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Old 10-04-2012, 10:40 AM   #21
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Velocity is speed in a certain direction. Seeings as the direction changes, i'd say that's your answer.
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Old 10-04-2012, 10:41 AM   #22
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Quote:
Originally Posted by Ssargentslayer
Your answer did not need to be this long.



It wasn't initially, half of it is me second guessing my first answer.
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Old 10-04-2012, 10:47 AM   #23
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ill give you an easy trick for you to use whenever you encounter such a problem:
whenever there's change in movement from the original direction to the opposed direction, velocity changes sign, and at the point where the change of directions happened, is whete the velocity is equal to zero.

other info, acceleration is always depending on an applied force, in your case gravity, and if there is no other forces applied during the movement, acceleration will stay constant, so in you case it will stay constant.
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Old 10-04-2012, 10:50 AM   #24
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Quote:
Originally Posted 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.

This. In much better words than I could put it in.

Although I'm still not convinced that gravity and accelleration are the exact same thing and not just correlated.

However, I'm a Communications major and haven't had to do this kind of shit in the greater half of a decade.
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Old 10-04-2012, 10:50 AM   #25
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Quote:
Originally Posted by mulefish
But wouldn't it need a positive acceleration to start with? Seeming as the ball is thrown?

Velocity definitely changes though.


These problems always start a the time when the ball or whatever has left whatever is producing a force on it. So, what is happening is that something (persons arm) is producing a force on it. Once it has left the arm, it is now in free 'fall' motion. There is nothing to accelerate it upward anymore, only gravity to pull it down. The higher the velocity the longer it will keep moving in the upward direction. So, you would most likely have an acceleration that gives the object speed but think of it as a force that creates acceleration F = ma.

Quote:
Originally Posted by snipelfritz
Although I'm still not convinced that gravity and accelleration are the exact same thing and not just correlated.


Gravity is a Force that causes acceleration on objects within it's field. F due to gravity = (G)m1m2/d^2.

G is universal gravitation constant, m1 and m2 are masses of two objects, and those three terms divided by the distance between them squared gives the force of gravity. Which you can then apply to F = ma (I'm pretty sure... it's been awhile since I've done this).
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Old 10-04-2012, 11:08 AM   #26
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Yeah, it's velocity.
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Old 10-04-2012, 11:30 AM   #27
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Velocity. I think everyone's covered why it's not acceleration, but not why it's not position.

Your problem states that it starts at Y=0, goes up and comes back to it's starting point at 0 therefore it was always positive and didn't change sign.
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Old 10-04-2012, 11:41 AM   #28
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Quote:
Originally Posted by magnus_maximus
Gravity is just an acceleration. Gravitational force is caused by distortions in space/time.

Magnus, what's your last name?
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Old 10-05-2012, 05:25 AM   #29
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Quote:
Originally Posted by magnus_maximus
Seximus Decimus Aurelius.

>_>

<_<

So it doesn't begin with a J?...

>_>
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Old 10-05-2012, 06:11 AM   #30
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Quote:
Originally Posted by magnus_maximus

Do you have....

Ginger hair? >_>
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Old 10-05-2012, 06:12 AM   #31
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acceleration stays at -9.81m/(s^2). velocity is the property that changes, because the direction in which the object is travelling changes. SPEED doesn't change, velocity does. look into the difference if you don't know it already.
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Old 10-05-2012, 06:14 AM   #32
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Originally Posted by magnus_maximus
No.

GTFO. That's just insulting brah.

Oh, okay. So your surname isn't Johnston(e)?
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Old 10-05-2012, 06:17 AM   #33
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Quote:
Originally Posted by laid-to-waste
acceleration stays at -9.81m/(s^2). velocity is the property that changes, because the direction in which the object is travelling changes. SPEED doesn't change, velocity does. look into the difference if you don't know it already.
Actually, the speed changes constantly as well. It just won't go beneath zero.
I realized halfway through writing this that this might actually have been what you meant, but meh... better to clarify it for confused TS (who really should have figured it out by now)
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Old 10-05-2012, 06:18 AM   #34
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Quote:
Originally Posted by magnus_maximus
Nope.


So close, so close.

Without the 't'!?
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Old 10-05-2012, 09:29 AM   #35
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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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Old 10-05-2012, 09:35 AM   #36
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That was awesome. Thank you for taking your time to do that, I appreciate it.

Also my physics class is calc based.
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Old 10-05-2012, 09:39 AM   #37
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For those of you doubting that gravity is the only acceleration, whatever force that caused the ball (or whatever, I can't remember) to start its trajectory happened BEFORE t=0 and isn't a subject of study for the problem. A lot of people get confused when a is working in an opposite direction to v.
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Old 10-05-2012, 10:12 AM   #38
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Quote:
Originally Posted by archenemyfan
That was awesome. Thank you for taking your time to do that, I appreciate it.

Also my physics class is calc based.


No problem. If you have any other questions, feel free to ask (although I may not get back to you right away since I have to go to class myself).
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Old 10-05-2012, 10:19 AM   #39
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Ok, not a physics expert here, so could someone explain to me (using short words and small sentences) why acceleration isn't changing sign?

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

?

As I've stated, this isn't my area of expertise, and while I have some small understanding of it, I'll not pretend to be an expert, so be gentle.
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Old 10-05-2012, 10:28 AM   #40
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Acceleration is always negative in this case. Initially it has an upward velocity because of the negative acceleration it will slow down until it is 0 m/s. At that point the negative accceleration will cause the velocity to go into the minus and accelerate it in the negative direction if you will.

To make it a little easier to understand maybe, flip the axis so the acceleration is positive and the particle travels in the +y direction. In that case the velocity would get more positive with time. This works exactly the same, but in the negative y direction instead of the positive.

Did this clear things up a bit?

Also noteworthy: Velocity is a vector quantity. Speed is the norm of that vector and is is a scalar that is always greater than or equal to 0. This means that the speed is the square root of the sum of the squared components of the velocity vector. This is the same as the square root of the dot product of the velocity vector with itself, for those with a mathematical background.

Edit: Clarification and typo's.

Last edited by ChaosInside : 10-05-2012 at 10:39 AM.
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