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Another physics question

Moderator: Torsten

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grunde
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Re: Another physics question

#41

Post by grunde »

"Essentially, all models are wrong, but some are useful."
George E. P. Box

Always question the assumptions!

Flycasting Definitions
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RSalar
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Re: Another physics question

#42

Post by RSalar »

So here is non mathematical proof that the inertia of the tip does not cause it to move backwards. In fact inertia actually resists that movement. I added 80 grains to the tip and did the same experiment multiple times. You can do the same experiment if you’d like to. Here is the same rod as in the previous video I posted. I did everything exactly the same — this time the tip doesn’t hit the workbench. It moves backwards less. Inertia resists the backwards movement that the wave in the rod is causing.



I hope someone will do the same experiment to verify the results.

Ron
FFI - CCI
Mangrove Cuckoo
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Re: Another physics question

#43

Post by Mangrove Cuckoo »

I'm sure I have a couple of Server's "thoughts" archived somewhere. None that I have are finished products. They were shared with Gordy Hill back when he was piloting a study group for FFF instructor candidates. Server openly states where he needed to do more work, and in many cases he was glossing over the more technical things like equations since the group was not engineers but fly casters.

Gordy shared them with his students not because they were important for passing the FFF exams, but because they were another of those "peeks behind the curtain" kind of things. If I remember correctly, Gordy even cautioned us to not mention some of these concepts if/when we were examined as they were not FFF "approved". :D
With appreciation and apologies to Ray Charles…

“If it wasn’t for AI, we wouldn’t have no I at all.”
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Merlin
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Re: Another physics question

#44

Post by Merlin »

I found the old video in my files, here is a picture frozen at sdm timing:
SS qt video.JPG
SS qt video.JPG (41.15 KiB) Viewed 1378 times
The definition of the Quick Time file is poor, I think the caster is Server himself, but I am not sure.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life
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Walter
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Re: Another physics question

#45

Post by Walter »

Yes, that’s Server at his home in Bozeman, Montana.
"There can be only one." - The Highlander. :pirate:

PS. I have a flying tank. Your argument is irrelevant.

PSS. How to generate a climbing loop through control of the casting stroke is left as a (considerable) exercise to the reader.
Torsten
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Re: Another physics question

#46

Post by Torsten »

Daniel,
Merlin wrote: Sun May 29, 2022 7:50 am
Otherwise is Ron correct with #21, a backward motion (of the tip) is not just related to inertia but requires a force too, and this force comes from the rest of the rod.
Really? There is no other external force than a torque at butt level. Are you speaking of an internal force then? This effect is directly related to inertia, until one can demonstrate the contrary. That is also why sometimes people refer to “waves”, and yes, it comes from flexural waves introduced at the butt of the rod. That’s also what is called the “whip” effect. Good luck for calculating the force in question.
Yes, I'd refer simply to Newton's first law, the tip has inertia, which is measured as mass and it requires a force to cause any motion from rest. In this case it's a internal force, when we consider the tip as part of the larger system (rod).
I don't see currently how it's possible to show that with a SDOF mass-spring system - but maybe you can convince me. I'd guess it's more complicated I'd argue perhaps with angular momentum, moment of inertia, or bending stiffness etc.
For laymen SDOF is single degree of freedom. And the issue is there: a moving spring has two degrees of freedom, one for each end, not like a spring with a fixed end. You should reread very carefully the document I wrote in 2016:
http://ffden-2.phys.uaf.edu/webproj/211 ... y_cast.pdf
The details are all in there, and for once a spring is modeled with a consistent expression for its kinetic energy as it is moved forward (or backwards).
One degree of freedom per node, but you're right we better call it something like a 2DOF system. I've quickly derived a few equations yesterday and indeed I get a dependency of the acceleration of the second node independent of the spring force - but only if I assume a consistent mass matrix, that means mass uniformly distributed between the nodes. I've yet to understand and visualize that for me, also I don't know if that's the same effect that we can observe in the videos - e.g. you can't model rotations with a 1D-system.
I had implemented a simple 1D 2DOF system about 15 years ago, maybe you can find the results somewhere in the board archive. But back then, I had only considered lumped masses.

Greetings,
Torsten
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Merlin
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Re: Another physics question

#47

Post by Merlin »

Hi Torsten
that means mass uniformly distributed between the nodes
Absolutely correct.
also I don't know if that's the same effect that we can observe in the videos - e.g. you can't model rotations with a 1D-system.
Correct again but one can make a 2D system incorporating the inertia issue.

I got inspiration from an academic paper giving various options for estimating the equivalent mass (at tip) of a cantilever beam. I then use the rotation at the butt and the rotation of the chord for speed parameters. It took several weeks, if not months, to correctly inegrate the various equations involved with a methodology using small variation in deflection to decribe speed in any point of rod shaft.

Today I can calculate MOI with this method and three equivalent masses (analogy with the spring) if I know the taper then deflection of a rod. I did the necessary checks for MOI and equivalent mass at tip (your lump mass). The only issue I failed to solve is a direct measurement of the other equivalent masses. We know how to measure MOI (e.g. Sexyloop method or pendulum method), we can estimate equivalent mass at tip with loaded frequency, but I failed to find a way to estimate one of the other two (equivalent mass at butt and the "transfer" one), bearing in mind that MOI results from the sum of those three equivalent masses. If we could measure one of the two missing equivalent masses, then the last one would be deducted from MOI value.

Merlin
Fly rods are like women, they won't play if they're maltreated
Charles Ritz, A Flyfisher's Life
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gordonjudd
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Re: Another physics question

#48

Post by gordonjudd »

One degree of freedom per node, but you're right we better call it something like a 2DOF system.
Torsten,
I suspect the complicated bendforms that result from the multiple vibration modes that are excited in a long tapered fly rod when there is a quick angular acceleration of its butt is going to be much more complicated than a spring-mass system with 2DOF.

When you look at the motion of the tip in a high speed video you will find the it has a slight upward bend at the end of the rod that causes the tip to move upward before it starts kicking down. I don't know if a 2DOF spring-mass system could capture that motion.
initial_up_tic_tip.jpg
Gordy
Torsten
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Re: Another physics question

#49

Post by Torsten »

Hi Gordy,

sure, for a more complex system it's plausible to explain with higher modes, but the 2DOF has AFAIK only one mode and somehow I still get a negative force, like for Daniel's model. I still have to understand that.

Thanks,
Torsten
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gordonjudd
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Re: Another physics question

#50

Post by gordonjudd »

but the 2DOF has AFAIK only one mode
Torsten,
I don't know how you have configured your 2 spring-2 mass mass system, but I would expect there to be two natural modes. One where the masses move in the same direction and a second where they move in opposite direction.

In general when the initial conditions are different than those given by the Eigenvalues the motion can be quite chaotic.

There are all kinds of examples on the web.

Gordy
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