applied. I'll post a diagram showing the path a mark takes tonight when I get home from work.

I'm not saying angular momentum isn't involved in the cast, but that I don't know how it is applied. I do see an acceleration of the leader as the loop consumes the line taper, so there is a force at work, for sure.

So I'd also like your thoughts on it James.

Cheers,
Graeme
FFi CCI

Paul Arden
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Joined: Thu Jan 03, 2013 11:20 am
Location: Belum Rainforest
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### Re: Angular Momentum

This is an interesting question actually, because if I was using a fly line and I wanted to crack it, I would use a level line and not add a tapered leader. Why the apparent reversal?

Cheers, Paul
It's an exploration; bring a flyrod.

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Graeme H
Posts: 1614
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Re: Angular Momentum

Hi James,

I think I can see where the angular momentum is playing a part in the cast now. In a standard cast, there is a very small part of the line's path which is following a curved path, just as the line enters the loop nose.

If we are making a snap cast, the length of that curve is increased because the line in the rod leg is moving against the direction of the fly leg, essentially doubling the length of line following a curved path. Instead of dropping vertically, the line would move left after passing through the loop nose in this video.

Cheers,
Graeme

FFi CCI

gordonjudd
Posts: 1255
Joined: Sat Jan 19, 2013 11:36 pm
Location: Southern California

### Re: Angular Momentum

I think I can see where the angular momentum is playing a part in the cast now.
Graeme,
It is even easier to see if you use a moving frame to analyze loop propagation.

Here is an example of how your clip 10 markers would appear to track for an observer in a moving frame whose x-velocity was equal to velocity of the loop. You can imagine what this "view from a moving car" might look like, but it may be easier to see it in the simulated video below.

https://vimeo.com/342277007

In this frame you can see a line marker just follows the line in front of it as it goes around the loop.

Gordy

Graeme H
Posts: 1614
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Re: Angular Momentum

Using a moving frame of reference is not only adding extra scope for confusion here, but it also inflates the apparent importance of angular momentum in the cast. The only part of the line's path that is subject to angular momentum in the real world frame of reference is that small curve just before the line stops its forward movement. If the loop shape approximates half an oval (cut through it's narrowest point), then the angular part of the path is half that again.

Screen Shot 2019-06-15 at 8.19.22 am.png (13.6 KiB) Viewed 448 times

A moving frame of reference centred on the nose of the loop gives the impression that all of the "half oval" is in motion and providing angular momentum. It's not.
Screen Shot 2019-06-15 at 8.22.08 am.png (18.48 KiB) Viewed 448 times

Using this moving frame of reference may also lead one to think there is a source of lift in the lower half of the loop. I can just imagine how confusing that would be.

Cheers,
Graeme
FFi CCI

John Waters
Posts: 632
Joined: Mon Jan 14, 2013 9:16 pm

### Re: Angular Momentum

What software are you using Graeme? Looks great.

John

Graeme H
Posts: 1614
Joined: Fri Jan 25, 2013 2:54 pm
Location: Perth, Western Australia

### Re: Angular Momentum

G'day John,

I'm using an iPhone 6 to capture slow motion footage at 240 frames per second.

I'm using iMovie to select the clip I want to use and set the replay speed.

The really useful software is called Tracker, a free piece of software for physics analysis of videos.

Screen Shot 2019-06-15 at 10.57.21 am.png (36.1 KiB) Viewed 439 times

Cheers,
Graeme
FFi CCI

Paul Arden
Posts: 12134
Joined: Thu Jan 03, 2013 11:20 am
Location: Belum Rainforest
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### Re: Angular Momentum

Interesting regards lift, Graeme. I had a good look at the loop propagation on the 170 video I posted recently. The line is very obviously being affected by gravity.

I still find it interesting that a whip is tapered to accelerate the loop whereas we taper the line to decelerate the line.

Is angular momentum the cause of the dolphin nose?

Cheers, Paul
It's an exploration; bring a flyrod.

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