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Is acceleration always positive

  1. Jul 19, 2012 #1

    Acceleration is defined as the change in velocity over the time interval. Is acceleration ever negative, or is it always positive?

    If I were to throw a rock up in the air, is it always accelerating at 10 m/s22 even up until it stops and starts falling back down?

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  3. Jul 20, 2012 #2


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    Yes, that's why it slows down and starts to fall. If there were no gravity, when you threw the rock up, it would keep travelling at a constant velocity.

    With gravity. The velocity would be negative in relation to gravity on its way up, so the acceleration due to gravity would chip away at its' upward velocity, until it reached zero, and then started to fall, and accelerate down.
  4. Jul 20, 2012 #3
    Is acceleration always positive? : no it can be zero.

    Is acceleration always non-negative? : well acceleration is a vector which always has a non negative magnitude. However it may act in one of two directions along its line of action. We denote this by using the positive or negative sign.
  5. Jul 20, 2012 #4
    The following statements apply to one-dimensional motion (so that a sign is sufficient to indicate the direction of a vector).

    The acceleration is positive when the object is moving in the positive direction and speeding up, or when moving in the negative direction and slowing down.

    The acceleration is negative when the object is moving in the positive direction and slowing down, or when moving in the negative direction and speeding up.
  6. Jul 21, 2012 #5


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    This thread made me think of another question. I'm going to post it here because it's closely related, but feel free to move it elsewhere.
    In the simplest possible terms (no math, please), how would one describe the acceleration of a satellite in an elliptical orbit? My instinct is that it's constantly changing, but I'm not sure.
  7. Jul 21, 2012 #6


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    Strictly speaking, acceleration, like velocity, is a vector quantity, with direction as well as magnitude. In the very limited "one dimensional" case, that direction can be interpreted as "positive" or "negative". So the answer to your question is "no, accleration is not always positive. It can be positive or negative". Or course, the magnitude of acceleration, like the magnitude of any vector, is always positive.
  8. Jul 21, 2012 #7
  9. Jul 21, 2012 #8


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    Thank you for the response, Studiot. That's a cool site. I've bookmarked it and suspect that I'll be spending a few idle hours playing around there. :smile:
  10. Jul 21, 2012 #9


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    Dearly Missed

    Wouldn't it, for example be, a=-Constant/r^2, where r is the distance to the focus where the Big Chunk is in place?
  11. Jul 21, 2012 #10


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    I'm not sure; Big Chunk isn't available here any more. That pisses me off, too, because it was my favourite chocolate bar. :grumpy:
  12. Jul 23, 2012 #11
    "Retardation" took me by surprise... lol
    A new word to my arsenal of physics vocabulary!
  13. Jul 23, 2012 #12
    as velocity can be negative, acceleration too

    you throw a rock upwards and its acceleration always points towards the earth vertically, the same direction the gravity is. acceleration is all about the force on it, per newton's laws.

    and the value of acceleration doesn't vary as long as there's no difference between the object and observer. say you can observe it from an elevator that's moving at a constant speed, the acceleration stands as it is despite how fast or slow you move
  14. Jul 23, 2012 #13
    I agree with this opinion, velocity and acceleration is a vector quantity that has direction. so the value of the velocity and acceleration, can be positive and negative depending on the direction.

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  15. Jul 24, 2012 #14
    Next time someone calls you a decelerant, you'll know what he means.
  16. Jul 24, 2012 #15
    Once again the answer is tied to the concept that acceleration is a vector concept possessing both magnitude AND direction. So even if the magnitude is constant, if the direction changes, as it does in circular or elliptical motion the acceleration always changes (provided that no other forces act on the object aside from the original one.)
    E.g. The Earth has a more or less constant speed in its orbit around the Sun but its direction is changing all the time so its acceleration alters, yet the magnitude of the force (and the acceleration) pulling it towards the Sun is more or less constant as the mean Earth-Sun distance doesn't vary greatly.
  17. Jul 25, 2012 #16
    accelleration ever negative?

    Think of accelleration as a vector. Is a vector ever negative?
    Robin Hood once said "I didn't shoot that arrow backwards. I shot it in a different direction!"
    If it seems to be negative it's just really positive but pointed in a different direction.
    But if you're stuck in Line-Land (1 dimensional) then yes, you have positive and negative.
  18. Jul 25, 2012 #17
    I think this is the gist of the whole thread here. On one hand it's an exercise in pedantry, on the other trying to be scientifically/mathematically precise using language which can often be ambiguous. :uhh:
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