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Homework Help: Kinetic Energy of Object Question

  1. Dec 2, 2007 #1
    [SOLVED] Kinetic Energy Question

    1. The problem statement, all variables and given/known data

    An object is dropped off a cliff of height h and is subjected to an average force of air resistance of F. If the object has a mass of m the KE it gains during the fall will be equal to.
    a. mgh
    b. mgh + f
    c. mgh -f
    d. mgh+ fh
    e. mgh - fh

    2. Relevant equations

    3. The attempt at a solution
    What I'm not sure about is does air resistance even factor in KE which would then make it answe A or does it add or subtract to it.
  2. jcsd
  3. Dec 2, 2007 #2


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    Homework Helper
    Gold Member

    The answer is not A. The force of air resistance will cause the object to speed up more slowly than it would without air resistance. In other words, the air resistance does work which removes energy from the object.

    So, using this knowledge and the definition of the work done by a force, what do you think is the correct response?
  4. Dec 2, 2007 #3
    If thats correct it wouldn't it be choice E then?
  5. Dec 2, 2007 #4
    external energy + potential energy1 + kinetic energy 1 = potential energy 2 + kinetic energy 2

    (-f*h) + m*g*h + 0 = 0 + kinetic energy2

    (-f*h) + m*g*h = kinetic energy2

    the 1 represent the ball at the starting position ( at h height) and 2 represents when it hits the floor(at h = 0)

  6. Dec 2, 2007 #5


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    Homework Helper
    Gold Member

    Yep. E is the correct answer.
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