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Homework Statement
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?
Homework Equations
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?