Negative acceleration Watch

JRGC
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Okay so I understand that acceleration is negative if its in the opposite direction to motion - i.e. when something is thrown upwards.

But say you throw a ball at angle x to the horizontal, before it reaches its max vertical displacement the acceleration will be constant at -9.81 (we're assuming no air resistance). But then, on its way down to the ground won't the acceleration be positive again?

I'm asking because if i'm using the constant acceleration equations to work out values for the whole flight, it says to take acceleration as negative... but is it not positive for half the flight. So why would I take it as being negative?

I'm sorry if this is a really stupid and trivial question, but its really bugging me. Also, I hope that all made sense! :P

Thanks
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Mark12345680
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That's why you need to assume at the beginning that which direction you are taking as positive/negative
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JRGC
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(Original post by Mark12345680)
That's why you need to assume at the beginning that which direction you are taking as positive/negative
If I do that, the ball is still moving in two different directions during its 'flight'. So why would I take negative acceleration for the whole flight?

I'm probably being really stupid....
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ilovedubstep
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if you assume that up is positive then acceleration is always down so is always negative.
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Stonebridge
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(Original post by JRGC)
Okay so I understand that acceleration is negative if its in the opposite direction to motion - i.e. when something is thrown upwards.
This isn't quite always true.
The sign depends on the direction. But...
If you decide that upwards is the positive direction then the object thrown upwards has a positive velocity (initially) and a negative acceleration.
The acceleration is always in the direction of the resultant force and that is also downwards. All the time. a=F/m and F is downwards and negative.
On the downwards journey the acceleration is still negative. The force doesn't change.
The velocity changes to downwards so it is now negative.
So on the downwards journey both the velocity and the acceleration are negative.
A negative velocity with a negative acceleration means the objects gets faster in the sense that its velocity gets ever more negative.
It's all a consequence of which direction you take as positive.
If you had taken down as positive then the acceleration is always positive. (The force is always positive/downwards)
The initial velocity is negative and on the downwards journey it is positive.
So on the way down the positive velocity becomes more positive under the influence of a positive force and acceleration.

Edit
Here's a summary

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JRGC
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(Original post by Stonebridge)
This isn't quite always true.
The sign depends on the direction. But...
If you decide that upwards is the positive direction then the object thrown upwards has a positive velocity (initially) and a negative acceleration.
The acceleration is always in the direction of the resultant force and that is also downwards. All the time. a=F/m and F is downwards and negative.
On the downwards journey the acceleration is still negative. The force doesn't change.
The velocity changes to downwards so it is now negative.
So on the downwards journey both the velocity and the acceleration are negative.
A negative velocity with a negative acceleration means the objects gets faster in the sense that its velocity gets ever more negative.
It's all a consequence of which direction you take as positive.
If you had taken down as positive then the acceleration is always positive. (The force is always positive/downwards)
The initial velocity is negative and on the downwards journey it is positive.
So on the way down the positive velocity becomes more positive under the influence of a positive force and acceleration.

Edit
Here's a summary

So is the resultant force downwards throughout the whole flight?
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Stonebridge
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(Original post by JRGC)
So is the resultant force downwards throughout the whole flight?
Yes. The gravitational force, and hence the acceleration it produces, is always downwards in direction.
And if you choose down to be positive, the acceleration is always positive.
If you choose down to be negative, the acceleration is always negative.
But remember, negative acceleration does not always mean an object is getting slower.
See my diagram in the previous post. (4th example)

Very often when you do questions on projectiles, you just choose the direction that is most convenient to be positive.
You just need to keep strictly to the direction rules when you plug values into the SUVAT equations. If up is + then down is -. This applies to velocity u,v, acceleration a, and distance/displacement s.
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JRGC
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(Original post by Stonebridge)
Yes. The gravitational force, and hence the acceleration it produces, is always downwards in direction.
And if you choose down to be positive, the acceleration is always positive.
If you choose down to be negative, the acceleration is always negative.
But remember, negative acceleration does not always mean an object is getting slower.
See my diagram in the previous post. (4th example)

Very often when you do questions on projectiles, you just choose the direction that is most convenient to be positive.
You just need to keep strictly to the direction rules when you plug values into the SUVAT equations. If up is + then down is -. This applies to velocity u,v, acceleration a, and distance/displacement s.
Ah yes, if the resultant force is downwards the whole time, everything makes sense.

Thankyou so much!

(I tried to rep you, but it said I needed to rep someone else first.... I think you helped me out on a question before!)
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Lyndon1504
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(Original post by JRGC)
Ah yes, if the resultant force is downwards the whole time, everything makes sense.

Thankyou so much!

