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Simple Acceleration Problem

  1. Sep 8, 2011 #1
    1. The problem statement, all variables and given/known data

    I'd just like someone to see if I answered correctly. I'm having a bit of difficulty grasping the vagueness of the displacement in these questions I've been getting in my textbook. From what I gather the displacement is relative to the vector points, (the two x points in kinematic equations). However, how am I to know what direction the car is travelling relative to the x plane if it doesn't specify the point in a question? I don't know if you guys get me.. The acceleration I found in the question below, could be positive or negative, it doesn't specify enough. Or am I wrong?

    A car slows down from 23 m/s to rest in a distance of 85 m. What was its acceleration, assumed constant?

    2. Relevant equations

    The kinematic equations for constant acceleration.

    I attempted to solve it with the following:

    finalV^2=initialV^2+2a(finalX-initialX)

    3. The attempt at a solution

    Here is an image of my attempted work. I got -3.1 m/s^2 as my answer.


    Physics_problem_solution.jpg
     
    Last edited: Sep 8, 2011
  2. jcsd
  3. Sep 8, 2011 #2

    lewando

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    Gold Member

    Good work by you! :smile:
     
  4. Sep 8, 2011 #3
    Yes! Thanks
     
  5. Sep 8, 2011 #4

    lewando

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    Gold Member

    Sorry, I did not read your question carefully enough.

    V0x = +23m/s (assumedly in the +x direction)
    x0 = 0
    x = 85

    Since the car is decelerating, the acceleration vector will be in the opposite direction of the velocity vector, hence the appropriateness of the "-" in front of 3.1m/s2
     
  6. Sep 8, 2011 #5
    Very helpful, thank you.
     
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