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Homework Help: Interesting Problem

  1. Sep 19, 2004 #1
    Ok. This one is giving me some trouble:

    A space person is motionless a distance of 500.0 m from the safety of the space craft. The person has exactly 11.32 minutes of air left, and the person's mass is 103.2 kg including the equipment. The person throws a phaser (for all you Trekkies) at a velocity of 50.2 km/h away from the space craft in order to get back. If the mass of the phaser is 5.3 kg, how many minutes of extra air does the person have?

    Heres what i have:
    I found the momentum of the phaser to be 73.67 Kgm/s
    Then the momentum of the person must also be 73.67 because of conservation of momentum.
    Solving for the velocity of the person you get -.7139.... inputing this into v=d/t, i get t= 700.38s.

    I think i may have made an error, or missed something because the question says how many minutes of extra air does the person have, and 11.32 m of air is 679.2 s, which is less than my t. therefore there is no extra air.
    Am i doing something wrong? or is it a trick question?
    Thanks alot for any help
  2. jcsd
  3. Sep 19, 2004 #2

    Looks good to me...
    I think your teacher just WANTS you to think you made a mistake; teachers do that...
  4. Sep 19, 2004 #3


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    I think he has about 0.25 minutes of air left! :-)
    Last edited: Sep 19, 2004
  5. Sep 19, 2004 #4


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    I think teachers want you to think! :smile:
  6. Sep 19, 2004 #5


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    It's hard to tell what error you made, if any, since you didn't tell us how you calculated those numbers. Did you remember to subtract the mass of the phaser from the mass of the spaceman?
  7. Sep 19, 2004 #6
    No, that's what he was doing wrong, judging by his anwers. You get approx. Tide's answer if you include that. Problem solved.
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