Dear last_lemming,
There is a very good answer to your question in another forum. Here is a quote from member "tnsilver":
"So effective mass in not mass - it's inertia! In fact, even the common measurement (in metric grams) is a misconception. This is brought to you here, by the tonearm manufacturers, as a curtsy to the layman. Effective mass, like any inertia, is measured in Kg/m/s2 (that is kilograms per meter per second squared). Since we're talking very small mass here - everything is divided by 1000 and so we're actually dealing with grams per millimeters per second squared. The general em formula relationships are manipulated such that we're left with grams only - but nevertheless it's Inertia!!!. Keeping that in mind it's easier to regard effective mass for what it is.
Another misconception is the relationship between 'effective mass' and mass. If you add 1 gram to the tip of the tonearm you do not add 1 gram of effective mass to the tonearm No way Jose!. You do not add a 1/3 or a half - none of it catches here. So, how much do you add? Well, that cannot be described in English, it can only be described in a math equation. This is what it looks like:
M(kg) = m(r²/L²) + (Z/3)
m is the counter weight mass
r is the counter weight distance from the pivot
L is the effective length (pivot to stylus tip)
Z is equal to twice the mass of the front end of the tonearm at the effective length. Your headshell mass is part of 'Z'.
M is the effective mass and the whole thing is in kilograms but it doesn't matter. This is just to demonstrate why the relationship between mass and effective mass is not as straight forward as one might think.
L (the leverage or effective length) will affect the importance of the real estate the most. In other words - the tip of the tonearm is the most strategic location where mass can affect inertia. Adding just a tiny amount of mass to that specific location might, just as well, be equivalent to the total effect the counterweight has on the effective mass of the tonearm. It's that important! This is where 'r' vs 'm' in the formula kicks in.
Having said that... movements of the counter weight back and forth across the back of the tonearm seldom changes effective mass by any significant amount. It's typically punched in and pre-calculated into the specs of the tonearm and it's a generic part of the given effective mass."
There is a very good answer to your question in another forum. Here is a quote from member "tnsilver":
"So effective mass in not mass - it's inertia! In fact, even the common measurement (in metric grams) is a misconception. This is brought to you here, by the tonearm manufacturers, as a curtsy to the layman. Effective mass, like any inertia, is measured in Kg/m/s2 (that is kilograms per meter per second squared). Since we're talking very small mass here - everything is divided by 1000 and so we're actually dealing with grams per millimeters per second squared. The general em formula relationships are manipulated such that we're left with grams only - but nevertheless it's Inertia!!!. Keeping that in mind it's easier to regard effective mass for what it is.
Another misconception is the relationship between 'effective mass' and mass. If you add 1 gram to the tip of the tonearm you do not add 1 gram of effective mass to the tonearm No way Jose!. You do not add a 1/3 or a half - none of it catches here. So, how much do you add? Well, that cannot be described in English, it can only be described in a math equation. This is what it looks like:
M(kg) = m(r²/L²) + (Z/3)
m is the counter weight mass
r is the counter weight distance from the pivot
L is the effective length (pivot to stylus tip)
Z is equal to twice the mass of the front end of the tonearm at the effective length. Your headshell mass is part of 'Z'.
M is the effective mass and the whole thing is in kilograms but it doesn't matter. This is just to demonstrate why the relationship between mass and effective mass is not as straight forward as one might think.
L (the leverage or effective length) will affect the importance of the real estate the most. In other words - the tip of the tonearm is the most strategic location where mass can affect inertia. Adding just a tiny amount of mass to that specific location might, just as well, be equivalent to the total effect the counterweight has on the effective mass of the tonearm. It's that important! This is where 'r' vs 'm' in the formula kicks in.
Having said that... movements of the counter weight back and forth across the back of the tonearm seldom changes effective mass by any significant amount. It's typically punched in and pre-calculated into the specs of the tonearm and it's a generic part of the given effective mass."