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Homework Help: Transposing V^2=u^2+2as

  1. Dec 16, 2008 #1
    1. The problem statement, all variables and given/known data

    Height reached by a ball if it is thrown vertically upwards with an initial velocity of 40m/s

    2. Relevant equations

    Using V^2=u^2+2as

    3. The attempt at a solutionI have been told to ignore air resistance and let g =9.8 m/s. Any help anyone?
  2. jcsd
  3. Dec 16, 2008 #2


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    Homework Helper

    Well you have the correct equation. Just plug in all the variables and solve for the displacement s.
  4. Dec 16, 2008 #3
    Consider that the balls's velocity of 40 m/s is being sapped away by gravity at a rate of 9.8 m/s/s. How much time (t) will it take for v to be reduced to zero?

    What will the ball's average velocity be over time period t? (Now you can solve for h.)

    (Also: you probably just made a typo but note that g is an acceleration and as such takes the unit "m/s/s", normally written "m/s2". "m/s" is a unit of velocity, not acceleration.).
  5. Dec 16, 2008 #4
    Thank you! Yes it was a typo! I am right that even if this question states initial velocity is 40m/s the actual value of u in this question is zero? Also my transposition of this equation works out to be V^2-u^2 divided by 2a=s?
  6. Dec 16, 2008 #5


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    Yes the final velocity is 0 for that equation.

    So for your equation:

    0 = 402 +2*(-g)*x

    (if +X is up, then g carries a - sign.)

    If time is all you want then you have an easier path with

    |V| = |g|*t

    t = V/g
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