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Very elementaria kinematics question

  1. Jun 19, 2007 #1
    1. The problem statement, all variables and given/known data
    A car starting from rest travels one quarter of a mile in 24.1 sec.
    a) What is the constant acceleration?
    b) What is the final speed?

    2. The attempt at a solution

    The solution is actually given in the book, but before posting that, I want to know why my attempt doesn't give me the same answer.

    Here is what I tried:

    vf = xf/t
    vf = (1/4)/24.1 = 0.01037 mi/s

    a=(0.01037 mi/s)/ 24.1s = 0.000430 mi/s^2

    The way the author found the unkown was using other kinematic equations:

    1) x = Vot + (1/2)(a)(t^2) where Vot = 0. To find the acceleration

    2)Then with that they find "Vf" by Vf= Vo + at

    I've just started to learn physics by myself and I knew it was going to be hard but I wasn't expecting getting stuck in simple problems like this one so please be patient and help me out. Maybe it's just some logic I'm not getting.

    Thank you!
  2. jcsd
  3. Jun 19, 2007 #2
    The equation v = x/t only works with constant velocity. For non constant velocity but constant acceleration, you can use the equation x=v0t + 1/2at^2. The 4 kinematic equations for constant acceleration can be easily derived using calculus.

    x=v0t + 1/2at^2
    v-v0 = 2ax
    a = (v-v0)/t
    v = (x-x0)/t

    The last 2 are for average values of acceleration and velocity
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