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Acceleration, screwed up vector?

  1. Sep 16, 2006 #1
    acceleration: screwed up vector??

    I need things cleared up when it comes to acceleration.
    Is acceleration negative when it points in a certain direction...or when an object is slowing down?
    I was working on a free falling bodies word problem:
    A diver springs upward with an initial speed of 1.8 m/s from a 3.0-m board.
    (a) find the velocity with which he strikes the water.[Hint: when the diver reaches the water, his displacement is y=-3.0 meters (measured from the board), assuming that the downward direction is chosen as the negative direction.]
    (b) What is the highest point he reaches above the water?
    I can get the final velocity from part (a) alright,
    To find the distance he jumps up, if I use a positive acceleration in the equation:
    then I get a negative distance...which I shouldn't if hes traveling UPWARD as he jumps.
    I use the same equation to get the final velocity as he hits the water:
    and I used both a negative displacement vector(-3.17m) and a negative acceleration vector(-9.8 m/s^2) and got + or - 7.9 m/s as the answer(I used the minus one, and it was the right answer)
    but my question is: when is acceleration negative?
    Thanks :smile:
    Last edited: Sep 16, 2006
  2. jcsd
  3. Sep 16, 2006 #2
    Actually nevermind!
    I did some research and found out that if an object is going in the positive direction and Deccelerating, then the sign of the acceleration is opposite of the direction.
    I think that's right?
  4. Sep 17, 2006 #3


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    Homework Helper

    The answer is actually right there in your post. You choose a coordinate system. The direction of the kinematic quantities are then either positive or negative with respect to the chosen coordinate system.

    It is true though that a deceleration means that the acceleration of the object is in the opposite direction of the motion of the object though. So if the object is moving in the negative direction (its velocity then being negative, but its position can be positive or negative), then its acceleration (or deceleration) will then be positive.

    Hope I have'nt confused you even more! If so keep the thread going!
  5. Sep 17, 2006 #4
    Yeah, I got through my physics h.w. much easier after having that cleared up.
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