1. The problem statement, all variables and given/known data A box of mass m rests on the flat floor of a truck. The coefficients of friction between the box and floor are mu_s and mu_k. The truck stops at a stop sign and then starts to move with an acceleration of a. If the box is a distance x from the rear of the truck when the truck starts, how much time elapses before the box falls off the truck? 2. Relevant equations F = ma 3. The attempt at a solution I have a few questions about the problem, mainly because I don't understand the situation. (Well I have started on it, but it turned out nonsense so I decided that I really need to understand the whole picture first.) These might be really stupid questions but it isn't clear for me.... First of all, the truck is driving, and THEN it stops right? It didn't start out at the stop sign and then start to accelerate? Second, the box on the truck, that's static friction, isn't it? Because the box is not moving. It takes static friction first to get it to fall off the truck?