Answer:
Taken from the answer of Horsin' Around.
p is used because the word "impetus" formally in place of "momentum" comes from the latin, "petere," to go towards or rush upon..........so therefore we get "p"
another way to look at it is q is used for the reaction and p is the mirror image of q so therefore since "to every action, there is an equal and opposite reaction," we choose p to go with q