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Homework Help: Thrust problem

  1. Sep 27, 2010 #1
    1. The problem statement, all variables and given/known data
    The first stage of a Saturn V space vehicle
    consumes fuel at the rate of 47400 kg/s, with
    an exhaust speed of 2820 m/s.
    The acceleration of gravity is 9.8 m/s2 .
    Calculate the thrust produced by these en-
    gines.
    Answer in units of N.

    (part 2 of 2) 10.0 points
    Note: You must include the force of gravity.
    Find the initial acceleration of the vehicle
    on the launch pad if its initial mass is 1.1 ×
    107 kg.
    Answer in units of m/s2.

    2. Relevant equations

    vrel*r= ma = thrust

    3. The attempt at a solution

    For the first part, I got 133668000 N, which is right. For the second part I divided the answer to the first part by the given mass. I got 12.51 m/s^2. Then to account for the acceleration of gravity, I subtracted 9.81 m/s^2 and got 2.3 m/s^2. However, that's not the correct answer. What am I doing wrong here? Can I not substract accelerations like that? It seems to me, I should be able to-- They're vector quantities acting in opposite directions.
     
  2. jcsd
  3. Sep 27, 2010 #2

    rock.freak667

    User Avatar
    Homework Helper

    You need to formulate the expression using Newton's 2nd Law. You'd need to divide by the mass of the fuel to get its acceleration.

    ma=Thrust-mg, find 'a'.
     
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