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Homework Help: Kinematics question - which equation to use?

  1. Nov 21, 2007 #1
    A ball is thrown upward with velocity 30 ft/s on the roof of a 50 ft building - find the total time until the object meets the ground.

    Heres my problem - I used the equation
    x = (xsub0) + (vsub0)t + 1/2(a)t^2
    I set xsub0 = 50, vsub0 = 30, and a = -9.8
    I solved for t when x = 0, but the book gave a totally different answer.

    Then I went back and used the equation given in the book:
    [tex]\Delta[/tex]x = (vsubnot)[tex]\Delta[/tex]t + 1/2(a)([tex]\Delta[/tex]t)^2

    I solved for [tex]\Delta[/tex]t when [tex]\Delta[/tex]x = 50, and my answer agreed
    with the textbook solution...

    My question is, why does the 2nd equation work in this situation, and how do I know when it is appropriate to use which?
  2. jcsd
  3. Nov 21, 2007 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Those equations are equivalent. Used properly, you get the same answer for each. (If you got different answers, show exactly what you did.) Note that with the usual sign convention (up is +), [itex]\Delta x[/itex] should be -50.

    Also: what are your units? If distance is in feet, then acceleration should be 32 ft/s^2, not 9.8 m/s^2.
  4. Nov 21, 2007 #3
    i think that was my error :blushing:
    thank you very much - it works out now
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