1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Rocket velocity/displacement problem, HELP

  1. Sep 21, 2009 #1
    1. The problem statement, all variables and given/known data
    A two stage rocket is launched with an average acceleration of +4 m/s/s. After 10 seconds, a second stage is activated and the rocket's acceration is now +6 m/s/s.

    Part A: Find the vertical displacement of stage one of the rocket, before accleration changes to 6.

    Part B: Find the final speed after 10 seconds of motion.

    Part C: The second stage is activated, find the total height the rocket ascends (its highest point) before it starts to travel back to earth.

    Part D: Find the displacement traveled by the second stage only.



    2. Relevant equations
    Vf = Vi + a[tex]\Delta[/tex]t

    [tex]\Delta[/tex]y = Vi[tex]\Delta[/tex]t + 0.5a([tex]\Delta[/tex]t)^2

    (Vf)^2 = (Vi)^2 + 2a[tex]\Delta[/tex]y



    3. The attempt at a solution

    For Part A I calculated displacement with [tex]\Delta[/tex]y = Vi[tex]\Delta[/tex]t + 0.5a([tex]\Delta[/tex]t)^2.
    y was my unknown variable and my Vi was 0, my t 10 seconds, and my a +4.
    I got 200meters.


    For Part B I calculated the first stage's final speed by using the equation Vf = Vi + a[tex]\Delta[/tex]t.

    I had a Vi of 0, an accleration of +4 and a t of 10. My velocity final was +40 m/s.

    As far as part c goes I plugged everything into
    (Vf)^2 = (Vi)^2 + 2a[tex]\Delta[/tex]y
    this time I had a velocity initial as +40 [part b] and i used 0 as my velocity final because i was solving for the peak. However, my negative signs got messed up. Here's where I need help. Am I on the right track??
     
  2. jcsd
  3. Sep 21, 2009 #2
    you need to know how long the second stage burns. When the second stage burns
    a = 6 m/s^-2 and you can compute the final speed and height as you did in part A and B.
    After the second stage stops burning the rocket is in freefall, but it will still move up for some time and you can use [itex] {V_f}^2 = {V_i}^2 + 2a \Delta y [/itex]
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Rocket velocity/displacement problem, HELP
Loading...