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Homework Help: Studying for exams need help very soon

  1. Dec 14, 2005 #1
    Studying for exams need help very soon!!!!

    On both of these I got the right answer but it was so long ago I have no idea how to do them.
    (Which equation am I to use?)(v=Vo+at? if so how?)
    1.Determine the stopping distances for an automobile with an initial speed of 91 km/h and human reaction time of 3.0 s for the following accelerations.
    (a) a = -4.0 m/s2
    ? m
    (b) a = -8.0 m/s2
    ? m
    (This one is beyond me I keep attempting to use equations but none of them are working)
    2.5. A football is kicked at ground level with a speed of 20.0 m/s at an angle of 38.0° to the horizontal. How much later does it hit the ground?
    ? seconds
  2. jcsd
  3. Dec 14, 2005 #2


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    a) use a kinematic equation to solve for "d"
    , then use a kinematic equation to solve for "time"
    then add reaction time.

    b) resolve the components (sin & cos)
    the y component give you time
  4. Dec 14, 2005 #3


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    Staff Emeritus
    Science Advisor

    The stopping distance consists of two parts, the distance traveled before the deceleration is applied, which is given by x1 = v tr, where r would be the response time, and then the distance traveled during the period of deceleration, which is governed by a different equation.

    see - http://hyperphysics.phy-astr.gsu.edu/hbase/mot.html#mot1
    for linear motion in one dimension.
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