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Homework Help: Kinematics Rocket Problem

  1. Sep 13, 2015 #1
    1. The problem statement, all variables and given/known data
    A 25 kg Rocket generates 400 N of thrust. It is designed to burn its fuel for
    40 seconds. If launched vertically what will its final vertical velocity be when the fuel runs out? How high can it fly? How long until it reaches the ground? Neglect mass of fuel, and air resistance.

    Please check over my work I feel like I may be making a mistake somewhere
    2. Relevant equations
    Kinematics Equations

    3. The attempt at a solution
    First I summed the forces in the Y direction
    ΣFy = Ft - Fg = ma
    = (400N - 245N)/(25kg) = a = 6.2 m/s^2

    Final Vertical Velocity
    Vfy = Voy + at
    Vfy = 6.2 m/s^2 * 40s = 248 m/s

    (Height when fuel runs out)

    Yf = Yo + Voy*t + (1/2)a t^2
    Yf = (1/2)*(6.2 m/s^2) (40)^2 = 4960 meters


    Vfy = Voy + at
    0 = 248 m/s - 9.8m/s^2 t = 25.31 seconds <---- Time of Final Height

    Yf = Yo + Voy*t + (1/2)at^2
    Yf = 4960m + 248 m/s * 25.31 s = 11385.7 meters

    (How Long until it reaches the ground)
    I'm confused how to solve this part....
  2. jcsd
  3. Sep 13, 2015 #2


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    Everything looks good up to here:
    What about the (1/2)at2 term in the equation above?

    Can you specify in more detail what is confusing your here?
  4. Sep 13, 2015 #3
    I forgot to include the negative acceleration due to gravity right?
    i'm not sure how to find the time of max flight....
  5. Sep 13, 2015 #4


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    Not sure what you mean by "max flight". Try to find the time from "burn out" until the rocket hits the ground. During this time the rocket is in free fall.

    {EDIT: Or, you can try to find the time from the point of max height until the rocket hits the ground. This will be easier. You already know the time to reach max height.}
  6. Sep 13, 2015 #5
    Nevermind I figured it out, thank you!
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