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Where and when does the car catch the truck?

  1. Sep 5, 2011 #1
    1. The problem statement, all variables and given/known data

    A truck starts from rest and accelerates at 1 M/S*2 . 4 s later, a car accelerates from rest at the same starting point with an acceleration of 2.7 M/S*2 .

    Where and when does the car catch the truck?
    2. Relevant equations



    3. The attempt at a solution
    i have tried many equations but nothing is working :s could someone please help
     
    Last edited: Sep 5, 2011
  2. jcsd
  3. Sep 5, 2011 #2

    lewando

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    Please clarify what you mean by "acceleration of ."

    Please show what you have done so that help can be provided.
     
  4. Sep 5, 2011 #3
    i clarified!
     
  5. Sep 5, 2011 #4

    lewando

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    Thanks, adoule. So what have you tried so far?
     
  6. Sep 5, 2011 #5
    i have tried s=ut + (1/2)at^2

    In 4 seconds, truck travels a distance of 0 (4) + (1/2) (1) (4)^2 = 0 + 8 = 8

    now, when car starts, truck is already 8 m ahead of it.

    Suppose they meet at time lapse of t seconds after car starts.
    So, car travels 8 m more than the truck in t seconds.
    distance travelled by car in t seconds = dist travelld by truck in t seconds + 8

    0 (t) + (1/2) (2.7) (t)^2 = 0 ( t) + (1/2) (1) (t)^2 + 8

    ==> (t)^2 = (160) / 17
    t = 3.06

    hence, car catches truck at 4 + 3.06 = 7.06 seconds after truck starts.
    distance travelled by car = 0 (7.06) + (1/2) (4) ( 160/17) = 18.8 metres
     
  7. Sep 5, 2011 #6

    lewando

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    I'm with you at this point.
    still with you...
    The RHS is not correct. You need to take into account the velocity of the truck (not zero!)
     
  8. Sep 5, 2011 #7
    ok so i corrected the velocity of the truck it gives me this
    0 (t) + (1/2) (2.7) (t)^2 = 4 ( t) + (1/2) (1) (t)^2 + 8
    and then it gives me an equation like this
    0= 0.85t^2- 4t-8
    and the anwser is 6.2192121 but its still nor correct :s
    and sorru about the other thread :p
     
  9. Sep 5, 2011 #8

    lewando

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    The answer you are getting, 6.2, is right for the equation, but you need to account for the 4s time shift. Need to add 4 s to it (you were trying to solve for a specific time interval τ, which began at t = 4).
     
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