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Homework Help: 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

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

    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...


    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



    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


    User Avatar
    Staff Emeritus
    Science Advisor

    Be careful with your rearranging. If



  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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