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Projectile Question

  1. Sep 21, 2014 #1

    HTT

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    1. The problem statement, all variables and given/known data
    A model rocket has an acc of 5m/s/s while its fuel is buring. Its fuel lasts for 10 sec. ignore the air resistance.


    2. Relevant equations
    a) max height above the ground of this rocket?
    b) the time this rocket in the air before it hits the ground?

    3. The attempt at a solution
    I have no idea about this question, and I'm gonna cry because it's due on tomorrow.... Please help me~
     
  2. jcsd
  3. Sep 21, 2014 #2
    You need to show an attempt. What equation will work for this question?
     
  4. Sep 21, 2014 #3

    HTT

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    Here are the equations I try to use:
    Vx =Vo+a*t
    displacement x = t/2(Vo+V) = Vo*t+(1/2)*a*t^2
    (Vx)^2 = (Vo)^2 + 2a*displacement
     
  5. Sep 21, 2014 #4

    mfb

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    2016 Award

    Staff: Mentor

    Those formulas are all you need. Split the motion into two parts - the time where the fuel is burning and the free fall afterwards.
    What are velocity and displacement after 10 seconds? What happens afterwards?

    You can assume that the rocket starts on the ground, at rest, and accelerates perfectly vertical. A good problem statement would have made this clear, but whatever.
     
  6. Sep 21, 2014 #5

    HTT

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    I'm not really understand this sentence from question, "Its fuel lasts for 10 sec" does it mean the time from the rocket start flying to hitting the ground. Or, other meanings...
     
  7. Sep 22, 2014 #6

    SteamKing

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    Homework Helper

    Assume at t = 0, the rocket is on the ground, and the fuel is ignited. The rocket takes off and flies straight up. At t = 10 s, the fuel is exhausted and the rocket is no longer accelerating due to the burning of the fuel. What happens next?
     
  8. Sep 22, 2014 #7

    HTT

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    Thank you so much, I think I got it~.
     
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