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 10-04-2012, 09:33 AM #1 archenemyfan UG Addict     Join Date: Apr 2010 For all you physics experts out there.. A ball is thrown upward from y=0 and then falls back to its starting poimt. Let up be Positiive. which of the following changes sign during the flight? Velocity Acceleration Y None So, my meagre knowledge of physics tells me that acceleration in this case is always negative, because it's gravity. But isn't position + during the flight up and - during the flight down? And isn't velocity the same? I can only choose one answer, I need halp!.
 10-04-2012, 09:37 AM #2 Colgate Total I <3 drugs     Join Date: Sep 2007 Location: Galloping Through Bastard Fires on My Metal Chariot Velocity is the speed; acceleration is the rate at which is speeds up or down. __________________ Like podcasts? Listen to these! BEER! VIDEO GAMES! MOVIES/GEEKY SHIT! _______________________________________________ Last edited by zappp : Today at 4:20 PM. Reason: Suck on my balls, UG
10-04-2012, 09:39 AM   #3
Weaponized
Creaky

Join Date: Mar 2011
Acceleration duh
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 10-04-2012, 09:40 AM #4 Portuguese_boy Registered User   Join Date: Dec 2005 Acceleration, you're right.
 10-04-2012, 09:41 AM #5 whoomit ⁣   Join Date: Jul 2006 Location: Chandler, AZ If I kick a ball straight up at 20 m/s, will it stay at 20m/s the entire journey?
10-04-2012, 09:42 AM   #6
Weaponized
Creaky

Join Date: Mar 2011
Velocity duh
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Quote:
 Originally Posted by Neo Evil11 at #33611170 If she is poor then she is a fucking idiot despite being an atheist.

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10-04-2012, 09:42 AM   #7
archenemyfan

Join Date: Apr 2010
Quote:
 Originally Posted by Weaponized Acceleration duh

So I'm confuzzled. Does acceleration change from positive to negative? I thought that gravity was always Negative.

Also I should've phrased it better when I said " isnt velocity the same". I meant doesn't velocity change from positive to negative also?

 10-04-2012, 09:43 AM #8 Kid_Thorazine Registered Sadist     Join Date: Jul 2004 Location: KY Velocity, assuming I'm understanding right, once the ball reaches it's apex and starts falling it will have negative velocity in terms of your grid, whereas acceleration will be a constant -g assuming you only track the ball once it's finished accelerating upwards, and Y would only be negative if the ball ended up at a lower point than where it started. I'm also operating under the assumption that the velocity and acceleration hitting 0 doesn't count. On the other hand it could be acceleration since depending on our frame of reference the acceleration becomes positive on the way down since it's adding to velocity instead of subtracting, actually that's probably what they are looking for. This is why my physics professor insisted on labeling opposing forces in degrees instead of dealing with positive and negative. __________________ make Industrial and/or experimental electronic music? Join my group! Last.fm Last edited by Kid_Thorazine : 10-04-2012 at 09:51 AM.
 10-04-2012, 09:54 AM #9 xyber56 ITS A MADHOUSE!   Join Date: Feb 2005 Location: Singapore answer is velocity Acceleration remains constant (-g) velocity starts of positive (as the ball goes upwards) and slowly decreases till a point in midair where for a very short moment velocity = 0 (when it is at it's maximum height) the ball then starts falling back down with negative velocity (because it is heading downwards) y does not change sign because negative would imply it ended up lower than the original position Last edited by xyber56 : 10-04-2012 at 09:56 AM.
 10-04-2012, 09:58 AM #10 Telecaster7 Call me Leon     Join Date: Aug 2007 Location: Perth, Australia Velocity will change constantly during the flight, but as long as it returns to its starting point it was end up having zero velocity. Acceleration will always be -9.8ms-2 down, as there is no longer any force exerted on the ball after it has left the launching device (your hand, I presume) __________________ Hey, I heard you want to dismantle a Stratocaster Oh, what's that? You have a fetish for dismantling Telecasters? Do you like talking about the stuff you make/break? GB+C Chat Thread!
 10-04-2012, 09:59 AM #11 Metallica1554 UG's Foo Foo Cuddly Poops     Join Date: Jan 2009 Location: ---- Lake Bodom ---- Join Date: May 2001 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. __________________ Oh Sheesh Y'all, 'twas a dream
10-04-2012, 10:11 AM   #12
archenemyfan

