Conservation of Momentum

[Walter]

Mike -

I was going to bring up the video but Magnus is being grumpy. Now that you have brought it up there are two things to note:

1 - when the fly leg crashes in to the board it doesn't suddenly accelerate to warp speeds as it gets shorter. It basically seems to keep going at the same speed.
2. Friction/drag from skin effects is pretty small. That's why boats don't travel sideways. Even if they are on pool tables.
3. The loop does reform but there is tension created by the moving fly leg being held back by the portion of the line on the board. I'm not sure if that's an external force or an internal force because I'm not officially a physicist like Grunde so I'm not allowed to have a copy of the Handbook of Defined Physical Systems, or is it the Physicist's Handbook of Defined Systems?
4. That's actually 3 points not 2.
5. Now it's 4. Actually 5 now... Anyway you get the idea...

Cheers - Walter

[Walter]

Magnus -

http://vimeo.com/17266598

[Magnus]

Hi Mike

Ok I have seen it now and I don't see "a part of the line slides passed the board, reforms a loop and carries on."

[Walter]

Hi Mike

Ok I have seen it now and I don't see "a part of the line slides passed the board, reforms a loop and carries on."

Seriously? Must be the excessive lentil consumption.

It happens twice - once at 30 seconds (although it's a pretty sloppy loop) and again at 43 seconds (a much better loop).

[Lasse Karlsson]

I can't say I've ever seen a loop cast where that is not true - I guess the minimum is using a thin shooting line behind a shooting head - but always there.

Magnus

Hi Magnus

Well minimum would be setting it free wouldn't it?





Gordy's experiment with the loop crashing into a wall and then forming another one is a great example, still it needs the initial tension to form (bunch of line piled up against the wall)




Cheers
Lasse

[gordonjudd]

I can't say I've ever seen a loop cast where that is not true

Magnus,
And here is an example that shows how quickly the loop falls apart and turns into flying spaghetti (overlay from yellow frame to overlay for red frame about 50 ms later ) when the short piece of running line on the back of this "head" came out of the guides.


This is the result that the wave equation would predict, i.e. no tension no loop propagation velocity.

In this case I was using about 10 meters of a #4 line tied to a piece of kite string. I did have a bit of headwind when this was done, but not enough to kill the loop if I did not release the kite string as shown below.


I have never been able to understand why Lasse was able to keep the loop going after his running line came out of the guides. He might have been using a heavier line with a larger linear mass density and no headwind.

Gordy

[Lasse Karlsson]

Hi Gordy

I've been using floating shootingheads made from either a 6 weight DT or a 8 weight DT, both have been around 10 meters long. I've done the full blown distance blast with a longer shootinghead with the same results. So far I have been casting in no wind or a back wind. With the MPR line it is very easy to make a loop either roll or never happen indoors handcasting.

Does the tail in your experiment have something to do with the collapse?

Cheers
Lasse

[Walter]

I have never been able to understand why Lasse was able to keep the loop going after his running line came out of the guides.

Gordy - Maybe it has to do with the fact that Lasse includes his rod tip as part of his system and you don't?

[gordonjudd]

Does the tail in your experiment have something to do with the collapse?

Lasse,
I don't think so. I think the collapse happens because there is no force on the back end of the rod leg once the string leaves the tip top and thus there is no guide friction to provide tension on the rod leg of the line.

No force from the rod means the tension in the rod leg where it joins the loop goes to near zero (there might be a small tension force due to the "shooting" of the rod leg that produces some F=ma type force and some small drag forces depending on how fast the kite string end of the line is accelerated and thus produces some velocity in the rod leg) but I don't think it would be nearly as big as the force you get by holding on to the string. That was the case in the second photograph where the loop propagated normally. Without that tension there is nothing to provide the force needed to support a momentum change involving the changing amount of moving mass (dm/dt) in the fly leg.

Thus the tendency is for the whole line to go forward with no momentum change and thus little or no line tension that can offset the drag forces. That would be similar to throwing a clump of line into the wind, where drag quickly kills the forward velocity.

I don't know if a heavier linear mass density will produce some angular momentum around the loop that can overcome the killing effects of drag, or you have more drag force on the rod leg. I imagine your rod leg was longer rod leg by the time your running line left the the rod tip and I suspect that the diameter of your running line was bigger than my piece of string. My string was only about 7' long and had a very small linear mass density. Sometime you might cast into a headwind and see if you get the same result as you do casting with the wind.

Gordy

[Walter]

Gordy,

That seems a pretty long way to say the same thing I did. Maybe you should try some of Grunde's bluntness lessons.