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Homework Help: Finding acceleration given

  1. Jun 1, 2016 #1
    1. The problem statement, all variables and given/known data
    he question is:
    A box is sitting on a board. The coefficient of kinetic friction between the box and the board is 0.4. One side of the board is raised so that the board is 36.8699 degrees from horizontal. This is the angle that the box starts sliding.

    There are two parts to the question, I figured out part A (What is the coefficient of static friction between the box and the board?) by setting Mk equal to Tan and got .75

    The part that throws me off about this equation is the last sentence " This is the angle that the box starts sliding"
    I'm pretty sure that if it weren't for that, I would use the equation gsinθ - ugcosθ = a however I tried that and didn't get the right answer
    2. Relevant equations
    mgsinθ - umgcosθ = ma
    gsinθ - ugcosθ = a
    mk=tan theta

    3. The attempt at a solution
    I tried plugging the values into the gsinθ - ugcosθ = a equation, I assumed I could just get rid of the mass variables because they'll all cancel out anyways
    gsinθ - ugcosθ = a = 9.8sin(36.8999 degrees) - .75(9.8)cos(36.8999 degrees)
    the answer I got doesn't my teachers solution of 2.744
    I'm think the part that's throwing me off is "this is the angle that the box starts sliding)
  2. jcsd
  3. Jun 1, 2016 #2


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    Staff: Mentor

    Hi alex_todd, Welcome to Physics Forums!

    Once the box is moving, what type of friction applies?
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