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Find deceleration, given initial velocity and final distance

  1. Jun 10, 2013 #1
    1. The problem statement, all variables and given/known data
    A car travelling at 88ft/s skids 176ft after its brakes are applied. The deceleration provided by the braking system is constant. What is its value?

    3. The attempt at a solution
    $$
    v_0=88 \\
    v_f=0 \\
    d_0=0 \\
    d_f=176
    $$
    $$
    d(t)=\int88\,dt \\
    d(t)=88t+c \\
    0=88(0)+C \\
    C=0 \\
    176=88t \\
    t=2s
    $$
    $$
    a=\frac{Δv}{Δt} \\
    =\frac{-88}{2} \\
    =-44ft/s
    $$
    The answer key of the textbook says that the deceleration is -22ft/s2. :(
     
  2. jcsd
  3. Jun 10, 2013 #2
    You have assumed motion with constant velocity in finding d(t). You need to use the kinematic relationships that deal with motion with constant acceleration:
    http://hyperphysics.phy-astr.gsu.edu/hbase/acons.html

    Those should give you 2 equations in 2 unknowns.
     
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