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Homework Help: Force Acceleration Weight

  1. Feb 5, 2007 #1
    1. The problem statement, all variables and given/known data
    A spacecraft descends vertically near the surface of Planet X. An upward thrust of 25.0kN from its engines slows it down at a rate of 1.20m/s^2, but if an upward thrust of only 10.0kN is applied, it speeds it up at a rate of .80m/s^2. Apply Newton's second law to each case, speeding up or slowing down, and use this to find the spacecraft's weight near the surface of Planet X.

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    Okay, I'm confused about how if 25.0kN force is applied, then it slows down but 10.0kN part will speed it up. I guess maybe that's why my method is wrong and I keep getting the wrong answer. This is what I'm doing: since I have the force and acceleration, I converted kN to N and divided by its acceleration to find the mass. Then I take the mass x gravity = weight, converting it back kN. Pls help me, where did I go wrong?
  2. jcsd
  3. Feb 5, 2007 #2
    Another poorly worded problem if thats quoted accurately. Is the right answers 2.13(7500)=15975?
    Last edited: Feb 5, 2007
  4. Feb 5, 2007 #3
    Yeah, I copied it word for word. I don't have the right answer but um, how did you get those numbers?
  5. Feb 5, 2007 #4
    well what the question supposes(and its poorly worded) is "it" is the planets gravitational pull. In one case it retards the spacecrafts upwards acceleration so that the net acceleration is up, and in the second case the gravitational pull overcomes the thrust of the rocket. In each case, try to develop an eqn for the given net a in terms of the rocket thrust and unknown g constant of the planet. Should have 2 eqns, 2 unknowns.
  6. Feb 6, 2007 #5
    Ah, if you didn't explain, I don't think I'd understand the problem...I got it now, thank you very much.
  7. Feb 6, 2007 #6
    My pleasure, I just can't resist a rocket problem!
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