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Homework Help: Equations of Motion

  1. Oct 7, 2008 #1
    1. The problem statement, all variables and given/known data
    The "reaction time" of the average automobile driver is about 0.700 . (The reaction time is the interval between the perception of a signal to stop and the application of the brakes.) If an automobile can slow down with an acceleration of 12.0 , compute the total distance covered in coming to a stop after a signal is observed (a) from an initial velocity of 15.0 (in a school zone) and (b) from an initial velocity of 55.0 .

    2. Relevant equations
    Equations of Motion

    3. The attempt at a solution
    v (not) =6.71 m/s
    v (final) = 0 m/s
    t=.700 s
    a= -3.6576 m/s^2
    x (final) = variable
    x (not) = 0

    x(final) = x(not) + v(not)t+(1/2at^2)
    x(final) = 0 + 6.71(.7) +(1/2*-3.6576)(.7)^2
    x(final) = 1.1126 meters

    What did I do wrong?
  2. jcsd
  3. Oct 7, 2008 #2


    User Avatar
    Homework Helper

    Welcome to PF.

    The .7 seconds is the reaction time. All that means is that there is a delay from when the driver observes the signal and when the deceleration begins. Hence to find the Total distance for each case just use the x = .7*V as the distance traveled before application of the brakes, and then use the V2 = 2*a*x to determine the remainder of the stopping distance.
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