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Help Intro Physic problem dealing with Kinetic

  1. Sep 7, 2009 #1
    1. The problem statement, all variables and given/known data
    A model rocket is launched straight upward with an initial speed of 46.7 m/s. It accelerates with a constant upward acceleration of 2.16 m/s2 until its engines stop at an altitude of 138 m.
    What is the maximum height reached by the rocket?

    How long after lift off does the rocket reach its maximum height?

    How long is the rocket in the air?



    2. Relevant equations
    What I found out from the given information above is that xi=o,xf=138m,vi=0m/s,vf=?,a= 2.16m/s2, and t= is ?.
    I used this equation = vf^2=2a(xf-xi)+vi^2 to find Vf and vf was 52.7 m/s. from this point on I do not know how to find the maximum height or the other two question? Please help thank you very much


    3. The attempt at a solution
     
  2. jcsd
  3. Sep 7, 2009 #2
    From the point where the engine stops, its acceleration becomes -9.8 m/s^2, so treat it as a free fall problem, in which Vi = 52.7 m/s, Xo = 0, Xf = ?, Vf = 0, a = -9.8 m/s^2, solve for Xf, and then add that height to 138, and that would be the maximum height.
     
  4. Sep 7, 2009 #3

    rl.bhat

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    Homework Helper

    In the given problem, the initial velocity vi is not zero. It is 46.7 m/s .
     
  5. Sep 7, 2009 #4
    He probably made a typing error, he got Vf for the first part correct...
     
  6. Sep 7, 2009 #5
    Yes, figure out the time first, using the equation Vf = Vi + at
     
  7. Sep 7, 2009 #6
    I found out t= 5.38s
    So I plug it in the equation: Xf = 52.7*5.38+(1/2)(-9.8)(5.38)^2 = 141.7 m
    Then 138+141.7 = 279.7m but it was not the right answer, am I doing this wrong?
     
  8. Sep 7, 2009 #7
    What does your book say is the right answer?
     
  9. Sep 7, 2009 #8
    It is not the book, it's an online homework
     
  10. Sep 7, 2009 #9
    Perhaps it's a rounding error; does it specify to round to the nearest tenth/hundredth/etc?
     
  11. Sep 7, 2009 #10
    I don't think so the computer is very easy on those if you a couple off the decimal it is fine. thanks for your help
     
  12. Sep 7, 2009 #11
    Another user (malik123) was having the some problem in a similar thread; maybe you guys are on the same online program?
     
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