Hey, I have this homework thing where I have to model an object's motion through a viscous liquid. I'm to assume that the resistive force directed upward is kv. Now, I'm going to take the downward direction as positive and the up as negative.. Should my force equation be mg - kv = m(dv/dt) or mg - kv = m(-dv/dt)? This is confusing me because I know that the acceleration opposes the direction of motion, so should it be dv/dt or -dv/dt? Thanks.