Editing Stock and flow (section)

==Calculus interpretation==
If the quantity of some ''stock'' variable at time <math>\,t\,</math> is <math>\,Q(t)\,</math>, then the [[derivative]] <math>\,\frac{dQ(t)}{dt}\,</math> is the ''flow'' of changes in the stock. Likewise, the ''stock'' at some time <math>t</math> is the [[integral]] of the ''flow'' from some moment set as zero until time <math>t</math>.

For example, if the [[capital stock]] <math>\,K(t)\,</math> is increased gradually over time by a flow of [[investment (macroeconomics)|gross investment]] <math>\,I^g(t)\,</math> and decreased gradually over time by a flow of [[depreciation]] <math>\,D(t)\,</math>, then the instantaneous rate of change in the capital stock is given by

: <math>\frac{dK(t)}{dt} = I^g(t) - D(t) = I^n(t)</math>

where the notation <math>\,I^n(t)\,</math> refers to the flow of [[investment (macroeconomics)|net investment]], which is the difference between gross investment and depreciation.