I agree it should be a=(f/m)-r regardless of whether it's travelled through a vacuum.Got it. But when we try to account for the effect of air resistance, do we do so by subtracting r from f or by subtracting r from a? Why is the equation, a=(f-r)/m, rather than a=(f/m)-r?
Suppose, for example, the force wasn't constant, as with gravity, but just an initial push (as with two sleds on a flat surface). The air resistance wouldn't affect the force, then, but it would affect the acceleration. (To maybe make the point clearer, let's say the sleds got the initial push, then went through a vacuum, then encountered air resistance.) Would it still make sense to subtract r from f?