DE: Resistant force, 2 equations

1. Oct 6, 2008

rocomath

So I'm given two equations, but not really told when it's appropriate to use one over the other.

1) The resistance is proportional to the velocity

$$F=-mg-rv$$

2) The magnitude of the resistance is proportional to the square of the velocity

"the magnitude" ... what's the difference?

$$F=-mg-k|v|v$$

2. Oct 7, 2008

gabbagabbahey

Are some of these quantities suppose to be vectors?