Is acceleration always positive

In summary, acceleration is a vector quantity that has both magnitude and direction. Therefore, it can be positive or negative depending on the direction of the object's motion. In one-dimensional motion, acceleration is positive when the object is speeding up or moving in the opposite direction and slowing down. On the other hand, it is negative when the object is slowing down or moving in the opposite direction and speeding up. It is important to note that even if the magnitude of acceleration remains constant, a change in direction will result in a change in acceleration.
  • #1
AbsoluteZer0
125
1
Hi,

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?

Thanks,
 
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  • #2
AbsoluteZer0 said:
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?

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 traveling 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.
 
  • #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.
 
  • #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.
 
  • #5
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.
 
  • #6
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
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:
 
  • #9
Danger said:
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.

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?
:smile:
 
  • #10
arildno said:
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?
:smile:
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:
 
  • #11
41burhan said:
no acceleration is negative when the velocity is decreasing... known as retardation.

"Retardation" took me by surprise... lol
A new word to my arsenal of physics vocabulary!
 
  • #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
 
  • #13
mikelepore said:
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.

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.

vBulletin Community Forum
 
  • #14
AbsoluteZer0 said:
"Retardation" took me by surprise... lol
A new word to my arsenal of physics vocabulary!

Next time someone calls you a decelerant, you'll know what he means.
 
  • #15
arildno said:
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?
:smile:
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.
 
  • #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.
 
  • #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:
 

1. What is acceleration?

Acceleration is the rate at which an object changes its velocity. It is a vector quantity, meaning it has both magnitude and direction.

2. Is acceleration always positive?

No, acceleration can be positive, negative, or zero. Positive acceleration occurs when an object is speeding up, negative acceleration (also known as deceleration) occurs when an object is slowing down, and zero acceleration occurs when an object's velocity is constant.

3. How is acceleration calculated?

Acceleration can be calculated by dividing the change in velocity by the change in time. The formula for acceleration is a = (vf - vi) / t, where a is acceleration, vf is final velocity, vi is initial velocity, and t is time.

4. What factors affect acceleration?

The factors that affect acceleration include the mass of the object, the force acting on the object, and the direction of the force relative to the object's motion. Inertia, friction, and air resistance can also affect an object's acceleration.

5. Can acceleration be constant?

Yes, acceleration can be constant if the object is moving with constant velocity, either at rest or in a straight line at a constant speed. In this case, the acceleration would be zero.

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