When one stock influences another, then that influence can be thought of as a force, controlled by a parameter equivalent to mass. Consider a force from one stock *y* to another stock *x*:

A full explanation of this model, and the concept of force, is given on the force page.

The parameter *b* controls how stock *x* responds to changes in stock *y*. The larger the value of *b* then the more the response of *x*. Thus *b* is like the inverse of the mass of *x, b = 1/m*. The smaller the mass of *x*, the larger the value of *b*, the more the response of *x*. That is, a “lighter” stock is more responsive to a force than a “heavier” one.

The model can be expressed mathematically:

Differentiating, and substituting for *b*, gives the acceleration of *x* as the equivalent of Newton’s second law of motion:

Considider a constant force *F* for four different values of mass *m*. The responses of *x* are compared in the figure:

The heavier the stock, the smaller the acceleration, i.e. the curvature in the time graph (mass = 4). The lighter the stock the more it is accelerated (mass = 1).

It is possible for a stock to be acted upon by different stocks in different units. In this case the stock could be thought of as having a different mass with repespect to each stock. In system dynamics the concept of mass is multidimensional.