I'm really stumped with this question. A block is held against the front of a vertical wall of a railroud car. The coeffiecient of friction between the block and the wall is 0.4. When the train betgins to accelerate the block is released and begins to slide down the wall with an acceleration of 9.0m/s^2. What is the horizontal acceleration of the train? I can't really show you much work that i've done because I can't really tell if i need to find the mass of the block.. I know that friction = 0.4*9.8*mass. since the block has a normal force of mass*9.8 I don't knwo the normal force and I have no idea how to find it. someone give me a clue!