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Direction and Average Acceleration

  1. Sep 7, 2015 #1
    You and your little brother are rolling toy cars back and forth to each other across the floor. He is sitting at x = 0, and you are at x = 4.0 m. You roll a car toward him, giving it an initial speed of 2.2 m/s . It stops just as it reaches him in 3.0 s.

    In what direction is the car's acceleration?
    A. The acceleration is in the +x direction.
    B. The acceleration is in the −x direction.
    I picked B but it was wrong. Can anybody explain it to me?

    What is the car's average acceleration?
    *My attempted answer: -0.28 m/s^2
     
  2. jcsd
  3. Sep 7, 2015 #2

    mfb

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    What is its initial velocity (note: this has a sign), what is its final velocity? Did the number increase or decrease?

    How did you get that number?
    With that acceleration, what is the velocity change in 3 seconds? Can that be right?
     
  4. Sep 7, 2015 #3
    I think its initial velocity is 2.2 m/s which probably has a positive sign. The final velocity would be 0 m/s. Since it moves from 4 m to 0 m, I consider it moves in negative direction.
     
  5. Sep 7, 2015 #4

    mfb

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    If it has a positive sign, the x-coordinate increases. That is not true.
     
  6. Sep 7, 2015 #5
    I found final velocity is 4/3 m/s then delta velocity 4/3 m/s - 2.2 m/s and divide that by 3 s to get the answer.
     
  7. Sep 7, 2015 #6

    mfb

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    The car stops. What is its final velocity?
     
  8. Sep 7, 2015 #7
    0 m/s ?
     
  9. Sep 7, 2015 #8
    So acceleration is simply 2.2 m/s divided by 3s ?
     
  10. Sep 8, 2015 #9

    haruspex

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    Yes.
     
  11. Sep 8, 2015 #10
    Thank you very much!
     
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