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Truck vs Tree (first week physics woes)

  1. Mar 4, 2010 #1
    1. The driver of a truck slams on the brakes when he sees a tree blocking the road. The truck slows down uniformly with an acceleration of -5.60 m/s2 for 4.20s making skid marks 62.4m long that end at the tree. With what speed does the truck strike the tree?



    2. Relevant equations

    I'm guessing that I have to use Vf2-Vo2=2as?

    I'm really not sure... I've been staring at this problem for hours, is It a trick question? Does it even hit the tree?
     
  2. jcsd
  3. Mar 4, 2010 #2
    You need

    [tex]
    x = x_0 + v_0 t + (1/2) a t^2
    [/tex]

    you know everyting except for the initial speed.
     
  4. Mar 4, 2010 #3
    Man i am defenitly going to need to work on knowing which formulas are for which... ummm using that...


    x=vo2+1/2at2



    vo2= x-1/2at2



    So vo = square root x-1/2at2


    square root of 62.4-(-5.6x4.22)/2


    = 10.573

    So he hits the tree at 10.58 m/s?


    EDIT: Wait did I miss out a t? let me try that again...
     
  5. Mar 4, 2010 #4
    x-(1/2)at2/t=vo


    62.4-(1/2)-5.6x4.22
    /4.2

    =

    26.617 m/s



    So thats his initial speed right?

    I still dont understand how to get his final speed?
     
  6. Mar 4, 2010 #5
    Wait V=Vo+at

    so 26.617+(-5.6x4.2) = 3.097

    does he hit the tree at 3.097 m/s?
     
  7. Mar 4, 2010 #6

    cristo

    User Avatar
    Staff Emeritus
    Science Advisor

    Be careful with your rearranging. If

    [tex]x=v_0t+\frac{1}{2}at^2[/tex]

    then

    [tex]v_0=\frac{1}{t}\Big(x-\frac{1}{2}at^2\Big)=\frac{x}{t}-\frac{1}{2}at[/tex]
     
  8. Mar 4, 2010 #7
    I think its just my formating making it look wrong? is 26.617 m/s the right answer?
     
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