Climbing loop = free energy?

[Walter]

Few,

Totally different use of free energy. The perpetual motion pundits talk about getting energy for free from their perpetual motion machines so its "free energy". I had the Helmholtz/Gibbs stuff buried so far back in my brain. Now I have to bury it again...

As you point out, I did contradict myself. The TE cannot change without outside influences but there is always an outside influence in the form of air resistance, and internal losses as well. These will only act to gradually deplete the TE in the system. From the time we launch the line in a tethered cast the TE is constantly decreasing. In order for the TE to increase there would need to be some additional input such a check mend of sufficient force to temporarily exceed the losses.

The KE vs speed thing is something to keep in mind. While the KE of the fly leg may be constant (or slowly depleting due to losses) its mass is constantly decreasing so it is possible to have an increase in speed but a drop in KE.

Thanks!

Walter

[Walter]

Paul,

Another thing that occurs to me is that when we look at the energy or momentum of the line it is typical to measure or calculate the energy or momentum of the rod leg, loop and fly leg individually and then add them together. That's a standard practice. But what we can't do is look at the motion of one part in isolation and make inferences about the other parts. We can't say, for example, that the fly leg is rising so the entire line is rising because the line as a whole, as determined by the motion of the center of mass, may be rising, falling or neither of the two.

[Paul Arden]

Thanks Walter, does the centre of mass always behave as a projectile despite what is going on in the respective legs?

For example is it possible that what is happening at the loop front could affect this centre or mass behaviour? Or will the centre of mass always behave the same?

Thanks, Paul

[Walter]

Paul,

The short answer is that things that happen elsewhere in the line are not going to change the motion of the center of mass.

In the shooting cast the center of mass would follow a ballistic trajectory similar to that of a thrown ball. Things that would be different between the ball and the line would be air resistance and the fact that the center of mass of the ball would be a fixed point in the ball but the center of mass of an unrolling would be constantly changing relative to any arbitrary point on the line and could, at any point in time, be anywhere on the line or a point in space somewhere between the two legs.

We like to think of things floating in space so if we have an astronaut floating in space and they bend and straighten their arm their whole body would move but if we observe their center of mass it’s position doesn't change but the location of their center of mass relative to a fixed point in their body does change.

There are some interesting things that can happen due to conservation of angular momentum like when we flip over a spinning wheel but I don’t see how that applies to fly line.

The tethered cast is something I’m still thinking about due a question from Mark but I think the trajectory of the center of mass for the tethered cast will be more like that of a falling rod hinged at one end rather than just that of a ball. It’s still a center of mass thing though.

[Mangrove Cuckoo]

So...

Does a cast that starts high from a rod tip that is held vertically, but is inclined toward the water, experience an increase in KE since its PE is decreasing?

And, if so, given general conditions like the usual 9' rod and say a 5wt flyline, is this something that could be discerned?

Or are we in the realm of the chalkboard in physics class while assuming no friction and perfectly spherical chickens?

[Merlin]

Hi Gary

KE increases since the vertical component of line speed increases, leading to a crash on water.



Merlin

[Walter]

Gary,

Merlin is right but I’m not sure if that’s what you had in mind.

Energy is a scalar quantity but the V part of 1/2 mv^2 is a vector and consists of a vertical and horizontal components. The conversion from PE to KE results in an increase in the vertical component only. It’s not going to affect the horizontal component.

There has been some discussion about the idea that gravity pulls down on the rod leg and creates extra tension in the rod leg (tethered cast, not shooting) and this tension can lead to acceleration of the fly leg beyond what the normal conservation of energy concept would. Looking at it from a purely KE/PE/TE point of view that would mean that either the TE of the line increases or we are causing the line to fall more quickly than it would otherwise in order to increase the rate of conversion from PE to KE while maintaining constant TE. My initial thoughts are that that isn’t happening.

[Mangrove Cuckoo]

Gary,

The conversion from PE to KE results in an increase in the vertical component only. It’s not going to affect the horizontal component.

Could not an increase in the vertical component of the rod leg increase the horizontal component of the fly leg?

What if the caster deliberately tries to create a "tailing" cast, where the rod leg might be inclining but the fly leg is parallel to the water?

But, again, are we talking about perceivable changes when using common fly tackle, or would this be such a minimal effect that it is only in the realm of philosophical discussion levels?

[Walter]

Gary,

I think what confuses people is that they see the loop as a pulley between the fly and rod legs. It does have elements of a pulley but it’s not separate from the fly and rod legs.

I can use a pulley to redirect forces acting on a line. If my line is hanging over a pulley the longer end is going to fall pulling the shorter end upwards. I can lay the short end (call it the fly leg) horizontally on a table so the pull of gravity on the long end (call it the rod leg) that pulls it vertically downward results in the fly leg being accelerated horizontally. At the same time the rod leg is not accelerating as quickly as it would if it wasn’t attached to the fly leg. The PE of the line is being converted to KE and some of that conversion results in the fly leg being accelerated vertically.

The problem with this analogy is that in the purely tethered cast the rod leg, fly leg and loop are all accelerating downwards equally due the force of gravity. Even if I threw the line upwards the elements are all slowing down and they are doing it together. The pulley is no longer fixed in position and there is no table preventing the fly leg from falling vertically. In the fixed pulley analogy the rod leg falls slower than it would on its own but if I could magically make the table disappear and allow the pulley to fall freely then without some sort of outside influence there is nothing that is going to cause the fly leg to accelerate horizontally in exchange for a slower rate of conversion from PE to KE.

Going one step farther the rod end in a tethered cast is not allowed to fall freely but it cannot prevent the rod leg from falling under the normal gravitational effect. It can’t slow its fall. All it can do is prevent a portion of the line from falling to the ground. Preventing that segment of line from rod tip to the ground from falling is going to cause a natural pull back of the line in flight. Imagine the line fully unrolls horizontally on the same level as the rod tip. Now as the line falls it is not going to fall perfectly vertically because the rod end is prevented from doing that. If we originally made our cast horizontally this natural pull back would be happening as the line is unrolling but don’t think this amount of pull back is going to significantly affect the speed of the fly leg.

[Merlin]

What if the caster deliberately tries to create a "tailing" cast, where the rod leg might be inclining but the fly leg is parallel to the water?

Gary

In that case there is some conversion of PE into horizontal KE through the pull back effect of rod tip (assumed to stay still). Gravity drives the line down whilst rod tip holds it back. Consequently there can be some horizontal acceleration of the fly leg.

Merlin