Subject: Connections between models and reality

Posted by Mohamad Morovati on 3/11/2004
In Reply To:Connections between models and reality Posted by Steve Kipp on 3/11/2004

Message:

Dear Steve,

Though the problem haz been solved, I guess furthur explanation may be useful. John has said:
" By definition, a flow is an amount of something that changes over time. You must have at least two measures of the amount (at two different times) in order to compute the change -- so, by definition, you cannot measure a flow instantaneously -- that is, with one measurement at one instant of time."

alternatively, the same concept is discussed in Physics and Calculus .
in Physics we have learnt the concept of average velocity and instantaneous velocity. we all know that the thing that can be measured in physics is average velocity and not the instantaneous. this is the reason that for practical purposes, we use a limit of average velocity to obtain the instantaneous. every physicist knows that instantaneous velocity is a mental concept and can not be measured as a physical parameter.

in calculus, the same debate applies. when we talk of diffrentiation we actually measure the slope of a line passing through two different points. the exact diffrentiative is defined through taking the limit of the slope, when the points become very near to each other. so we do not measure the exact differentitive but the slope.

thinking of flows as differentiatives of stocks, makes the discussion obvious.

it worths mentioning that because of the above (that flows can not be measured directly), each flow in a model should depend only on stocks of the system not the flows. the reason is obvious: for defining the flows you can just reluy upon stocks of the system, because you cannot measure the flows.