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Homework Help: Seems simple but strange

  1. Mar 11, 2006 #1
    We all know that the formulae for velocity is v=x/t (x=distance) & that acceleration= (v-u)/t
    know as we know x=ut+(at^2)/2 lets replace this value of x in the formulae of velocity & make "a" the subject of furmaulae, What I got was

    a=2(v-u)/t istead of a= (v-u)/t Whats my mistake?????:surprised
  2. jcsd
  3. Mar 11, 2006 #2


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    Science Advisor
    Homework Helper

    Your variables don't mean the same things:

    Average Velocity = Total Displacement / Elapsed Time
    Average Acceleration = (Final Velocity - Initial Velocity) / Elapsed Time
    If acceleration is constant: Total Displacement = (Initial Velocity x Time Elapsed) + 0.5[Constant Acceleration x (Time Elapsed)²]

    When acceleration is constant, the average acceleration is the (constant) acceleration, so it's okay to use a for both. However, you're using v to denote both the average velocity and the final velocity, and this is where you're getting your problems. What you're getting is:

    [tex]a = \frac{2(v_{avg} - v_{initial})}{t}[/tex]

    compared to:

    [tex]a = \frac{v_{final} - v_{initial}}{t}[/tex]

    Note that both are correct when acceleration is constant.
  4. Mar 12, 2006 #3
    Thanks a lot
    The problem is solved
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