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Static friction problem for person to walk up plank

  1. Jun 18, 2012 #1
    1. The problem statement, all variables and given/known data
    hHjgP.png


    2. Relevant equations
    See my work.


    3. The attempt at a solution
    sh0W2.jpg
     
  2. jcsd
  3. Jun 18, 2012 #2
    You used the wrong trig function, you multiply all forces by sin40 except for friction going up, that's when you use cos40 .
     
  4. Jun 18, 2012 #3
    Shouldn't the weights be multiplied by cos(40) because they are vertically downwards, so the moment arm would be the horizontal distance?
     
  5. Jun 18, 2012 #4
    You're right, my mistake.

    The answer I get still doesn't match yours, but I can barely see the text, do you have a clearer picture of the solution?
     
  6. Jun 18, 2012 #5
    Yeah my bad, the quality is horrendous. Here's a way better one:

    hBrCp.jpg
     
  7. Jun 18, 2012 #6
    You used the wrong equation during the final step. Set 165.22s + 70.81 equal to equation 2. That should get you the correct answer.
     
  8. Jun 18, 2012 #7
    Wow that was such a silly mistake haha. Thanks for pointing it out! I got 1.00m as the answer.
     
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