Conservation of Momentum

[Magnus]

In the Hang Time thread we had Paul talking about Conservation of Momentum (CoM), using it vaguely to explain all sorts of loops. The way Paul uses it does nothing to convince me that CoM is the best/most complete way to describe how loops work.

I'm not offering a better explanation, if anything I am asking for the technically inclined to give me a better explanation of the applicability/adequacy of CoM and/or how CoM should be applied to understanding loop mechanics

I'll start this off by saying that when I make a snap I am doing work - I am increasing the KE in the line and forcing (sic) it to move. I am applying a force to the line between the tip of my rod and the fly. So I have a problem with Paul's explanation of a snap which seems to consist in saying ""Alejandro, Torsten and Aitor told me it's CoM (so you must be wrong)"
(apologies in advance if those were not the names he quoted.)

In the case of a snap I am applying a force, doing work, increasing the KE in the line - that sounds to me like I am casting and as I read the concept of CoM those mean it either does not apply or should be applied with caution.


Next, we have this bland idea that as line in the fly leg reaches the loop front it slows. So the fly leg is losing mass, apply CoM and hey presto the fly leg must accelerate to maintain the same momentum in that leg.

This strikes me as bland and frankly wrong. That would be me saying that as I roll along in my car - free wheeling, no force on the car but friction - if I lob the seats and other passengers out of the car it will accelerate. Nonsense!

So, without dazzling us with formulae and graphs, I'd like to hear Paul's explanation of the part CoM plays in an adequate description of casting and loops.

Magnus

[TrevH]

Don't forget, when you make a cast, you speed up the rotation of the earth (or slow it down, depending on the direction), but don't worry, when you arrest the progress of the line at the end of the cast, you set it back to normal again :-)

So not all of the momentum is conserved in the fly leg. But there must be some. I just can't work out what proportion

A bear of small brain xxx

[Magnus]

Now Trev....

If that were true, casting from a boat/tube/kayak would be interesting as we moved ourselves by the mystic power of casting alone

[Unregistered]

So I have a problem with Paul's explanation of a snap which seems to consist in saying ""Alejandro, Torsten and Aitor told me it's CoM (so you must be wrong)"
(apologies in advance if those were not the names he quoted.)
Magnus

Apologies accepted.

[Magnus]

Before I offend anyone else,

So there are two things here:

1) the momentum change at the loop front generates the Tension that allows a loop to propagate

2) as the fly leg shortens, its mass decreases and therefore - without air resistance - would accelerate to infinity.

Many engineers have used 2), Alejandro, Torsten, Walter and so on - so maybe they can help us out here!

Cheers, Paul

and

Yes I happy to argue this, Magnus! We've had enough people here argue this topic, including Torsten and Alejandro, so I feel that I can call upon a graph or an equation if required. The Snap Cast is virtually all about Conservation of Momentum (about 90% according to Torsten and Brian Ellis, on the previous Board, or maybe even the previous Board but one).

In fact it was the first thing Alejandro explained to me when I met him for the first time in Spain. Of course I disagreed with him at the time because I had no idea what he was talking about.

Cheers, Paul

Which of course, does not mean what Paul says is what they said or thought - which is why I asked:
"So, without dazzling us with formulae and graphs, I'd like to hear Paul's explanation of the part CoM plays in an adequate description of casting and loops."

NB - in the second quote, the meaningless figure of 90% was of course pulled by Paul from thin air.

[grunde]

Even without air resistance momentum is not conserved when the loop propagates.

Energy will however be conserved if there is no air resistance.

Cheers,
Grunde

[Bernd Ziesche]

Hi Magnus,
as far as I understand it, the conservation of momentum principle indeed does only apply to a closed system. I agree with you in true fly casting we do have external forces and therefore it seems not to make much sense to built up a (theoretical) closed system to explain fly casting.
During unrolling of the loop we have friction, drag, gravity and the (non constant) force permanently added by the caster (via rod tip).

