1. The problem statement, all variables and given/known data A projectile is launched horizontally. A retarding force acts on the projectile of the form F = -Ae^(cv). Find it's speed as function of time. 2. Relevant equations F = ma 3. The attempt at a solution m(dv/dt) = -Ae^(cv) e^(-cv) dv = (-A/m) dt I integrated the left side between intitial velocity and final velocity, (v_i and v_f) and the right side between 0 and t. After evaluating the elementary integral and doing a small amount of algebra we come to the following: e^(-c*v_f) = (t*A*c)/m + e^(-c*v_i) taking the natural log of both sides.... -c*v_f = ln ((t*A*c)/m + e^(-c*v_i)) so... v_f = (-1/c) ln((t*A*c)/m + e^(-c*v_i)) which is what I wrote for my answer. Hurray. Except an online source that is pretty dependable gives a different solution. They have the following: v_f = v_i - (1/c)*ln(1+(A*c*t*e^(-c*v_i)/m)) Anyone wanna shed some light on my mistake or tell me my other source is incorrect?