Calculating final speed using momentum

  1. calculating final speed using momentum [solved]

    1. A 73.8 kg girl runs at 4 m/s and jumps onto a 3.4 kg stationary skateboard. Ignore friction and calculate the final speed of the girl.

    * DO NOT include units in your answer.
    * Give your answer with one decimal.
    * Answers will be accepted if they are within 0.2 of the correct answer.
    2. I understood that this uses the law of conservation of momentum, but couldn't get past this basic equation without getting it wrong.
    m1v1i + m2v2i = m1v1f + m2v2f

    Assuming object 1 is the girl, and object 2 is the skateboard, v2i would equal 0 (stationary skateboard), v1i would equal 4m/s (girl), and you would plug in the masses (m1=73.8, m2=3.4kg). I'm not sure what to put in for v2f, becuse it doesn't specify in the problem the final velocity of the skateboard. I'm pretty sure I'm missing the big picture.
    Last edited: Jan 20, 2009
  2. jcsd
  3. rock.freak667

    rock.freak667 6,231
    Homework Helper

    The girl and the skateboard move with the same final velocity.
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