I understand that without drag and friction (in a horizontal cast) the fly-leg would increase its speed significally due to the fact that most (not all) of its momentum would be conserved while the mass of course would decrease all the time until the end of unrolling. Since momentum equals mass times velocity... speed would have to increase a lot.

Back to real casting conditions there seems to be a balance between mainly friction, drag, the force added by the caster and the change in mass (length + taper based) of the fly-leg, which mostly results in nearly constant (fly-leg) speed for most of the life cycle of the fly-leg. Well, at least that is what slomos make me think (I agree with Paul here).

Personally I think, that we also have to take into account that the air resistance increases squared to speed. So, if the fly-leg will speed up at some point during unrolling, drag and friction would increase significally at the same time. This probably might keep the fly-leg from further acceleration again.

To quantify anything here (to quantify the balance or better each part of it) seems to be senseless to me (inpossible anyway?) since all casts are highly different often.
Of course beside the taper of the fly-leg the leader and the fly design will also have their impact on the whole process of unrolling.

This may not be the detailed explanation you have been looking/hoping for, but I hope you can agree with it so far? (If not am happy to learn...)
Best
Bernd

[Walter]

This strikes me as bland and frankly wrong. That would be me saying that as I roll along in my car - free wheeling, no force on the car but friction - if I lob the seats and other passengers out of the car it will accelerate. Nonsense!

Magnus - Actually it does work like that (sort of). Let's change the scale a bit first - imagine floating in space and throwing a heavy weight. The weight will go one way and I will go the other. If I'm currently moving and I throw the weight ahead of myself I will slow down. If I throw the weight behind me I will speed up. I can calculate my change in velocity by using conservation of momentum.

Now for the "sort of" part. There needs to be some sort of application of force for this to happen. I push against the weight and it pushes back with an equal amount of force (equal but opposite reaction). If I simply shed pounds by releasing the weight it does nothing. This is where the original "kinetic chain" discussion got all f$%^&ed up when somebody introduced some marketing crap from a self appointed PhD about how simply stopping one body part makes all the others in the chain go faster.

Think of the concept of cracking the whip with a line of ice skaters. They are all skating down the ice in a line and suddenly the one at the end comes to a stop and the rest speed up. This continues with one skater dropping out at a time until the final skater is travelling as fast as one of Paul's casts. There are two ways for the skater at the end each time to come to a stop. The first is to simply apply the brakes and let go of the rest of the line. In this case the rest of the line does not get accelerated. The second way is for the skater to either push or pull on the rest of the line until he comes to a stop (or even reverses direction) - this will add his/her momentum to the rest of the line and the line will go faster as a result.

Back to your car analogy - If I simply drop a passenger out of the car it will do nothing to change the momentum of the system. If I forcibly throw him from the car then I get to use his change of momentum to either speed me up, change direction, or slow down.

When it comes to a fly leg increasing in speed as it gets shorter you have to ask where the application of force is happening in order for the change of velocity to be happening. It comes from me applying a retarding (not retarded ) force on the line with the rod tip. This pulls on the rod leg which pulls on the fly leg via the loop. Now think of how I can make the fly leg really go fast and how do I know it is getting faster. One way is to create a whip crack sound with my line. I know this indicates a large increase in speed because it is supposedly the end of the line exceeding the sound barrier that causes this. This is not necessarily correct but the whip crack does indicate a high fly leg speed. Can I do it while shooting line? No - at least I've never been able to do it. Can I do it by making a very high speed cast and coming to a totally dead stop? Try it - I doubt you'll have much success. Can I do it by making a high speed cast and then pulling back after the stop? Yes - and for even greater effect don't have a fly or yarn on the end of your leader - in fact remove the leader.

So I can get the fly leg to accelerate using conservation of momentum but only under limited conditions.

Cheers!

Walter

[Magnus]

Hi Bernd

It's Paul's account I would like to read - thanks all the same.

[Magnus]

Grunde

Thank you!