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Homework Help: Stop Sign Question

  1. Jan 16, 2012 #1
    1. The problem statement, all variables and given/known data
    When Bob applies the brakes to his car, it slows uniformly from 20.0 m/s to 0.0 m/s in 4.0 s. How many meters before a stop sign must he apply his brakes in order to stop at the sign?


    80 m

    60 m

    40 m

    20 m



    2. Relevant equations

    d=(20+0)/2(4)
    d= 40 m

    3. The attempt at a solution

    Bob needs 40 m to stop his car.

    Would that be correct?
    1. The problem statement, all variables and given/known data



    2. Relevant equations



    3. The attempt at a solution
     
  2. jcsd
  3. Jan 16, 2012 #2

    SammyS

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    Yes.
     
  4. Jan 16, 2012 #3
    Distance = ??
    Time = 4s
    Velocity initial = 20m/s
    Velocity final = 0m/s
    Acceleration = ??

    A = Vf-Vi / T
    A = -20 / 4
    A = -5m/ss

    D = ViT + ATT/2
    D = (20)(4) + (-5)(4)(4) / 2
    D = 80 + -40
    D = 40m

    yes, you're correct
     
  5. Jan 16, 2012 #4

    SammyS

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    @ billybobay,

    It's perfectly fine to work it out the way you did, using average velocity.
     
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