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Homework Help: Free Fall Question [ Need Assistance]

  1. Sep 15, 2009 #1
    1. The problem statement, all variables and given/known data
    It is desired to have a pile driver strike a pile with a speed of 20 m/s. Assuming the driver to be in free fall, from how high above the pile should be released?

    2. Relevant equations
    Used v^(2) = V(o)^(2) + 2ax
    v=final velocity
    V(o) =initial velocity

    3. The attempt at a solution
    I set up my problem as (20 m/s)^(2) = (0m/s)^(2) +2(-9.8 m/s^(2))x finding position (x).
    Next step looks like this 400 m/s^(2) = -19.6 m/s^(2) (x)
    Last step looks like (400 m/s^(2))/(-19.6 m/s^(2)) = x
    Answer: -20.4 = x
    Answer is negative and units are wrong. I believe all the units would cancel out and the answer would be unitless.

    I was also thinking about using the equation x=V(o)T + 1/2at^(2) but problem is I wasn't given a time unless I assume time to be 0.

    Please Help, Thank You.
  2. jcsd
  3. Sep 15, 2009 #2


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    Science Advisor
    Homework Helper

    Your units aren't quite right, v^2 is (m/s)^2 = m^2/s^2
    acceleration is m/s^2, not quite the same thing.

    You are free to pick any direction for your acceleration and distance, as log as they agree. If you want acceleration down to be negative, that means displacements down are negative.

    ps. Extra marks for checking (and noticing) the units ;-)
  4. Sep 15, 2009 #3
    Awesome thanks a bunch! Is there a rating system or something somewhere on this site to rate people's help?
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