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Relationship Between Kinetic Friction and Mechanical Energy

  1. Mar 18, 2012 #1
    1. The problem statement, all variables and given/known data
    Which of the following statements about friction is correct?
    I. Kinetic friction always decreases the mechanical energy of a system.
    II. The static friction force cannot change the mechanical energy of a system.
    III. The force of friction always points in the direction opposite to the way that a system moves.
    None of the statements is correct.
    Statement III only
    Statements I & II only
    Statement II only
    Statements I & III only
    Statements II & III only
    Statement I only.
    All three statements.




    2. Relevant equations
    W = F * D
    Mechanical Energy = KE + Ug


    3. The attempt at a solution

    I cannot find the culprit. All appear to be correct and I cannot form any counter-examples. Kinetic friction is a non-conservative force, so it must decrease the mechanical energy of a system where friction is present. Static friction does not do work, so it must not affect the mechanical energy of a system. For the third; I cannot imagine a system in which friction points in the direction of motion. Kinetic friction is caused by this motion.
     
  2. jcsd
  3. Mar 18, 2012 #2

    cepheid

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    So you think that the answer is "all of the above?"

    So what's the problem? Have you been told that this answer is incorrect?
     
  4. Mar 18, 2012 #3
    Exactly. All of the above was taken as incorrect by the online homework program I am using. I am now stumped as to which one is the odd problem out.
     
  5. Mar 18, 2012 #4

    cepheid

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    Hmm, okay, let's think this through.

    If I push a box across a flat surface at a constant speed, is the mechanical energy of the box changing? Is kinetic friction present?
     
  6. Mar 18, 2012 #5
    There would be a change in kinetic energy but no change in potential energy. When pushing a box across a floor with friction, kinetic energy is lost to the frictional force which runs parallel to the direction of motion?
     
  7. Mar 18, 2012 #6

    cepheid

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    No, pay closer attention to the scenario I am proposing:

     
  8. Mar 18, 2012 #7

    cepheid

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    Actually let's consider a couple of other situations:

    1. Walking
    2. A car moving down a road.

    For situation 2, what direction does the frictional force from the road ON the tires point? What direction does the system move?

    For both situations: is the friction causing the mechanical energy of the system to decrease?
     
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