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Car acceleration problem

  1. Feb 6, 2015 #1
    1. The problem statement, all variables and given/known data
    A car enters the freeway with a speed of 6.4 m/s and accelerates uniformly for 3.2 km in 3.5 minutes. How fast in m/s is the car moving after this time?

    2. Relevant equations
    DeltaX=1/2(vi+vf)(DeltaT)
    Displacement=1/2(initial velocity + final velocity)(Time interval)

    3. The attempt at a solution
    3.2=1/2(6.4+x)(3.5)
    x=-4.6
    ? Why is this negative? What is the final velocity?
     
  2. jcsd
  3. Feb 6, 2015 #2

    Nathanael

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    Homework Helper

    It is important to be aware of which units you are using!
     
  4. Feb 6, 2015 #3
    What do you mean? Do we need to convert the km to m first?
     
  5. Feb 6, 2015 #4

    Nathanael

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    You need to convert everything into the same units, It only makes sense to add/subtract/cancel units which are the same.

    For example, if I'm traveling 50 miles per hour, in 1 second will I have travelled 50*1=50 miles?
    Of course not, because seconds are not the same as hours. (This is essentially what you did wrong.)
     
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