Cartridge Loading and Compliance Laws

No. Because your model does not hold true in real life. In real life, the tonearm must permit the cartridge to trace the groove from outside to inside, and so it cannot be fixed in space, which is where your model falls short. In real life, the cantilever has to move the effective mass of the cartridge body and the arm wand up to the pivot point. The cantilever of a high compliance cartridge would be more prone to flex in response to the need to move this mass in addition to its need to flex in response to groove modulations, with a high mass tonearm vs a low mass one. This could produce a low level signal that is not part of the music signal, i.e., distortion. The best simple analogy I know is to think of a high mass arm as a truck and a low mass arm as a sports car. Have you ever seen a truck with bad shocks (high compliance/low damping) when it travels over a bump? It will be set into harmonic motion by the bump more easily than will a sports car with much smaller, higher compliance shocks, because the latter has so much less mass. Conversely, really hard shocks (low compliance) on a sports car will knock your teeth out. Even this model has flaws, I admit, but I hope I made my point.

Manitunc, You wrote, "To use your truck/sports car analogy however, a truck would be less likely to be knocked off its straight path than a sports car hitting the same bump. I dont know that the analogy works because the tonearm is not moving, and therefore has no momentum inertia of its own, only its fixed inertia as an impediment to motion."

I don't really see what you are getting at. The tonearm most certainly does move, in the lateral plane it has to move in order for the stylus to trace the groove, in the vertical plane we don't want it to move (up and down), but it will to a degree that is directly dependent upon the compliance of the cartridge (the springiness of the cantilever) and the mass of the whole ensemble of the tonearm/cartridge. Obviously, the less vertical motion of the tonearm wand, the better.

As an aside, "momentum" and "moment of inertia" are two different things. Don't know what "momentum inertia" is. Finally, for both a truck and a sports car, the correct shock absorber damping will result in the least reaction to a "bump". (I don't really like my own sports car/truck analogy so much, either.)

Dear Manitunc, My point is that you cannot make the general statement that a heavy tonearm is "better" than a light one, or vice versa, because there are other factors involved and for sure there are conditions within which either proposition is the correct one. And the model of a cartridge held static in space is a bad one to start from, I still say.

"An object in motion tends to stay in motion". Yes, that is inertia.
"Momentum" is a quantity applicable to a body in motion. Momentum is equal to mass X velocity. Inertia also says that a body at rest will tend to stay at rest. A body at rest has no momentum.

"heavy arm = low compliance cartridge, light arm = high compliance cartridge"
Yes, that is a mantra of the internet that I read very often, too. As I am sure you know, that formula is meant to set the low frequency resonance at a frequency between 8 Hz (so as not to excite resonance via footfalls and other very low frequency sources) and 12 Hz (so as to stay away from the audio bandwidth). The formula for that resonant frequency can be found on Vinyl Asylum and in other places, but tonearm effective mass and cartridge compliance are its principle determinants. So this is why the gurus tell us, "heavy arm = low compliance, etc". However, there are guys on this discussion group, most notably Raul, who point out that there are many other factors that determine the overall "goodness" of a match between tonearm and cartridge and that sometimes one can and should overlook the mantra in search of good sound. One example is that several owners of the Fidelity Research FR66, a 12-inch arm with very very high effective mass, claim it sounds fantastic with MM and MI cartridges that have very high compliance. Conversely, some others who own very light tonearms like the ADC and the Black Widow like to use them with MC cartridges that have low compliance. To all this I say, "go figure". The take home lesson I think is if you have a tonearm you like a lot, don't stop using it just because it might technically be a mismatch with your new cartridge, on the basis of its effective mass vs the cartridge compliance. All bets are off.

Why this is true is worthy of another separate discussion, but I have my ideas.

Ideas why cartridge/tonearm mating may defy the predictions:
(1) The compliance of the actual cartridge sample could vary significantly from what the manufacturer has published, especially if the cartridge has aged.
(2) Many systems cannot reproduce much bass below 30-40Hz and so a resonant peak above 12 Hz is not much of a problem and/or the turntable is well isolated and in a very stable environment, so resonance below 8 Hz is also not a problem.
(3) Little tricks used in certain tonearms to dampen resonance and/or to reduce effective mass, such as moving the counter-wt very close to the pivot point.
(4) Inaccurate data on tonearm effective mass.
(5) When one spends a lot of money for a tonearm and cartridge, one is predisposed to like the result.