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Homework Help: Linear Momentum Conservation

  1. Oct 14, 2008 #1
    1. The problem statement, all variables and given/known data
    A rocket of total mass 3180 kg is travelling in outer space with a velocity of 115 m/s toward the sun. It wishes to alter its course by 35.0 degrees, and can do this by firing its rockets briefly in a direction perpendicular to its original motion. If the rocket gases are expelled at a speed of 1750 m/s, how much mass must be expelled?
    Answer is 140kg

    2. Relevant equations


    3. The attempt at a solution
    http://uploader.neoextreme.com/files/1274/haba/physicsproblem13ch7.jpg [Broken]

    First off, I want to know if that picture is correct, otherwise i'm heading in the wrong direction with this.

    of course, momentum is conserved, so whatever is on one side has to be on the other. The rocket has an initial (r for rocket (mri*vri))
    then the gases expelled have a certain (g for gas NOT gravity(mg * vg))
    and the rocket will have a new (mrf * vrf)

    mri*vri = mg*vg + mrf*vrf

    The vectors in the picture have to be spilt up into their x and y components

    I seem to be going wrong here. I don't get a correct answer
    Last edited by a moderator: May 3, 2017
  2. jcsd
  3. Oct 15, 2008 #2


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    Science Advisor
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    Hi pinkerpikachu! :smile:
    Yes! :smile:
    Yes … and the x component isn't changing, so you can forget that. :wink:

    But I can't really follow what you've done :redface:

    can you write it out in full for us, with the numbers?

    (and you are remembering that the mass of the gas equals the mass lost by the rocket, aren't you? :wink:)
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