1. The problem statement, all variables and given/known data A block of mass m is at rest on an inclined plane of angle θ with respect to the horizontal. There is friction between the block and the inclined surface (μk and μs). Now, assume that the value of θ is less than the value in part A) (so that the block will remain stationary if no other forces act on it). What is the maximum magnitude of a horizontal Force F that(with respect to ground) can be applied without causing the block to move up the incline? 2. Relevant equations 3. The attempt at a solution I have found a similar thread here: https://www.physicsforums.com/threads/block-wedge-problem.698343/#post-4424430 and here: https://www.physicsforums.com/threa...-friction-min-and-max-forces-involved.619902/ I understand everything except one significant detail. I have to choose between several options: mg(sinθ−μ*cosθ)/(cosθ+μ*sinθ) or mg(sinθ+μ*cosθ)/(cosθ+μ*sinθ) Friction is static. I don't understand the numerator. Why is there -/+μ*sinθ or -/+μ*cosθ?? Could you give me a hint?