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Acceleration of ball due to gravity

  1. Aug 27, 2015 #1
    1. The problem statement, all variables and given/known data
    So I know this is a super basic question, but my teacher and I have been going back and forth on this. If a ball is thrown up into the air, with no other forces acting on it, what is its acceleration? Isn't it - 9.8 m/s^2 because it is acting against the force of gravity and slowing down, so it should be NEGATIVE 9.8 m/s^2 right? My teacher keeps saying that if a ball is thrown into the air, the acceleration is 9.8 m/s^2 , and if a ball is dropped then it is NEGATIVE 9.8 m/s^2. So which one is it?
    2. Relevant equations


    3. The attempt at a solution
     
  2. jcsd
  3. Aug 27, 2015 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Acceleration is a vector. It has a magnitude and a direction. Start by defining the coordinate axes, and then just use the fact that the acceleration due to gravity points toward the center of the Earth.

    So if you define your coordinate axes in the traditional way for problems of projectile motion, the Z axis points up, so the acceleration due to gravity is 9.8m/s^2 pointing in the negative Z direction. In your equations you would write that as -9.8m/s^2.

    Hope that helps. :smile:
     
  4. Aug 27, 2015 #3

    jbriggs444

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    Science Advisor

    Your teacher is wrong.

    The sign on acceleration depends on the chosen coordinate system. That choice need have nothing whatsoever to do with the motion of the ball. To be charitable, your teacher may be operating on the assumption that the proper coordinate system to use is one in which the ball's current velocity (in the case of a thrown ball) or its future velocity (in the case of a dropped ball) are positive. But that is an arbitrary choice, a biased choice and not the only possible choice.

    More reasonable is to pick a single coordinate system [as you have] where positive accelerations are upward and negative accelerations are downward.
     
  5. Aug 27, 2015 #4


    I appreciate your help, I'm not looking too into it though. So the teacher is wrong for saying that the dropping balls acceleration is negative 9.8 because it is actually going with the force of gravity?
     
  6. Aug 27, 2015 #5

    jbriggs444

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    As Berkeman points out, first you need to choose a coordinate system. One would normally pick a coordinate system fixed to the Earth, not one fixed to the ball.
     
  7. Aug 27, 2015 #6

    mfb

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    2016 Award

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    The sign depends purely on the coordinate system. There is no "right" and "wrong" sign unless you fix the coordinate system.
     
  8. Aug 27, 2015 #7
    What do you mean by coordinate system? I have been in my physics class for 2 days only...
     
  9. Aug 27, 2015 #8

    berkeman

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