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Homework Help: Distance Between Stop Signs

  1. Oct 5, 2007 #1
    A car starts from rest at a stop sign. It accelerates at 4.20m/s^2 for 7.10s, coasts for 2.10s, and then slows down at a rate of 3.10m/s^2 for the next stop sign. How far apart are the stop signs?

    Please help me start this problem. Not sure where to begin. Thanks
     
  2. jcsd
  3. Oct 5, 2007 #2

    Kurdt

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  4. Oct 6, 2007 #3
    1st Interval

    Vf= 0 + 4.20m/s^2(7.10s)
    = 29.82m/s

    29.82m/s x 7.10s
    = 211.72m

    2nd Interval

    29.82m/s x 2.10s
    = 62.622m

    3rd Interval

    Vf = 29.82m/s - 3.10m/s^2(t)
    -29.82m/s = -3.10m/s^2(t)

    9.62s = t

    29.82m/s - 3.10m/s^2(9.62s)
    ??

    Well, do I have the first two intervals correct?

    And I am stuck on the third interval... Does it just equal 29.82m, which means the distance between the two stop signs is 211.722+62.622+29.82= 304.164m

    Any help would be greatly appreciated.
     
  5. Oct 6, 2007 #4

    learningphysics

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    interval 1 is not right...

    Remember this is accelerated motion... d = vt doesn't work here unless by v you mean "average velocity"...

    what displacement formulas do you have for accelerated motion...

    try to apply one here...
     
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