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Homework Help: Motion problem for constant acceleration of truck

  1. Sep 20, 2010 #1
    1. The problem statement, all variables and given/known data
    A truck travels at a constant speed of 28.0 m/s in the fast lane of a two-lane highway. It approaches a stationary car stopped a the side of the road. When the truck is still 125 m behind the car, the car pulls out into the slow lane with an acceleration of 2.6 m/s2. How long will it take the truck to reach the car? (Answer is 6.32 s)


    2. Relevant equations
    dt = dc + 125
    dc = 1.3t2


    3. The attempt at a solution
    Okay I got the answer, but it's in quadratic form so:

    1.3t2 - 28.0t + 125 =

    t = 15.2 s or t = 6.32 s

    How do you know which answer is the correct one?
     
  2. jcsd
  3. Sep 20, 2010 #2
    Hello? Please, I need help :frown:
     
  4. Sep 21, 2010 #3
    How did you come up with your equation? You must have started from the basic kinematic equations, right?

    Go back to those and try both values of t to see which one works/makes more sense.
     
  5. Sep 21, 2010 #4
    Think about what this means...

    At the end of t=6.32s, how fast is each vehicle moving (what are their velocities)?
    What is the distance covered by each in this time interval?

    What about at t=15.2s?
     
  6. Sep 21, 2010 #5
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