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Kinematics, Acceleration on negative side of x axis

  1. Feb 24, 2010 #1
    The question is to find the acceleration of a certain section of a velocity-time graph, state its acceleration, if its speeding up or slowing down, and to state if its going north or south.
    (It states North = Positive)

    I was fine answering all of the questions until it went under the x axis.
    |
    |
    |
    |______________
    |\
    | \
    |
    |
    Something like that, now, because acceleration is a vector, and i include the fact that it is going South. Does the answer need to be a negative.

    I know for a fact that the slope is 12 m/s^2, so do i write the answer as
    12m/s^2 Increasing speed, moving South
    Or as
    - 12m/s^2 increasing speed, moving south

    Because in my mind, - 12 m/s^2 moving south, seems like a double negative, like for position (12 m [N] is the same as - 12 m ).
     
  2. jcsd
  3. Feb 24, 2010 #2

    kuruman

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    Homework Helper
    Gold Member

    Maybe this will help: Regardless of how you define the positive and negative directions, when the acceleration and the velocity point in the same direction the speed will increase and when the acceleration and the velocity point in opposite directions, the speed will decrease.

    In the v vs. t example shown, the velocity is negative (below the time axis) and the acceleration (slope) is also negative. Therefore the speed will increase.
     
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