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Negative acceleration

  1. Oct 6, 2013 #1
    I know that "negative acceleration" can be slowing down, or going backward and speeding up. I read the old posts here and I am still confused.

    If no velocity are given, and you only know that the acceleration is, say, a= -5 m/s^2, how do you know the object is going forward or backward?
  2. jcsd
  3. Oct 6, 2013 #2


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    Staff: Mentor

    Hi sallychan. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif [Broken]

    You don't know anything about the current velocity (or displacement) when all you are told is acceleration.
    Last edited by a moderator: May 6, 2017
  4. Oct 7, 2013 #3


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    What is "positive" direction and what is "negative"?
    That is an arbitrary choice WE make!
    When we use the concept of RETARDATION then that means the acceleration is in opposite direction of the velocity, where "opposite" means along the same line the velocity lies, but in the other direction (that is, 180 degrees shifted from the velocity direction). Retardation always reduces the SPEED of the object.

    If the acceleration is ORTHOGONAL to the velocity, that is with a direction 90 degrees to the direction of the velocity, the acceleration will not change the speed of the object, but will make its trajectory curved, the sign of the acceleration telling us to "which side" the object will curve. We call such acceleration centripetal acceleration.
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