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Homework Help: Finding Time If you know Accel, Vi and distance

  1. Sep 12, 2011 #1
    1. The problem statement, all variables and given/known data
    Some aircraft are capable of accelerations
    of magnitude 42.5 m/s2. If an aircraft starts
    from rest, how long will it take the aircraft
    to travel down the 2.6-km runway?

    2. Relevant equations
    d= vi(t) - 1/2 a(t)2

    I am assuming that you need to solve for t. I had trouble altering the equation to do that.

    3. The attempt at a solution
    since velocity initial is 0m/s, I keep getting divide by zero error, and I get a cube root.
    Last edited: Sep 12, 2011
  2. jcsd
  3. Sep 12, 2011 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to PF student54321,

    Divide by zero error???

    If vi = 0, then vit = 0, and the equation becomes:

    d= 0 + (1/2)at2

    Note that the second term should have a + sign, not a - sign, although typically 'a' also has an intrinsic sign depending on whether the acceleration is positive or negative.

    This becomes:

    d= (1/2)at2

    Now can you solve for t?
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