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Velocity to acceleration then conversion from km/h to m/s2 confusion

  1. Sep 21, 2013 #1
    A car travels in a straight line for 5.3 h at a constant speed of 60 km/h.
    What is its acceleration? Answer in units of m/s2

    So far, to my understanding so far acceleration is change in velocity over time. So 60 over 5.3, which is 11.32 km/h2

    Then I converted to m first, which is 1132 m/h2, then from h2 to sec, which is .00524 m/sec2.

    But that isn't the answer, what did I do wrong?
     
  2. jcsd
  3. Sep 21, 2013 #2

    arildno

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    "a constant speed of 60 km/h.
    What is its acceleration?"

    What does CONSTANT speed mean, when you are travelling along a straight line? In particular, what is the acceleration then?
     
  4. Sep 21, 2013 #3
    I'm not understanding what your asking. The acceleration is (60-0)/time right?
     
  5. Sep 21, 2013 #4

    arildno

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    What was the INITIAL speed?
    What is the FINAL speed?

    In particular, in those 5.3 hours, does the speed change?
     
  6. Sep 21, 2013 #5

    tiny-tim

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    hi survivorboiii! welcome to pf! :smile:
    hint: what does "change" mean? :wink:
     
  7. Sep 21, 2013 #6
    The car probably started at 0km/h, then speeds up to 60km/h where it stayed for 5.3hr
     
  8. Sep 21, 2013 #7

    arildno

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    "0km/h, then speeds up to 60km/h "
    Really?
    What does the book say about the speed during the time interval we're looking at?
     
  9. Sep 21, 2013 #8
    Enlighten me, I don't have my book with me. I guess I'm still confused about speed, vel, and acc
     
  10. Sep 21, 2013 #9

    arildno

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    What does the word CONSTANT mean?
     
  11. Sep 21, 2013 #10
    constant velocity means acceleration is 0 doesn't it because the velocity isn't changing
     
  12. Sep 21, 2013 #11

    arildno

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    Correct!

    So what was wrong with your two statements:

    1. " The car probably started at 0km/h, then speeds up to 60km/h where it stayed for 5.3hr"

    2. "The acceleration is (60-0)/time right? "

    In particular:
    How should you reframe the equation in 2. ?
     
  13. Sep 21, 2013 #12
    Would it be 0-60 over time?
     
  14. Sep 22, 2013 #13

    arildno

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    Why?
    You said yourself:
    "constant velocity means acceleration is 0 doesn't it because the velocity isn't changing "

    How can the speed change from 0 to 60 if it isn't changing?
     
  15. Sep 22, 2013 #14

    tiny-tim

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    (just got up :zzz:)

    average acceleration = final velocity minus initial velocity divided by time

    instantaneous acceleration (usually just called "acceleration") = final velocity after an extremely short time minus initial velocity divided by that extremely short time = the slope of the velocity-time graph :smile:

    this question is asking for acceleration​

    so draw a graph … what is the slope? :wink:
     
  16. Sep 23, 2013 #15
    IT just hit me...the acceleration is 0 because the vel is constant! :) Thanks both of you!
     
  17. Sep 24, 2013 #16

    arildno

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    :smile:



    PS:
    Remember to give your answer as 0m/s^2, because the exercise specifically asked you to include units
     
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