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The magnitude of acceleration?

  1. Oct 18, 2012 #1
    1. The problem statement, all variables and given/known data
    A particle moving along the x axis has a position given by
    x = 24t - 2.0t3 m, where t is
    measured in s. What is the magnitude of the acceleration of the particle at the instant
    when its velocity is zero?

    2. Relevant equations



    3. The attempt at a solution

    v = dx/dt = 24 - 6t2 = 0 =>t2 = 4 => t= ± 2

    a = dv/dt = d2x/dt2 = -12t

    => a = -12 (2) = -24 m/s2

    or a = -12(-2) = 24 m/s2

    the right answer is a = 24 m/s2
    but why??
  2. jcsd
  3. Oct 18, 2012 #2


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    Gold Member

    Time cannot be negative so your second solution, although meaningful in a mathematical sense, can be discarded. But this observation not withstanding, the problem asks for the magnitude of the acceleration.
  4. Oct 18, 2012 #3
    Yes, that's it.
    Thank you.
    the magnitude is 24 whether time = 2 or -2.
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