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Kinematics: Rocket/Free Fall Problem

  1. Feb 13, 2012 #1
    1. The problem statement, all variables and given/known data
    A one stage booster rocket has a Xo of 0m and a Vo of 0 m/s. The rocket takes off with a constant acceleration until 5 seconds when the boosters turn off and goes into free fall. The rocket is set to explode the instant it hits it's max height. The max height is 1200m.

    What is the acceleration for the first 5 seconds?
    What is the Vf for the first 5 seconds?
    What is the total time of flight?


    2. Relevant equations
    Xf=Xo+Vo(t)+(.5)a(t^2)
    Vf=Vo+at
    (Vf^2)=(Vo^2)+2a(Xf-Xo)



    3. The attempt at a solution

    1st Part
    Xo1=0m
    Xf1=Xo2=
    Vo1=0m/s
    Vf1=Vo2=
    a1=
    t1=5s

    Part 2
    Xo2=Xf1=
    Xf2=
    Vo2=Vf1=
    Vf2=0
    a2=-9.8 m/s^2
    t2=

    Also we know: Xf1+Xf2=1200m

    I kept trying to get to an equation where I could boil it down to one variable but I cant seem to do it... I have used a lot of substitution, please help....


    THANK YOU FOR YOUR TIME!
     
  2. jcsd
  3. Feb 13, 2012 #2
    You've got a good start. For part 1 you can simply say vf=a*t. Now the problem is really what I'm sure you've done over and over, just a kinematic problem with an initial velocity and height. Use that "final velocity" as an initial velocity in problem two. Then in problem two you can use the distance kinematic equation, and you can get time from the velocity kinematic equation. So on and so forth. Try it out with some confidence and come back if you're stuck.
     
  4. Feb 14, 2012 #3
    got it. I used a m/s vs t graph and solved for Vf1.... 1200= (5*Vf1)/2 + (Vf1*(Vf1/9.8))/2
     
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