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Homework Help: A simple momentum question

  1. Apr 17, 2012 #1
    A 60kg person is holding two 10kg blocks and she throws them one at a time at 5m/s.
    What is her final speed?

    What I get is,
    after she throws the first rock 0=70v-50 so her speed is 5/7 m/s,
    and after she throws the second 5/7 *70=60v-50 so her final speed is 5/3 m/s (1.67 m/s)

    But the answer says 1.55 m/s which is less than the speed she would gain if she throws the two rocks simultaneously (5/3 m/s).
    What am I doing wrong?
  2. jcsd
  3. Apr 17, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    The speed of each thrown block is 5 m/s with respect to the person. As the person's speed changes, so does the speed of a thrown block with respect to the ground.
  4. Apr 17, 2012 #3
    Ofcourse... thank you!
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