In 1975, using a similar argument, Dorigo and Tobler (1983) expressed:

Also

where M ij refers to migration from place i to place j. R refers to rejecting, repelling, or repulsing, “push” away factors. E refers to enticing or “pull” toward factors. U, W and L refer to unemployment rate, wage rate and number of people in the labour market.

One can change the variables in the model, delete or add some, transform variables to mathematical or trigonometric form, but as long as the logic developed by Zipf in suggesting gravity models is correct, the above types of models would remain relevant.