(I tried to rep you, but it said I needed to rep someone else first.... I think you helped me out on a question before!)
I repped Stonebridge for your, he deserves it he's always so helpful!
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RK92
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(Original post by Lyndon1504)
I repped Stonebridge for your, he deserves it he's always so helpful!
i did too, i was just thinking that hed explained it way better than i could have hoped to!
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Lyndon1504
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(Original post by RK92)
i did too, i was just thinking that hed explained it way better than i could have hoped to!
Agreed, I see the title of the thread and think I can help, but then I read Stonebridge had already replied and explained it far better than I ever could!
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v-zero
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(Original post by JRGC)
If I do that, the ball is still moving in two different directions during its 'flight'. So why would I take negative acceleration for the whole flight?

I'm probably being really stupid....
Because on the way down it will also have negative velocity.
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py0alb
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(Original post by Stonebridge)
Yes. The gravitational force, and hence the acceleration it produces, is always downwards in direction.
And if you choose down to be positive, the acceleration is always positive.
If you choose down to be negative, the acceleration is always negative.
But remember, negative acceleration does not always mean an object is getting slower.
See my diagram in the previous post. (4th example)

Very often when you do questions on projectiles, you just choose the direction that is most convenient to be positive.
You just need to keep strictly to the direction rules when you plug values into the SUVAT equations. If up is + then down is -. This applies to velocity u,v, acceleration a, and distance/displacement s.
Its worth clarifying this:

Kinda like the difference between speed v and velocity v, its important to understand that sometimes people are talking about a (scalar acceleration) and sometimes about a (vector acceleration). This is where the OP's confusion lies.

In everyday life we instinctively talk about acceleration as a scalar variable, so if an object is slowing down it is negative, if it is speeding up its positive. In a question like the one above however, its is necessary to either use a or at very least to apply a little common sense in using a.
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Stonebridge
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(Original post by py0alb)
Its worth clarifying this:

Kinda like the difference between speed v and velocity v, its important to understand that sometimes people are talking about a (scalar acceleration) and sometimes about a (vector acceleration). This is where the OP's confusion lies.

In everyday life we instinctively talk about acceleration as a scalar variable, so if an object is slowing down it is negative, if it is speeding up its positive. In a question like the one above however, its is necessary to either use a or at very least to apply a little common sense in using a.
Unfortunately, a lot of the problems students have are a result of applying "everyday life" definitions to terms that are used more specifically within physics. (And maths).
If we take the "everyday life" notion, then because an object thrown upwards decelerates initially we assign it a negative acceleration, but after it reaches the top and starts to gain speed on the downwards journey up we assign it a positive acceleration. This is fundamentally wrong. The acceleration is constant. It doesn't suddenly switch from plus to minus. If, for example, you apply a SUVAT equation to the path of this object, or any projectile, you give it one and only one value of acceleration. The sign will depend only on its direction. Its direction will be that of the resultant force.
That is what needed clarifying. If one doesn't keep to this notion, more complex dynamics problems will come seriously unstuck.
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py0alb
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(Original post by Stonebridge)
Unfortunately, a lot of the problems students have are a result of applying "everyday life" definitions to terms that are used more specifically within physics. (And maths).
If we take the "everyday life" notion, then because an object thrown upwards decelerates initially we assign it a negative acceleration, but after it reaches the top and starts to gain speed on the downwards journey up we assign it a positive acceleration. This is fundamentally wrong. The acceleration is constant. It doesn't suddenly switch from plus to minus. If, for example, you apply a SUVAT equation to the path of this object, or any projectile, you give it one and only one value of acceleration. The sign will depend only on its direction. Its direction will be that of the resultant force.
That is what needed clarifying. If one doesn't keep to this notion, more complex dynamics problems will come seriously unstuck.
Yes, and you're talking about vector acceleration and thats fine. But be careful: its no more "wrong" to talk about scalar acceleration any more than it is "wrong" to say "the ball came down again at the same speed that I threw it up."

I could happily (and perfectly accurately) say "the ball decelerated on its way up and then accelerated again on its way down" and everyone would understand exactly what I meant.

I don't think I ever bothered using vector acceleration, I just remembered which way the object was moving and used my common sense.
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Zishi
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(Original post by Stonebridge)
Yes. The gravitational force, and hence the acceleration it produces, is always downwards in direction.
And if you choose down to be positive, the acceleration is always positive.
If you choose down to be negative, the acceleration is always negative.
But remember, negative acceleration does not always mean an object is getting slower.
See my diagram in the previous post. (4th example)

Very often when you do questions on projectiles, you just choose the direction that is most convenient to be positive.
You just need to keep strictly to the direction rules when you plug values into the SUVAT equations. If up is + then down is -. This applies to velocity u,v, acceleration a, and distance/displacement s.
Wow! Thanks a lot from my side too! I was in the same confusion and my concept is now clear.
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