1. The problem statement, all variables and given/known data Two crates of mass m are stacked on the back of a flat-bed truck of mass M. The truck accelerates from rest with a uniform acceleration a. How does the force exerted by the road on the truck change when the masses are stationary vs. when the masses are sliding on the floor of the truck? 2. Relevant equations Fn = (2m+M)g Ff = μs(2m+M)g as = μg (as is value of acceleration beyond which crates begin sliding, and coefficient of static friction between crate and truck is μ) 3. The attempt at a solution The first two equations above are the equations I got for the force exerted by the road when the masses are stationary, but I'm not sure how this would change when the masses are sliding. It doesn't seem to me that they would, but maybe I'm missing something. The part of the question asking for force exerted by the road when the masses are sliding comes after a question about the value of as so I'm thinking that maybe that has something to do with it?