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Potential energy and conservation of energy problem

  1. Nov 10, 2013 #1
    1. The problem statement, all variables and given/known data

    An outfielder throws a baseball with an initial speed of 79.0 mi/h. Just before an infielder catches the ball at the same level, its speed is 65 ft/s. How much of the ball's mechanical energy is dissipated by air drag? (The weight of a baseball is 9 oz.)

    2. Relevant equations

    ke=1/2mv2
    ug=mgh
    Emec = ΔU + ΔK

    ANSWER IS: 80.4 ft*lb

    3. The attempt at a solution
    Converting knowns to metres/second:
    79.0 mi/h (0.44704) = 35.32 m/s
    65 ft/s (0.3048) = 19.81 m/s

    When they say "weight" do they mean
    fg=9 oz. = 2.5 N
    or
    mass= 9 oz. = 0.255 kg

    ΔKE=1/2mvf2-1/2mvi2
    =1/2(0.255)(19.81)2-1/2(0.255)(35.32)2
    =50.0356-159.0565
    =-109.02 J

    This is where I wasn't sure how to continue..
    Any help is appreciated, thanks!
     
  2. jcsd
  3. Nov 10, 2013 #2

    tiny-tim

    User Avatar
    Science Advisor
    Homework Helper

    hi magnolia1! :wink:
    in the ft-lb system, the pound (and the ounce) is a force

    so "weight" is used instead of mass (and has the g already in it)

    (the unit of mass is the slug)

    anyway, why have you converted to metres etc when the required answer is in ft-lbs ?? :confused:

    start again :smile:
     
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