We describe students revising the mathematical form of physics equations to match the physical situation they are describing, even though their revision violates physical laws. In an unfamiliar air resistance problem, a majority of students in a soph
omore level mechanics class at some point wrote Newtons Second Law as F = -ma; they were using this form to ensure that the sign of the force pointed in a direction consistent with the chosen coordinate system while assuming that some variables have only positive value. We use one students detailed explanation to suggest that students issues with variables are context-dependent, and that much of their reasoning is useful for productive instruction.
