We all know in phenomenological equations we like to add a [itex]-\gamma v[/itex] effective force term to Newton's equation to include frictional effects with the environment. Why this specific shape? I understand it's not fundamental, and that it is not always appropriate, but still it seems to be a good approximation in many cases. Why is this? I understand the intuitive idea that a particle moving gets more collisions head-front and as such will experience a backwards force, but why directly proportional to velocity (and not, say, a square root or a quadratic or... etc)?