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Motion problem Final velocity of a dropped stone

  1. Jul 23, 2013 #1
    Hi everybody

    I got this from a book

    A stone is dropped from a height of 50 m and it falls freely. Calculate the velocity of the stone when it reaches the ground.

    Assuming g = - 9.8 ms2 ,negative as its going downwards .

    Using v2 = u2 + 2a(x - x0)

    v2 = 02 + 2(-9.8)(-50-0)

    I got v = 31.30 ms1

    But the book have an answer of 9.9 ms1

    so whats wrong with my process or the answer of the book is wrong ?

    Thanks
     
  2. jcsd
  3. Jul 23, 2013 #2

    Zondrina

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    Homework Helper

    Careful picking your directions.

    g = 9.8 m/s2 [Down] = -9.8 m/s2 [Up]

    Your answer is correct though, maybe the book had a typo.
     
  4. Jul 23, 2013 #3
    @Zondrina
    I learnt its a free choice i.e.. I can choose my positive and negative direction..
    Yes it make sense that acceleration due to gravity is always downwards so that's its positive direction,but again mathematically I can choose it negative,if I see the motion in a Cartesian coordinate system.Am I wrong ?

    Thanks

    I found a notice,its not a homework so I didn't posted it in homework section.
     
  5. Jul 23, 2013 #4

    berkeman

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    Staff: Mentor

    At the PF, all schoolwork-type questions need to go in the Homework Help (HH) forums. You should also use the HH Template when you post such questions (it is provided for you when you start a new HH thread) -- the Template helps to organize the way you present the question and your work on the solution.

    Here is the text from your Notice:

    And here is a thread that should help you understand why the PF HH rules are the way they are:

    https://www.physicsforums.com/showthread.php?t=373889

    :smile:
     
  6. Jul 23, 2013 #5

    berkeman

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    Staff: Mentor

    Oh, and we ask that you please use very descriptive thread titles, to make it easier for other folks on the forum to decide if they are interested in clicking into your thread. I have added some more descriptive words to your thread title. Cheers! :smile:
     
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