The Big Loading Concept

[gordonjudd]

If I wanna know how much is done by only straightening of the rod, I think I can't rotate the rod!?

Bernd,
That is correct. If you just want to restrict the input energy source to just the PE in the rod then you must restrict the distance the tip moves as it unloads to be equal to the initial tip deflection. Clamping the rod butt so it cannot rotate should do just that.

As you know not all of the PE in the rod is going to be converted to KE in the line. in your experiment some of the elastic energy in the rod will go to moving the mass of the rod. I think Merlin gets an estimate of that loss by comparing the effective mass of the rod (m0) to the mass of the line. Thus if you were casting 7 g of line and the rod had a m0 of 3 g then about 3/(3+7) or 30% of the PE in the rod would be lost in terms of converting it to line speed.

Same thing with the deflection of the rod in casting. Some of the deflection that you see results from inertial bending of the rod due to its own mass. Thus the actual acceleration force on the line is less than the F= -k*x spring force you measure from static deflection measurements. Dr. Spolek has estimated that around 30% of the deflection in a cast comes from inertial bending. Thus the actual acceleration force being applied to the line is around F=-k*(.7*x).

I did not take that factor into account when I computed the energy curves shown earlier. That is why the apparent work energy computed by the force over distance calculation (the dark blue curve below) was some 3.4/2.6 (30%) higher than the KE of line shown in the magenta curve. Drag losses on the line and the rod would also be part of that difference.

I was surprised that no one asked about the "why" of those energy curve differences earlier. It makes me wonder if most readers did not understand what those curves were trying to show.

Otherwise I would have hold the rod and add rotation, too.

That is also true. When you hold the rod while making a bow and arrow cast you will instinctively rotate the rod butt forward when the line is released. That will add to distance the tip travels as it unloads and thus will produce more line speed than had it been clamped as you did in your experiment.

Gordy

[gordonjudd]

PS What is work energy?

Vince,
Here is one of many discussion about the force applied over distance work principle you get by using "work energy" into Google.

The work done by a constant force of magnitude F on a point that moves a distance d in the direction of the force is the product,

W = Fd.

For example, if a force of 10 newton (F = 10 N) acts along point that travels 2 meters (d = 2 m), then it does the work W = (10 N)(2 m) = 20 N m = 20 J.

Gordy

[VGB]

Gordy

Thank you for Googling it.

It appears that the term "work energy" you are using that you actually mean "work" and not energy. If you follow your reference down a bit you will see why I asked the question:

Work is closely related to energy. The conservation of energy states that the change in total internal energy of a system equals the added heat, minus the work performed by the system

Vince

[Alejandro]

I understand that you think a slow stop in some way added elastic energy to the rod, right?

Alejandro,
No.
Grunde's simulation showed that because the force applied from the spring in the rod was stretched over a longer distance for the slow stop it would apply more work energy to the line.
The force over distance concept has nothing to do with the elastic energy stored in the rod. The deflection force from the rod and the distance the caster moves that force is important, but the PE in the rod never enters into that calculation.

Gordy,
Thank you very much for your reply.

So you are saying that we have the same amount of energy, but when we use it slowly produce more work? This makes no sense if we start from energy definition as the capacity to produce work. I think Vince has similar doubts.

If the elastic energy does not enter in the equation imagine that we have no a spring. We're casting with a broomstick, If we stop the broomstick slowly we cast further? Or if I'm on the roof of a car and the car brakes slowly, I go further?

But I am not talking about Grunde´s simulations, I speak of the fact.

In 30 years of fly-casting I have never cast a greater distance using a slower stop. In fact rather the opposite happens to me, except when I use 170 style in which the stop speed seems lack importance. The biomechanical observations made by Moran and Kyte point in the same direction, this is what they write:
“Any hand movement or change in rod butt angle during the stop phase represents a softening of the stop movement and involves some release of energy down through the hand This would result in a less efficient utilization of energy stored in the bent rod. We measured the stop in terms of the degrees of rod butt angle change between the point of maximum rod deflection to the point at which the first bends downward. This is when the energy stored in the bent rod is released to the fly line. This is sometimes referred to as the point of turnover. The most successful distance casters stopped the rod so abruptly that the butt moved barely 1 degree. The elite group restricted rod butt movement to 6 degrees, as compared to a mean of 1 1 degrees of movement for the good group”.

