An 18.8kg box is released on a 38.0o incline and accelerates down the incline at 0.281m/s2. What is the magnitude of the friction force impeding its motion. F = m*g*cos(38) sum forces parallel to the plane m*g*sin(38) - (mu)*(F) = m*a, or m*g*sin(38) - (mu)*m*g*cos(38) = m*a masses cancel out [9.81*sin(38)-(.281)]/[9.81*cos(38)] = .75 = mu but it says i'm wrong. i cant be. i only have one try left, and that's it.