I have two separate problems that I think require essentially the same equations and thought process. So if I can get one I should be able to get the other: First problem 1. The problem statement, all variables and given/known data A bat crashes into the vertical front of an accelerating subway train. If the coefficient of friction between bat and train is 0.89, what is the minimum acceleration of the train that will allow the bat to remain in place? 2. Relevant equations Friction=[mu]*N F=ma kinematics equations ??? 3. The attempt at a solution No idea how to even start this one. Second problem 1. The problem statement, all variables and given/known data At the end of a factory production line, boxes start from rest and slide down a 30 degree ramp 5.7 m long. If the slide is to take no more than 3.5 s, what is the maximum allowed frictional coefficient? 2. Relevant equations same as first problem, i think? 3. The attempt at a solution First I thought that I need to find the acceleration of the box over 5.7 meters in 3.5 seconds. I got about .931 m/s^2. I'm not sure where to go from there. One big question I have is how does the 30 degree angle play into the question? Thanks!!