1. The problem statement, all variables and given/known data You testify as an "expert witness" in a case involving an accident in which car A slid into the rear of car B, which was stopped at a red light along a road headed down a hill. You find that the slope of the hill is θ = 12.0°, that the cars were separated by distance d = 24.0 m when the driver of car A put the car into a slide (it lacked any automatic anti-brake-lock system), and that the speed of car A at the onset of braking was v0 = 18.0 m/s. (a) With what speed did car A hit car B if the coefficient of kinetic friction was 0.60 (dry road surface)? v = ? m/s 2. Relevant equations F = ma Kinetic Friction Ff = μFn 3. The attempt at a solution I've actually figured out where I went wrong, I just don't know why my way was wrong. I set up my FBD with the positive x direction going up the hill so that my acceleration and frictional force (Ff) is positive and going down the hill is negative x direction so that mgsinθ is negative. This would give me Fnet x components as Ff - mgsinθ = m(+a) But apparently this is wrong as I only get the correct answer if I switch the signs on the x axis. Why should going down the hill be positive and going up be negative?