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When is g (9.8) negative or positive?

  1. Oct 22, 2013 #1
    when is g (9.8) negative or positive?
    i thought that g would be positive when its going towards the ground, and negative when it is being thrown in the air or against gravity.
     
  2. jcsd
  3. Oct 22, 2013 #2

    Nugatory

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    Staff: Mentor

    You can do it either way. If you drop a rock into a hole ten meters deep, you could say that the rock experienced positive acceleration in the downwards direction and ended up ten meters more deep; or you could say that the rock experienced negative acceleration in the upwards direction and ended up ten meters less high. The math works out the same either way, as long as you're careful to be consistent about whether negative means up or negative means down.
     
  4. Oct 23, 2013 #3

    Dale

    Staff: Mentor

    The acceleration due to gravity is a vector. It has a magnitude of 9.8 m/s², and its direction is downward. Whether or not that is negative or positive depends on your choice of coordinate system. If you had a coordinate system where up is positive then gravity would be negative since its direction is the opposite of your positive direction. If you had a coordinate system where down is positive then gravity would be positive since its direction is the same as your positive direction.

    As long as you are careful and consistent you can get the correct answer either way. I tend to choose a positive=up convention.
     
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