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Homework Help: Can someone help me with this static friction problem?

  1. Mar 12, 2012 #1
    1. The problem statement, all variables and given/known data

    A 45-kg crate is placed on an inclined ramp. When the angle the ramp makes with the horizontal is increased to 24 degrees , the crate begins to slide downward. What is the coefficient of static friction between the crate and the ramp?

    2. Relevant equations



    3. The attempt at a solution

    I started by drawing a free body diagram. My weight came out to be -441 N. I have the angle of my weight at 24 degrees. The normal force at mgcos24, for which I got 187N. mgsin24 came out to be 399N which is also the static friction force. Then I just substituted into the static friction equation and my coefficient of static friction came out to be 2.13. I entered this into the homework problem which we do online and it is wrong. Did I miss a step? What did I do wrong?
  2. jcsd
  3. Mar 12, 2012 #2
    Your values for mgcos24 and mgsin24 are incorrect. Your calculator appears to be in radian mode.
  4. Mar 12, 2012 #3
    The applied formula is right.The math is wrong.

    the weight u calculated using m as 450 and g as 9.8 (all in SI units) is right as 441 N.

    As u said normal force is mgcos24.
    and mgsin24 as component of weight parallel to inclined plane.

    However, the calculated values are not right.

    Hint(for angles less than 45 degrees cos x is always greater the sin x.

    However, here your mgcos24 is less than mgsin24)
    Last edited: Mar 12, 2012
  5. Mar 12, 2012 #4
    Wow, thanks a lot Calc class, lol.
    Got the answer, thanks for your help guys
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