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A Simple Kinematics Question

  1. Mar 26, 2008 #1
    Well, I think this one is pretty simple, but still, I don't know how to solve it.

    We all know that for uniform velocity in a straight line the following equation sets the relationship between time and distance traveled: S = So + Vt (Eq. 1)

    When it comes to uniformly accelerated motion, the only difference is that the velocity is changing constantly, according to the following equation: V = Vo + at (Eq. 2)

    Now, if you insert Eq. 2 in Eq. 1 you get: S = So + Vot + at^2 (Eq. 3)

    But we all know the correct equation is S = So + Vot + at^2/2! Besides, the second derivative of Eq. 3 is 2a, when the correct one is, by definition, a, obviously. So it's clearly wrong. What's the deal with this?
  2. jcsd
  3. Mar 26, 2008 #2

    Doc Al

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    Staff: Mentor

    In Eq. 2, V stands for the final speed after some time. To use it in Eq. 1, you'd have to replace V by the average speed, since that equation only applies for constant speed (or average speed). Since the acceleration is uniform, the average speed is just (Vo + Vf)/2 = (Vo + Vo + at)/2 = Vo + at/2. Plug that into Eq. 1 and see what happens.
  4. Mar 26, 2008 #3
    Damn it, that's beautiful. Thank you very much.
  5. Mar 30, 2008 #4
    The reason for the wrong answer is that ;
    v = u + at is equation to find final velocity in constant acceleration, and you are putting this final velocity in 2nd equation (BUT VELOCITY IS CHANGING AT EVERY POINT)


    S = So + v1t1 + v2t2 + ...(where t1 + t2 +...= t)

    so,in general,u MUST USE,

    dS = v.dt
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