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?
The Attempt at a Solution
I have found a similar thread here:
and here: https://www.physicsforums.com/threads/box-on-an-incline-with-friction-min-and-max-forces-involved.619902/
I understand everything except one significant detail. I have to choose between several options:
Friction is static. I don't understand the numerator. Why is there -/+μ*sinθ or -/+μ*cosθ?? Could you give me a hint?