1. The problem statement, all variables and given/known data The coefficient of static friction between a block and a horizontal floor is 0.40, while the coefficient of kinetic friction is 0.15. The mass of the block is 5.0 kg. If a horizontal force is slowly increased until it is barely enough to make the block start moving, what is the net force on the block the instant that it starts to slide? 2. Relevant equations Fx= F-coefficient of static friction*m*g=0 3. The attempt at a solution F= 0.4*5.0 kg*9.8m/s^2= 19.6N However the answer on the key is 12N???? I am not sure how to get to 12N? Please help... My thought was, when the block start moving, I have to use static coefficient. Your help is greatly appreciated!