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Homework Help: Forces and Newtons Laws

  1. Oct 3, 2005 #1
    Can some one help explain?
    A 20.0kg sled is being pulled across a horizontal surface at a CONSTANT VELOCITY. The pulling force has a magnitude of 80.0 Newtons(N) and is directed at an angle of 30 degreesabove the horizontal. What is the coefficient of kinetic friction?
  2. jcsd
  3. Oct 3, 2005 #2


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    Homework Helper

    Do a FreeBody Diagram and apply Newton's 1st Law.
  4. Oct 3, 2005 #3
    what? can you explain?
  5. Oct 3, 2005 #4


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    Homework Helper

    Do you know Newton's 1st Law?

    Do you know what is a Freebody diagram?
  6. Oct 3, 2005 #5
    Since the object is dragged along the horizontal, the normal reaction will be equal and opposite to the object's weight (W = mg).

    The force is applied at an angle to the horizintal, but the friction will only act across the horizontal, so you need to find the horizontal component of the force (Fx = F cos theta).

    As the object moves at a constant velocity, there are no unbalanced forces acting on the object, so the friction force must be equal in magnitude to the pulling force.

    The friction force of a sliding object is proportional to the coefficient of kinetic friction and the normal reaction, so:

    coefficient = -(F cos theta) / -(mg),

    F being the applied pulling force.
  7. Oct 3, 2005 #6
    still not working

    It says that I should come up with an answer of 0.444 but I keep getting 0.357, What is going wrong?
  8. Oct 3, 2005 #7

    Doc Al

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    Staff: Mentor

    Since the 80N force is applied at an angle, the normal force is not mg. The normal force is reduced by the vertical component of the applied force.
  9. Oct 3, 2005 #8
    just write it out for him

    Just write the equation out i don't think he understands!
  10. Oct 3, 2005 #9
    *^$*! Very true. Sorry, Phun, I forgot that part. You'll need to subtracted F sin theta from -mg. Thanks, Doc Al, for spotting my clumsiness.
  11. Oct 3, 2005 #10
    He does, I think, I just missed off part of the solution and confused him. Entirely my fault, not his.
  12. Oct 3, 2005 #11
    got it

    Thats much better! Thanks guys!
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