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Homework Help: Finding initial and final velocities of projectile

  1. Mar 26, 2012 #1
    1. The problem statement, all variables and given/known data

    I had to calculate initial and final velocities for a rocket launch. Data are as follows:

    horiz displacement = 17.5m
    time = 3.5 s
    launch angle = 40 deg
    angle to apex from 20m = 11 deg
    net vertical displacement = 0

    2. Relevant equations

    I used the following equations:

    d = vit + (a[t^2])/2
    vfy^2 = viy^2 + 2ad

    3. The attempt at a solution
    calculations for initial velocity


    dx = vixt + (aav[t^2])/2
    17.5 m = 3.5s(vix) + (-9.81 m/s^2[3.5s^2])/2
    77.6 m/3.5 s = vix
    22.2 m/s = vix

    0m = viy(3.5s) + (-9.81 m/s^2[3.5s^2])/2
    60.1 m/3.5s = viy
    viy = 17.2 m/s

    then final velocity

    vfy^2 = viy^2 + 2ady
    = (17.2 m/s)^2 + 2 (-9.81m/s^2)(0)
    = -17.2 m/s ?

    I used d = vit -(a[t^2])/2 to calculate this again and got the same answer.

    It doesn't seem right that the final velocity would be the same magnitude as the initial velocity... is this answer correct? It also doesn't make sense that we had to measure the launch angle and angle to the apex if they aren't required to solve the problem

    Also, since horizontal velocity is constant, wouldn't the final horizontal velocity be the same as the initial horizontal velocity?

  2. jcsd
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