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Friction and Tension Force

  1. Dec 15, 2004 #1
    Question #1) A boy pulls a sled of mass 5.0 kg with a rope that makes an 60.0degree angle with respect to the horizontal surface of a frozen pond. The boy pulls on the rope with a force of 10.0 N; and the sled moves with constant velocity. What is the coefficient of friction between the sled and the ice.

    My answer: 10 * Cos 60 = 5 = Ff
    5 * 9.80 = 49 = Fn
    5 / 49 = .102
    .102 = coefficient of friction (My answer)
    .12 = (Teacher answer)

    Question #2) A rock is suspended from a string; and it accelerates downward. Which one of the following statements concerning the tension in the string is true?
    A) The tension points downward
    B) The tension is less that the weight of the rock
    C) The tension is equal to the weight of the rock.
    D) The tension is greater that the weight of the rock.
    E) The tension is independant of the magnitude of the rock's acceleration.

    The answer is B, and I don't know why. Please help explain.
     
  2. jcsd
  3. Dec 15, 2004 #2

    HallsofIvy

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    The tension in the string is the force pulling up on the rock. The weight of the rock is the force pulling down. You are told that the rock acclerates downward. What does that tell you about the net force?
     
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