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Final velocity

  1. Feb 9, 2010 #1
    just a quick question! will the final velocity of a free falling object always be negative even if the answer is positive?
     
  2. jcsd
  3. Feb 9, 2010 #2

    cepheid

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    Hi, welcome to PF!

    Sign conventions are arbitrary. It's up to you whether you define "up" to be the positive direction, or "down" to be the positive direction.

    That having been said, if you have defined "down" to be negative, and you know that an object is supposed to be moving downward, yet you get a positive answer for its velocity, then your answer is incorrect given the sign convention you chose.
     
  4. Feb 9, 2010 #3

    Doc Al

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    :confused:

    Not sure what you mean by 'final' velocity, but something in freefall can be moving up or down depending upon the exact problem. (Or not moving at all, for an instant.)
     
  5. Feb 9, 2010 #4
    thanks so much! that's what i thought, but then there's this question i was trying out (don't worry, it's not homework. found it online so i could practice):

    A cliff diver from the top of a 100 [m] cliff. He begins his dive by jumping up
    with a velocity of 5 [m/s]. What is his velocity right before he hits the water?

    if down is defined as negative and the equation used is vf^2=vi^2+2ad then the answer should be 44.6m/s but the answer on the site was -44.6m/s.
     
  6. Feb 9, 2010 #5

    Doc Al

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    He's moving down so his velocity should be negative. The site's answer is correct.
     
  7. Feb 9, 2010 #6
    i MUST be doing something wrong.@_@ or i'm inputting something wrong in my calculator. so the square root of 5^2 + 2(-9.8)(-100) is positive?
     
  8. Feb 9, 2010 #7

    Doc Al

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    The calculator only gives you the positive square root of a number. You have to supply the negative sign based on your understanding of how he's moving. (Don't forget that a negative number squared is positive.)
     
  9. Feb 9, 2010 #8
    that's totally the answer to my first question.=D thanks!
     
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