Hello. I've got a simply ballistics program with a very simply algorithm to deal with air friction. However, I am trying to find a mathematical forumula equivalent. Here is the algorithm: G = accel due to gravity (universal constant) F = projectile air friction factor (constant for a given projectile) T = delta T (delay between loops, varies depending on frame rate but is about 0.01) Projectile starts out traveling at velocity Vo fired at angle Ao. Velocity is broken into a vector made up of: Vx and Vy Loop Ax = Vx * F Ay = Vy * F * G V = [Vx*Ax*T, Vy*Ay*T] Wait T goto Loop For any given projectile, I will know Vo and F. For now, I want an equation where I can specify a point (x,0), and it will give me the required Ao so that the trajectory intersects that point. Basically, given a specific distance, I want to know the angle required in order to land the projectile there (assuming the ground is flat and projectile is fired from y=0). I the approach I'm trying to take is to find equations that give the projectile's current position (x,y) based on the variables Vo, Ao, and F, and the constant G. I can do this easily in an Excel spreadsheet, but I'm not sure how to do it in an equation. I've come up with an acceleration forumula: Ax = Vx^2*F Basic physics says that velocity is the integral of acceleration. However, I'm confused about if/how I can integrate this, since acceleration depends on velocity. Can anyone help me out here? Thanks!