We may agree or disagree with the explanations of Moran and Kite, but the fact are that in their test the casters who made stronger stops achieved greater distance.
If we accept this fact and also think that a slower stop apply more work on the line (although I do not know still how this is possible), we could infer that there is something that prevents that the greater work will become greater momentum on the line in the rigth direction. In any case the assumption that a slower stop has advantages in the fly casting seems is not met in the reality.

Cheers
Alejandro

[Unregistered]

I think most students are more interested in the "how" of casting rather than the "why", so generally there is no need to discuss physics principles in a casting class.

I'm sure that's so, but some of us have a theoretical leaning style and for us the why matters. If I can get one, I like to have a model in my head of what's supposed to be going on to act as a framework for the endeavour. Help me understand why it went right, why it went wrong, what I have to achieve to get from where I am to where I want to be. It seems to me that the 'how' only gets you so far, as a beginners 'how' is different to an intermediate's 'how', is different to an expert's 'how', but a good theoretical model will go with you all the way.

Exactly my view.

[Alejandro]

Gordy, a small correction: the work is not force x distance. Is force x distance x cosine of the angle between the force vector and the displacement vector. In fly casting consider the vectors angle can be essential.
I know that you know it well, but it is possible that readers of this forum who do not have physics knowledge are mistaken after reading your message.ç

Alejandro

[Walter]

Alejandro,

Merlin's model in another thread showed that if the casting arc is kept the same from the start of the casting stroke to RSP1 then you get the greatest line speed with the hardest stop. This is in keeping with what Gordy is saying here and it also agrees with what you are saying. Gordy is saying that the greater distance over which I apply a force to an object (in this case a line) the greater the kinetic energy the line object will attain and this translates to higher line speed. Merlin's model and your observations show that a hard stop allows you to apply force to the line over a greater distance because part of your casting arc is taken up by the stop and a slow stop takes up more of the casting arc than a slow stop. Adding a bit of arc to make up for the slow stop opens your loop and results in lost energy.

What Grunde showed is that in the 170 cast I can apply force to the line for an even greater distance and get even greater line speed than if I had stopped the rod earlier.

Where is the difference? In the early stop cast we concentrate on getting the rod tip to move as much as possible in a straight line. Then we stop the rod and the line overtakes the rod tip. In the 170 cast we move the rod tip in a straight line for as long as possible but eventually the rod tip begins to move in an elliptical path. Even though the rod tip hasn't slowed down it is now moving in another direction and the line over takes the rod tip. We lose much of the PE that is stored in the rod bend with the 170 cast but if we are good we make up for it by having a longer distance to apply force to the line.

The bending of the rod, other effects due to the rod's springiness, the relative mass of the rod and the line, losses due to friction, angles of the rod relative to the line as you mentioned and the rod as a third class lever make this a lot more complicated than it sounds.

[Torsten]

Alejandro, just to correct the correction:

Gordy, a small correction: the work is not force x distance. Is force x distance x cosine of the angle between the force vector and the displacement vector.

The work is defined as scalar product of the force and the distance only if the moved object travels on a straight path. In this case we can write W = F * s where F is the force vector and s is the displacement vector. Of course this can be expressed as W = |F| * |s| * cos(angle(F,s)), where || indicates the vector norm.

The general definition of the work is W = curve integral ( F(s) ds )
This way you can compute the work if your object travels around any path.

[Bernd Ziesche]

In 30 years of fly-casting I have never cast a greater distance using a slower stop. In fact rather the opposite happens to me

Hi Alejandro,
I add another 20 years . 50 now :b.
Greets
Bernd

[Merlin]

Where have anyone seen that a slow stop is better than a sharper one? It all depends on which ground you make the comparison. How does a 170 cast compares with a 100 one? It is a question of both maximum rotation speed and timing.

Merlin