The Board

VGB wrote:Science is a wonderful thing

Indeed it is. Got some handy? I saw some around here once ...

why is the skin drag in the loop not pulling it all the way over the top? Is the loop just too narrow to do that?

Graeme,
I don't understand what "pulling it all the way over the top" means, but I don't think drag could pull anything. Drag just opposes the line's movement through the air. Could you expand on the point you are trying to make?

2: When he aims the string straight up, how is the string maintaining a height of ~8'?

2. Because the rho_l*v.^2 tension in the string where the loop joins the rising leg is greater than the weight of the string + the downward skin drag on that leg. The height could be much higher for the tangential velocity he produced, but is limited by the length of his loop.

Q3: Would you like to predict the outcome of him pointing his string shooter at the wall, destroying the loop? Would the string just fall to the ground straight out of the motors without a loop? Or would the upper leg maintain trajectory initially imparted by the string shooter?

3. I don't know how big of mess their would be on the return leg after the loop hits the wall, but assuming it would feed through the pinch rollers ok, then it should shoot towards the wall with a trajectory that would be similar (although maybe not exact because there will be a tension difference) to the case with a loop.

Gordy

I don't understand what "pulling it all the way over the top" means, but I don't think drag could pull anything. Drag just opposes the line's movement through the air. Could you expand on the point you are trying to make?

Someone told me that drag can produce lift in the line. I just wanted to make sure the drag on the line wasn't making the loop rise through the air. If you say drag can't pull anything, I guess that other person was wrong.

2. Because the rho_l*v.^2 tension in the string where the loop joins the rising leg is greater than the weight of the string + the downward skin drag on that leg. The height could be much higher for the tangential velocity he produced, but is limited by the length of his loop.

So tension is holding the loop 8 foot up? What's making the rising leg rise? Tension in the loop?

3. ... then it should shoot towards the wall with a trajectory that would be similar (although maybe not exact because there will be a tension difference) to the case with a loop.

And how would that trajectory differ? Would it go up, down, faster, slower, straighter, more curved? Without tension from the loop, what will happen?

Cheers,
Graeme

gordonjudd wrote: It might work in a vacuum, but that does not explain the difference in the width of the legs and the shape of the loop when he changes the launch angle of the shooter. I would think those changes are only effected by the angle of the forces produced by the skin drag on the line.

If it was just the momentum change at the loop that was producing the tension in the string, then I would think the axis of loop would rotate in the plane that was created at the source of the shooter. It doesn't do that as you can see the loop remains aligned in a vertical plane even though the angle of the shooter was changed as shown here:


Maybe this is another interesting experiment for Dr. John Biggins to explain based on some real physics as he did with the chain fountain.

So what force (or forces) do you think is offsetting the force of gravity on the mass of the spinning loop? It can't be momentum.

Gordy

I hope Dr Biggens discovers gravity when he does real physics, that would be really cool. Perhaps he will be able to gather a team of the finest minds to look at what happens to tension in the loop when the motor is switched off:

after watching these videos on youtube I wanted to ask you experts if you think that this phenomenon can be related to the fact that the fly line seems to defy gravity and stay airborne much longer than we would expect when unrolling after the shot, in particular during a distance cast.

So what about the slinky then? Is it true that the lower end doesn't drop due to gravity because the upper end hasn't told it yet?

Hi David and Paul

I take the risk to be pedantic but questions on this forum deserve an answer, at least by politeness.

The slinky issue has nothing to do with the flight of the fly line. It is a completely different problem: different object, different conditions, even if there is some “likeness” in the apparent “aloft” conditions.

That being said, Paul’s comment make me think that even if you have a fantastic software to mimic the problem (fourth video posted by David), it does not give you the clues for understanding what is happening. I would say that the more sophisticated it is, the less you really understand. The slinky problem looks simple but it is not. As far as I remember the post which James deleted, the basic principles were stated: you can consider two simultaneous motions, the fall of the center of mass (g acceleration), and the unloading motion of the slinky around the center of mass. I think that all engineers can agree on these basics. The devil is in the details, as usual.

If you look at the videos you can realize that the slinky does not behave like a simple spring following Hook’s law (deflection proportional to applied load). The center of mass is well below the expected position (middle of the deflected object). It looks as if the slinky has a non linear stiffness. Considering the motion around the center of mass, you need to know the free-free mode for the slinky, and you have no idea about that. For a simple spring, the frequency of this mode is the double of the frequency in the fixed–free mode (thanks to Mr. Lagrange) which you have learned at school. You also have to consider a transitory motion, and not a standing mode of vibrations. All those hurdles prevent a simple demonstration confirming the standstill of the lower end of the slinky. Nothing “simple” seems to work. I am not going to go deeper (that would obviously show my pedantry again), the slinky is a highly non linear object, and by consequence its behavior has something to do with magic.

Merlin

While searching for some documents about springs on the web, I found a Swiss one written in 1932, with a detailed explanation of static and dynamic behavior of helicoidal springs. Just awesome, I wait eagerly for someone who can write the same for the slinky. It is written in French.

Merlin

Why do you think you are considered the pedant, I suspect that a pedant would get excited about trivia like someone writing momentum instead of momentum change.

The slinky issue has nothing to do with the flight of the fly line. It is a completely different problem: different object, different conditions, even if there is some “likeness” in the apparent “aloft” conditions.

Given the forums history of trying to read across studies of bead chains, javelins and string shooters to the flight of a fly line, why not throw in a slinky?

Just awesome, I wait eagerly for someone who can write the same for the slinky.

A slinky model and a description of the associated dynamics is shown below:

https://pdfs.semanticscholar.org/9549/b ... 0bbbf6.pdf

It’s well worth skimming through the references, there’s been many analyses of a slinky

Vince

Thanks Vince

A compliment: you are always very good at finding documents.

A reproof: you should have posted it as soon as David raised his question, then Paul (and others like me), would have understood the wavy aspect of the problem (perhaps).

Some consolation: I found 0.32s for the contact time of a slinky stretched by one meter, not 0.28s, not that bad.

Some disappointment: it is more complicate than I expected.

I leave everyone read (easily) the publication and appreciate the conclusion.

The modeling of the process presented here is also relatively simple and should be accessible to undergraduate students.

Of course Sir

Merlin

I did not check but I think that some of David’s videos are mentionned at the end of the publication found by Vince.
My twisted mind let me think that this might not be by chance. But maybe I am too tricky.

Merlin

Merlin wrote: A reproof: you should have posted it as soon as David raised his question, then Paul (and others like me), would have understood the wavy aspect of the problem (perhaps).

Merlin, I am not retired and it is the fishing season, there are only so many tangents that it’s possible to cope with simultaneously and I was interested in Dirks analysis of the snap. Having raised the question, it would have been rude to ignore it and I’m painfully aware of the time video analysis takes.

Vince

Thanks guys, I figured there was more to it! I had failed to appreciate how the Slinky hangs.

Just back from fishing. I see you guys have been having a nice time.

Cheers, Paul