REPLY Meaning of Stock/Level (SD6954)

[SDMAIL Dhr. Nijland]

Posted by  "Dhr. Nijland" <lukkenaer at planet.nl>

Jack Harich wrote:
 > Maybe it's more like nouns versus verbs. Variables that are verbs
 > (rates) have a unit per time period, like persons/year. Nouns have only
 > the unit, like persons. Verbs cannot be a stock, but nouns can
 > optionally be a stock. It's just a question of which nouns you want to
 > emphasize in the model's structure by making then stocks.
 
Two remarks:
1) I think, in some cases, a variable may be represented as "state 
variable" and "rate variable" in the same model, even without 
representing "material" stocks and flows.
Studying the dynamics of position, velocity, acceleration and mass 
of satelites in a gravitational field by means of a numeric simulation 
model: 
The "velocity" (m/sec) of the satelite may be modelled as a rate 
variable with respect to "covered distance (or position)" (m), the 
latter being a state variable with respect to the first.
The same rate variable "velocity" (m/sec), however, is a state variable 
with respect to another rate variable "acceleration of the satelite" 
((m/sec)/sec).
It may be noted that the "mass of the satelite" according to relativity 
theory variates with "velocity of the satelite", but there is /_no need 
to model this mass variable as a "state"_,/ though it evidently 
represents "matter".  At their turn "mass" and "position" 
determine "gravitational force", and "gravitational force" determines 
the rate variable "acceleration". So three causal loops are formed:  1) 
velocity > position > gravitational force > accelleration > velocity; 2) 
velocity > mass > gravitational force > acceleration > velocity; 3) 
velocity > mass > acceleration > velocity. Is this model conceptually 
correct?
So the material variable "mass" is not represented as a state variable, 
while the non-material variables "acceleration" and "velocity"  are 
reprented as rate and state.  
 
2) Cannot any variable in a model formally be represented as a 
combination of state variable and rate  variables? And any constant too? 
For a constant may be regarded as a state variable of which the sum of 
inrates and outrates are zero.
 
Best regards,
Geert Nijland
Posted by  "Dhr. Nijland" <lukkenaer at planet.nl>
posting date  Mon, 21 Apr 2008 20:43:04 +0200