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Quote by Arby911
Seems to me when the object leaves the starting point it's accelerating at -9.8 m/s^2 (or as the rest of us call that, decelerating). It then reaches apogee where it's at 0 m/s, and begins on its downward path at an acceleration of +9.8 m/s^2

Acceleration is a vector quantity( meaning it can be broken down into componants like left and right, or up and down, in other words, you can describe any 2 dimensional motion in terms of so far to the left/right and so far up/down.)

In physical terms, a change in sign means a change in direction. If the acceleration changed on the trip back down that would imply the now gravity is pulling you up instead of down.
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Quote by J-Dawg158
Acceleration is a vector quantity( meaning it can be broken down into componants like left and right, or up and down, in other words, you can describe any 2 dimensional motion in terms of so far to the left/right and so far up/down.)

In physical terms, a change in sign means a change in direction. If the acceleration changed on the trip back down that would imply the now gravity is pulling you up instead of down.

Would it be correct to say that what I was describing above was the effect of acceleration, but not acceleration itself, and thus my error, or am I still not seeing it?
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Quote by Arby911
Would it be correct to say that what I was describing above was the effect of acceleration, but not acceleration itself, and thus my error, or am I still not seeing it?
Well, you were decently correct though. After it reaches 0 m/s it will accelerate downwards at 9.81 m/s^2. But since it is going in the -y direction it will accelerate at -9.81 m/s^2.
Quote by Arby911
Would it be correct to say that what I was describing above was the effect of acceleration, but not acceleration itself, and thus my error, or am I still not seeing it?

If you refer to it's effect on velocity, sure, but it's a little cumbersome. Essientially what you're trying to say is when velocity is positive and acceleration is negative then you decelerate (velocity is decreasing due to opposite directions), but when velocity and acceleration are both negative, it accelerates (velocity is increasing, but in negative direction.)

It's that whole rule of a negative times a negative equals a positive that causes this, but when it comes to physics if you don't pay attention to the direction(signs) then you open yourself up to many careless errors like inadvertantly adding two things that should have been subtracted.
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Quote by J-Dawg158
If you refer to it's effect on velocity, sure, but it's a little cumbersome. Essientially what you're trying to say is when velocity is positive and acceleration is negative then you decelerate (velocity is decreasing due to opposite directions), but when velocity and acceleration are both negative, it accelerates (velocity is increasing, but in negative direction.)

It's that whole rule of a negative times a negative equals a positive that causes this, but when it comes to physics if you don't pay attention to the direction(signs) then you open yourself up to many careless errors like inadvertantly adding two things that should have been subtracted.

Quote by ChaosInside
Well, you were decently correct though. After it reaches 0 m/s it will accelerate downwards at 9.81 m/s^2. But since it is going in the -y direction it will accelerate at -9.81 m/s^2.

Great, thanks.

I've actually learned something useful in the Pit...not sure if I'm amazed or disappointed...
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Quote by J-Dawg158

I wonder if that 'The More You Know' campaign is the longest running PSA?

I know it's been around for a very long time, probably longer than most of the residents here...
“Ignorance more frequently begets confidence than does knowledge.”
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Perhaps a better way to think about it is in terms of direction rather than positive and negative. Rather than thinking of negative acceleration as 'slowing down', we think of it as 'accelerating downwards'. Going by this definition, if you're traveling upwards but accelerating downwards, then yes, your speed is going to decrease, but once you turn around, you're still accelerating downwards, so you speed up.

The speed graph I drew earlier reflects this, with the speed going down and then back up as it changes direction. The velocity graph actually shows the exact same thing, only that, after t1, it's going higher in the negative direction. I suppose the final idea is that the negative sign is an indicator of direction rather than actually meaning 'less than 0'.
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Quote by Colgate Total
Velocity is the speed; acceleration is the rate at which is speeds up or down.

not QUITE right. Velocity is speed in a direction and acceleration is the rate at which velocity changes. This is why something going in a circle is always accelerating even it's moving at the same speed.

OP: Acceleration.
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It's rather astonishing how long this thread for a rather basic physics-question has been going...
Quote by oneblackened

OP: Acceleration.

You sure? Seems several folks above disagree?
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Quote by magnus_maximus
It's velocity, for crying out loud.

I'm a third year physics student, listen to me.

I'm not arguing the point, I'm asking oneblackened if he/she/it is sure?

And I'm a left-handed insomniac twice removed on my mother's side...unprovable credentials mean shit on the internet...
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Forgive the red marker, I'm all out of other colors.

Anyway, as you can see from the graphs, Velocity is the only quantity whose sign changes. I avoided using dx/dt and dv/dt for velocity and acceleration respectively since I doubt your Physics class is Calculus-based, but for those of you who do know Calc I, derivatives are what I was approximating with my Δy/Δt and Δv/Δt.

I also marked the time t1 at which the object reaches its highest point so you could see how different quantities are changing (or not changing) at that time.

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Just leave the feckin ball where it is and eat something.
Last edited by kertets at Oct 5, 2012,
Only velocity...

position changes within the range of 0 and max distance (which is postivie)

velocity starts out upward, shrinks, becomes null at max height, and augments negatively (downard vector)

acceleration is always downward, by which I mean gravity, so contrary to the starting motion and negative at all times...

Was an expert really needed for this?
Quote by Metallica1554
The answer is velocity because it is a vector quantity, hence speed and direction are taken into account. The direction that the ball is travelling in changes throughout the flight, so on the way back down the velocity becomes negative.

well done well done.
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^ acceleration is also a vector quantity, it's just unchanged in this case (assuming there's no wind resistance)

edit: even wind resistance would not invert the accel. sign, just diminish or increase it's value a bit
Last edited by kalnoky7 at Oct 5, 2012,