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Homework Help: Initial velocity when deacelerating at a constant rate

  1. Sep 20, 2009 #1
    1. The problem statement, all variables and given/known data
    You are driving along a straight section of roadway (x-axis). You spot a police officer and apply the brakes, slowing down at a constant rate. Your positions (in meters) at successive time intervals of 0.435 s are tabulated below as function of time. (1) Calculate the initial speed (i.e., the speed at t=0, the time at which the brakes are first applied.). (2) Calculate your acceleration along the x-axis, ax.
    x (m) 7.00 19.73 31.92 43.55
    t (s) 0.000 0.435 0.870 1.305

    2. Relevant equations

    v= (d2-d1)/t
    x= Vo(t) +1/2at^2
    V= Vo + at

    3. The attempt at a solution

    For the initial velocity I tried using the equation x= Vo (t) +1/2 at^2
    the problem says slowing down at a constant rate so I put a= 0, leaving x=vo (t) I subtracted 43.55-7 to get 36.55m over a period of the 1.305s. Dividing 36.55/1.305 i got 28 m/s but the correct one is supposed to be 29.9m/s. What did I do wrong. And for part B of the problem would I just be able to use the initial velocity from part A and the same equation I mentioned above to find a?
  2. jcsd
  3. Sep 21, 2009 #2

    Andrew Mason

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    Science Advisor
    Homework Helper

    Why would a = 0 if the car is slowing down? The rate of change of a may be 0 but not a.

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