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Homework Help: Simple Kinematics problem

  1. May 12, 2009 #1
    1. The problem statement, all variables and given/known data

    A small steel ball, dropped from rest, takes 0.5s to fall through a vertical distance of 1.25m.
    Determine the actual speed of the ball when it has fallen through a vertical distance of 1.25m, assuming that acceleration is constant.

    2. Relevant equations


    3. The attempt at a solution

    Long ago I was helped by a friend to get the result
    ( 1.25m x 2 ) / 0.5s = 5ms-1

    but I've forgotten how did he come to that conclusion, and the reason behind (1.25mx2), help is much appreciated .
  2. jcsd
  3. May 12, 2009 #2


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    Homework Helper

    The formula [itex]s=vt[/itex] is only valid for constant speed. Does an object fall with constant speed?

    Your friend used the following, correct equations:

    1) [itex]s=\frac{1}{2}at^2+v_0t+x_0[/itex]
    2) [itex]v=at+v_0[/itex]

    These are valid for constant acceleration. You will need to combine both of them to get the answer.
  4. May 12, 2009 #3
    s = t*(u+v)/2
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