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Homework Help: Help with frictional problems

  1. Feb 3, 2010 #1
    Some sliding rocks approach the base of a hill with a speed of 12 m/s. The hill rises at 36 degrees above the horizontal and has coefficients of kinetic and static friction of .45 and .65, respectively, with these rocks. Start each part of your solution to this problem with a free-body diagram. (a) Find the acceleration of the rocks as they slide up the hill. (b) Once a rock reaches the highest point, will it stay there or slide downhill? If it stays there, show hy. If it slides down, find its acceleration on the way down.



    2. Relevant equationsa(x)=g(sin(degrees)-(kinetic coefficient)(cos degrees)) ??????



    3. The attempt at a solutionacceleration= 9.8 (.22)
    =2.19 m/s^2

    Not sure, how to do the second part, or whether my answer to the first part is right. Thanks for any help.
     
  2. jcsd
  3. Feb 3, 2010 #2
    Anyone?
     
  4. Feb 3, 2010 #3

    Redbelly98

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    The rocks are sliding in the uphill direction. Look at your free-body diagram (I am assuming you drew one, if not then please do draw one :smile:) and answer this: do the two force components, mg·sinθ and the friction force, act in the same direction or in opposite directions? I.e., do they point uphill or downhill, or does one point uphill while the other points downhill?

    Would this change the equation you have written above?
     
  5. Feb 4, 2010 #4
    Well, for something going downhill, that's the equation, so I thought it would work the same way going uphill, no?
     
  6. Feb 4, 2010 #5

    Redbelly98

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    No. Draw the free-body diagram. Think carefully about the direction of the friction force.
     
  7. Feb 4, 2010 #6
    Is it a(x)=-g(u(k)cos(degrees)+sin(degrees))?
     
  8. Feb 6, 2010 #7
  9. Feb 6, 2010 #8

    Redbelly98

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    Yes.
     
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