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The rocket problem: please help.

  1. Dec 6, 2013 #1
    1. The problem statement, all variables and given/known data
    Consider a rocket in space that ejects burned fuel at a speed of v_ex= 2.0 km/s with respect to the rocket. The rocket burns 10 % of its mass in 340 s (assume the burn rate is constant).

    (a) What is the speed v of the rocket after a burn time of 170.0 s? (suppose that the rocket starts at rest; and enter your answer in m/s) ?
    (b) What is the instantaneous acceleration of the rocket at time 170.0 s after the start of the engines?(in m/s^2 ) ?

    2. Relevant equations

    v_f = -v_ex*ln(m_final/m_initial) - g*burn time

    3. The attempt at a solution

    I plugged in the data, getting -2000*ln(0.95) - g*170, but got the velocity wrong. I have no idea where I went wrong, and I am stuck here, along with finding the acceleration. Can someone please help me get unstuck here. Thanks and sorry to be a bother.
     
  2. jcsd
  3. Dec 6, 2013 #2
    EDIT : What I wrote here was wrong
     
    Last edited: Dec 6, 2013
  4. Dec 6, 2013 #3
    What is g in space?
     
  5. Dec 6, 2013 #4

    mfb

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    In addition to BOYLANATOR's post, where does the 0.95 come from?
     
  6. Dec 6, 2013 #5
    0.95 comes from the final mass over initial mass, since after 170.0 seconds, you have 95% of the original mass left (other 5% mass was burned already)
     
  7. Dec 6, 2013 #6

    mfb

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    Oh 10% in 340s... okay, I missed that part. Where is the point in the second 170 seconds?
     
  8. Dec 6, 2013 #7
    what you mean?
     
  9. Dec 6, 2013 #8
    Because, 10% of mass burned every 340 seconds, so 5% of mass burned every 170 seconds, therefore I got the 0.95
     
  10. Dec 7, 2013 #9

    mfb

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    I agree, I didn't read the problem statement careful enough.
    I was just wondering why the problem statement gives the fuel burnt in 340 seconds and then just asks for the rocket after 170 seconds.
     
  11. Dec 7, 2013 #10
    Did you manage to solve the problem?
     
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