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Static friction and maximum/minimum force

  1. Jan 24, 2015 #1
    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?

     
  2. jcsd
  3. Jan 24, 2015 #2
    Why don't you draw a Free Body Diagram of the block on the incline
     
  4. Jan 25, 2015 #3
    Oh, you are right. I did it. I understand it now. Silly me.
    Many thanks. :)
    Block and wedge.png
     
  5. Jan 25, 2015 #4
    You did it yourself mate! Cheers!
     
  6. Jan 25, 2015 #5
    Could you please explain that Fcos(theta) bit?
     
  7. Jan 25, 2015 #6
    Nevermind, I've understood it now.
     
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