Coam again

[Walter]

Question, what do you use for h in either version? I could also integrate from bottom to top as well. Any method will give us the same answer. I don’t find any of the methods particularly difficult but sometimes a different view can make it easier for someone to visualize.

Thanks for your input.

[Merlin]

Usually h is taken from the ground.

Merlin

[Torsten]

Hi Walter,

I don't want to hijack your thread, just one comment;

In order to determine if it has KE we have to look at the motion of it's center of mass. Since the rod leg is constantly getting longer it's center of mass is moving and if we look at the motion of it's center of mass and it's mass we can determine the amount of KE that the rod leg actually has. Similarly, if we want to determine the KE of the rod leg we need to use it's center of mass to determine it's actual velocity for purposes of determining its KE.

The center of mass is useful, when you want to compute the kinetic energy of a rigid body (and you'd assume the mass is constant). In the case of the earth-frame the rod leg has no velocity and thus zero kinetic energy. If you have trouble to understand this, you can alternatively determine the kinetic energy of the rod leg by integrating all the mass points of the rod leg - and because all mass points have zero velocity, the net result is zero too. My comment might be irrelevant, if you mean a different reference frame.

Greetings,
Torsten

[Walter]

Torsten,

You need to integrate the entire line, not just one leg or the other to determine the ke of the entire line.

[Torsten]

Hi Walter,

this is correct, my comment was related to the rod leg only. For the whole line you need to add Ekin of all parts of course; I did this already, maybe you can find in the old board my computations. OK I'll see that I can put together the necessary steps to produce the equations of motion for the analytic model, because I'm checking the equations of one work already.

Greetings,
Torsten

[Walter]

Hi Torsten,

We have two different models. One where KE is concentrated in the fly leg and is based on a fixed frame summing all parts of the line and one where KE is distributed in the line based on a center of mass calculation. Both models conserve energy. In the absence of all other forces in one model the fly leg has the potential to accelerate to infinity and in the other the fly leg speed stays unchanged. I know the concept of things accelerating to infinity in the absence of outside forces isn’t a new one in physics models but I don’t care for it when I see it.

I’m willing to concede that my concept is a bit off the wall and could easily be wrong. On the other hand, I’ve stated my issues with the “standard” model. And I should forewarn you that I’m not sure I’ll be convinced by an analysis based on a model that makes the assumption up front about one mechanism over the other. Maybe you have something in mind but I’m being up front.

I’m trying to think of a way, possibly (hopefully) experimentally that would give a definitive answer one way or the other, or possibly even a totally different direction. Unfortunately, we don’t have a vacuum chamber to experiment in. (Although I’m sure we could convince Paul to make the cast if one was available ). The only other idea I have is measuring the amount of energy in the cast when the line fully unrolls but that’s a bit sketchy. Do you have any ideas you would be willing to share?

Thanks

Walter

PS: I know we have some test experts out there so feel free to chime in.

[Graeme H]

I can't think of a way to quantitively test the models but there is a way to qualitatively test for the influence of drag on the fly line: video casts with and against the wind with an anemometer in the frame. If one accelerates significantly more than the other when similar initial line speeds are used and wind speed is added to the analysis, we can say drag has a significant effect. It's not a vacuum chamber, but maybe it will help.

The hard part is making two similar casts.

At least we can then account for one of the external forces (or remove it from the models.)

Cheers,
Graeme

(I'm considering drag here because I don't rate it highly as a factor in casting. Anecdotal evidence of the fly leg taper, leader, tippet and fluff flapping around freely during the cast suggests these things aren't a parachute for the fly leg.)

[Merlin]

We have two different models. One where KE is concentrated in the fly leg and is based on a fixed frame summing all parts of the line and one where KE is distributed in the line based on a center of mass calculation.

Hi Walter

I think you are going to hit the wall with the second model. Good luck for writing the equations, which anyway must be the same in both cases.

I’m willing to concede that my concept is a bit off the wall and could easily be wrong. On the other hand, I’ve stated my issues with the “standard” model. And I should forewarn you that I’m not sure I’ll be convinced by an analysis based on a model that makes the assumption up front about one mechanism over the other. Maybe you have something in mind but I’m being up front.

Where do you see that there is a preconceived idea in “standard” models? They have been designed using Newton laws, nothing else. Checking models with a nice lab test is unfortunately a holy Grail quest.

Merlin

[Paul Arden]

I suppose if you’re going to hit infinity then the drag forces must be quite significant. That would apply to slow casts too right? You could throw a really slow cast in Space and WHAM!! infinity!!

This is highly disappointing if true. I was really excited to imagine casting infinity loops in Outer Space and was curious if the Universe would explode, or my arm fall off. It could be some sort of space travel mechanism. “Get ready for infinity speed, wake the fly caster out of bed”.

Outer Space lumiline fights would have been to the death. Like in Star Wars, opponents could be vaporised.

And now you’re suggesting all this might not be true, Walter? Bummer.

Cheers, Paul

[Merlin]

Walter

I am cross checking your curve for line speed under constant KE conditions and cannot match your graphic. What size of loop are you using?
If the asymptotic value of speed is infinity it is because you are not using air drag, hence the lack of comparison with reality. Maybe also you are not considering the KE of the loop.

Merlin