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Help on Physics homework

  1. Mar 18, 2008 #1
    A rocket is fired in deep space, where gravity is negligible
    If the rocket has an initial mass of 6000kg and ejects gas at a relative velocity of magnitude 2000m/s , how much gas must it eject in the first second to have an initial acceleration of 25.0m/s^2 ?

    im stuck on this problem, can anyone help me on it thanks!
     
  2. jcsd
  3. Mar 18, 2008 #2
  4. Mar 18, 2008 #3
    You can figure out the total force using F=ma

    Then you can use the impulse-momentum theorem to calculate the amount of fuel needed to be ejected in the first second.
     
  5. Mar 18, 2008 #4
    the mass loss of the rocket is big enough to be significant, given the fact that the
    acceleration was given with 3 significant digits.
    Given this, it think "how much gas must it eject in the first second to have an initial acceleration of 25.0m/s^2" is ambiguous.
    I think "how much gas must it eject in the first second to have a constant acceleration of
    25.0m/s^2 during this time" is the most reasonable interpretation.
    You will need the rocket equation v = v_ex ln(m_0/m) that you gave in your other post for this. you know v, v_ex and m_0 (initial mass of the spaceship).
     
  6. Mar 18, 2008 #5
    sorry, this is my first time using this website, so i don't know all the rules, but it's is extremely helpful
     
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