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Homework Help: How To Know When To Use Which Motion Equation

  1. Jan 26, 2012 #1
    1. The problem statement, all variables and given/known data
    I never know when to use either Distance = Velocity x Time or when to use Distance = v1t + 1/2at^2. Please help me and explain when i have to use which
  2. jcsd
  3. Jan 26, 2012 #2


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    Staff: Mentor

    The second equation adds in the last term to account for the effect of a constant acceleration "a". If there is no acceleration, there is no change in velocity. If there is a constant acceleration (like from gravity), you need that extra term.
  4. Jan 26, 2012 #3


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    Science Advisor
    Homework Helper

    hi justinh8! :smile:

    they're the same …

    if you put a = 0 into s = v1t + 1/2at2,

    you get s = v1t (which is really s= vt, since v is constant if a = 0) :wink:

    so you can always use the 2nd one, and you can use the first one if the speed is constant (ie if the acceleration is 0) :smile:
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