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Homework Help: Test Question

  1. Mar 6, 2008 #1
    1. The problem statement, all variables and given/known data
    Ahh just had a test and i thought i did this question correctly as it turns out though most of the other people in the class got a different value than me so i'm just wondering if someone could show me where i went wrong :D
    Bascially the question is a person drops a TV from a window and then throws a converter at 20m/s down it takes the converter half the amount of time to reach the ground how high is the window

    so i listed the given values for the TV with down as positive:
    v1=0 v2=x a=9.8 d=? t=?
    and the converter:
    v1=20 v2=x a=9.8 d=? t=?

    then i used the TV to find an equation for time since v1=0
    [tex]d=V_{1}t\frac{at^2}{2}[/tex]
    then just rearrange end up with t=[tex]\sqrt{\frac{2d}{a}}[/tex]

    so i sub that into the converter equation get...
    [tex]d=\frac{20\sqrt{\frac{2d}{a}}+1/2a\frac{2d}{a}}{2}[/tex]

    then i subbed my values and got something like 3/4d^2+20.7d=0 (not sure of exact values..)
    d(3/4d+20.7)=0
    20.7(4)/3=d
    27.6=d
    so you get d=0 and d=27.6 so its obviously 27.6 but other people got 33.6 or something where did i go wrong :|
     
  2. jcsd
  3. Mar 6, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Good.

    Redo the substitution more carefully. The converter equation is:
    [tex]d = v_0t + 1/2at^2[/tex]

    You need to substitute:
    [tex]t = (\sqrt{\frac{2d}{a}})/2[/tex]
     
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