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Angular Momentum

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Paul Arden
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Angular Momentum

#1

Post by Paul Arden » Thu Jun 13, 2019 8:42 am

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
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James9118
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Re: Angular Momentum

#2

Post by James9118 » Thu Jun 13, 2019 11:43 pm

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?

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Graeme H
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Re: Angular Momentum

#3

Post by Graeme H » Fri Jun 14, 2019 12:56 am

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
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Paul Arden
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Re: Angular Momentum

#4

Post by Paul Arden » Fri Jun 14, 2019 5:28 am

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
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Graeme H
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Re: Angular Momentum

#5

Post by Graeme H » Fri Jun 14, 2019 11:12 am

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

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gordonjudd
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Re: Angular Momentum

#6

Post by gordonjudd » Fri Jun 14, 2019 5:03 pm

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

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Graeme H
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Re: Angular Momentum

#7

Post by Graeme H » Sat Jun 15, 2019 12:30 am

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
Screen Shot 2019-06-15 at 8.19.22 am.png (13.6 KiB) Viewed 158 times
Screen Shot 2019-06-15 at 8.37.22 am.png

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
Screen Shot 2019-06-15 at 8.22.08 am.png (18.48 KiB) Viewed 158 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
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Re: Angular Momentum

#8

Post by John Waters » Sat Jun 15, 2019 12:53 am

What software are you using Graeme? Looks great.

John

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Graeme H
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Re: Angular Momentum

#9

Post by Graeme H » Sat Jun 15, 2019 3:01 am

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
Screen Shot 2019-06-15 at 10.57.21 am.png (36.1 KiB) Viewed 149 times

Cheers,
Graeme
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Re: Angular Momentum

#10

Post by Paul Arden » Sat Jun 15, 2019 5:02 am

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 flyrods.

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