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Is the Velocity negative or positive?

  1. Feb 5, 2015 #1
    • Member warned about re-posting the same problem statement in new threads. Infraction points awarded.
    1. The problem statement, all variables and given/known data
    If you were to throw a large log over the edge of the Grand Canyon and it took 5.65 seconds to hit the ground, calculate the velocity of the log at impact in m/s and calculate the distance the log fell in feet.



    2. Relevant equations
    vf=vi+at



    3. The attempt at a solution
    So I get vf=(9.8)(5.65)= 55.4 ...But then I think about it, and the object is going downwards so shouldn't it's velocity be negative (South)..? Also, is 9.8 supposed to be positive or negative?
     
  2. jcsd
  3. Feb 5, 2015 #2

    NascentOxygen

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    Staff: Mentor

    You can choose g to be positive or negative, whichever you wish. The final velocity here will take on whatever sign you give to g. The important thing is that you remain consistent! :)
     
  4. Feb 5, 2015 #3
    So there's always two right answers?
     
  5. Feb 5, 2015 #4

    NascentOxygen

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    Staff: Mentor

    Not really. It's final velocity is 55.4 m/s towards the centre of the Earth. That's one answer. :)

    I'd be concerned about your omission of the units when talking of velocity, though.
     
  6. Feb 5, 2015 #5
    Sorry, haha *m/s. One last question, is acceleration the same for all objects?
    Well, it is, due to gravity (9.8m/s^2) ..but then again I get confused because there's so many different word problems with different accelerations as answers, obviously. What would be the final, true answer though?
     
  7. Feb 5, 2015 #6

    NascentOxygen

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    Staff: Mentor

    g is the same for all objects in a particular location. Whether the object is accelerating depends on the net force. Gravity is only one of many forces that may be acting simultaneously on an object.

    The answer to what?
     
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