Why is the price of new tonearms so high

I think this is a similar issue that the motorcycle industry had to solve! In the case of shaft drive motorbikes, if they have a simple swingarm for the rear wheel, when power is applied there is what is called "drive shaft jacking" where the rear of the bike will rise up as power is applied. It takes some getting used to.

The solution was a parallelogram swingarm.

Seems to me that the spring that applies the tracking force could be mounted on a parallelogram device (independant of the arm gymbals) that articulated as the arm rides inconstancies in the LP surface. Then the variations in the spring tension could be substantially reduced (not eliminated).

I imagine such a device would raise the cost of the arm :)

*** Yes, we are indeed talking about the same thing. The "effective mass" of a tonearm is the inertial mass of the end where the cartridge bolts on. The interchangability of inertial mass and gravitational mass is fundamental to classical physics . . . as P=mv and F=ma . . . mass = inertia. ***

We are not yet talking about the same thing.
Small derivations, but our models are different.
And I seem to be unable to illustrate what I mean.
We have a force of inertia and we have a moment of inertia.
If we do not set up a model which takes speed into calculation we do not reach the point.

I do not mean audio modulations in the vertical mode - the tonearm is moving vertically - not just the cantilever.

Or better. it should, but due to its moment of inertia it can't follow the counter-movement in zero time but changes the VTF and compresses (and other way around milliseconds later again) the cantilevers suspension and thus moving the attached coil out of the optimal position due to constant increase and decrease in VTF.

**** The angular force vector around the vertical bearing will of course change with all different manner of tonearm-design factors (including effective length) but is irrevelant to the cartridge between two tonearms that have the same effective mass. ****
So the moment of inertia is independent from the distance of the majority of the moving mass to the center of movement ?
Only if the moving mass is homogenous distributed in the whole moving corpus - which is not the case in a tonearm with mounted cartridge.
Brings up again the picture of the Micro Seiki and other turntables which increased their moment of inertia by moving most of the mass towards the outer rim.

We already have different calculations for the force inertia of cylinders, balls and sticks - to name but a few.
I am not questioning your thoughts, I just think we didn't have set up the correct model yet.

**** Oder . . . herrum sitzen und daumen drücken? ****
Well - wer sitz herum und drückt die Daumen wem ?

"Quod erat demonstrandum in realitas mobilis versus modelus in spiritus ?"

Which was to be demonstrated in actual motion (real life), compared to a mental model (i.e. theoretical only))?

Latin can be handy, but mostly in legalistic situations --these days...
Are we now getting ready to go in the dock over: moment, and mass of inertia (effective mass)?

It makes for a degree of interesting and slightly confusing reading, yet we seem no iota closer to the 'true' sonic effects of gravitational vs. spring force VTF...

Wo hängt denn jetzt der Hammer?
A.
PS: AND cart compliance must be part of the equation, I say.

Well it comes down in several aspects to the static vs dynamic model....
Aber es hat weder mit dem Hammer zu tun, noch damit, wo er hängt (im Zweifel immer an der Wand...).

I don't get your last comment (unless Mark's comment that effective mass = moment of inertia divided by square of effective length is wrong). If Mark's equation is right, the two could be different and still result in an identical third (effective mass) value.

No, the equation isn't wrong; I believe that Mark was using it to make a point about how little variation there is in the deflection of a dynamic VTF spring, and thus a correspondingly extremely small change in the VTF at the end of the tonearm as a result of this change in spring deflection. For this calculation, the tonearm length is of course relevant.

The misconception that I understood from Dertonarm's comments is that given two tonearms of different effective length, but identical effective mass . . . that they will somehow present different forces against the cartridge when tracking vertical undulations and warps:

Only if the moving mass is homogenous distributed in the whole moving corpus - which is not the case in a tonearm with mounted cartridge.
Brings up again the picture of the Micro Seiki and other turntables which increased their moment of inertia by moving most of the mass towards the outer rim.

You are correct in that the effective mass at one end of a tonearm is the result of its static mass, length, and distribution of mass along the length. But the 'effective mass' specification takes all that into account, and if you change two of these factors (i.e. length and static mass) to acheive the same effective mass at the cartridge . . . the cartridge doesn't care one iota.

To use the platter analogy . . . you can either increase the rotational mass by adding a BUNCH of weight close to the center, or a whole lot less weight at the edge. But which ever way you do it (practical considerations aside), the platter can have the EXACT same rotational mass either way. As long as the "effective mass" at the circumference of the platter is the same, they will exhibit the same inertial characteristics.