Join Date: Apr 2010
Quote:
 Originally Posted by Kid_Thorazine I'm also operating under the assumption that the velocity and acceleration hitting 0 doesn't count. On the other hand it could be acceleration since depending on our frame of reference the acceleration becomes positive on the way down since it's adding to velocity instead of subtracting, actually that's probably what they are looking for. This is why my physics professor insisted on labeling opposing forces in degrees instead of dealing with positive and negative.

But if velocity is negative and acceleration is also negative doesn't it add to the velocity because they're both in the same direction?

And thanks for the replies , i'm convinced that Velocity is indeed the right answer.

 10-04-2012, 10:11 AM #13 captsnow Registered User   Join Date: Jul 2008 Using a simple model for this s=u*t +0.5*a*(t^2) where s is displacement u is initial velocity t is time a is acceleration The only force involved is the force due to gravity (constant) at 9.81ms^-2 in the negative direction Differentiating the previous equation gives an equation for velocity v=u+a*t Clearly at t=0, v=u, i.e. the velocity is equal to the initial positive value If you plotted this equation then you would find a negative slope starting at v=u, t=0. Then crossing the axis at v=0, t=u/a i.e the point at which the ball is stationary. it would then become negative as the ball accelerates towards the ground. tl;dr acceleration is constant, displacement (y) it always positive, velocity goes from positive to negative.
10-04-2012, 10:11 AM   #14
JagerSlushy
duckin the 12 olol

Join Date: Mar 2009
Velocity references speed and direction. Acceleration is a change in the rate of speed, which is always positive.
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 Originally Posted by Tone Deaf Someone has had too much jager in their slushy. :/
Quote:
 Originally Posted by CL/\SH First person on UG to be a grammar nazi and use the correct form of "your" in the correct context. + 70 virgins to you, my good sir.

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 Originally Posted by Fassa Albrecht Girls DO fap...I don't though.

Last edited by JagerSlushy : 10-04-2012 at 10:12 AM.

10-04-2012, 10:13 AM   #15
JagerSlushy
duckin the 12 olol

Join Date: Mar 2009
Quote:
 Originally Posted by magnus_maximus You are wrong.

shit, I was hoping nobody saw it before I did. but yeah, I caught that right after I posted it

leave it to a mechanical engineer to fuck up a physics question...
__________________
Quote:
 Originally Posted by Tone Deaf Someone has had too much jager in their slushy. :/
Quote:
 Originally Posted by CL/\SH First person on UG to be a grammar nazi and use the correct form of "your" in the correct context. + 70 virgins to you, my good sir.

Quote:
 Originally Posted by Fassa Albrecht Girls DO fap...I don't though.

Last edited by JagerSlushy : 10-04-2012 at 10:15 AM.

 10-04-2012, 10:20 AM #16 mulefish UG Addict     Join Date: Mar 2007 Location: Australia But wouldn't it need a positive acceleration to start with? Seeming as the ball is thrown? Velocity definitely changes though. __________________ RIP Gooze cats
10-04-2012, 10:24 AM   #17
JustRooster
Shoulder Shrugger

Join Date: Jan 2005
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.

I was waiting for someone to say it.
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10-04-2012, 10:24 AM   #18
Ssargentslayer
Registered Abuser

Join Date: Sep 2007
Quote:
 Originally Posted by Kid_Thorazine Velocity, assuming I'm understanding right, once the ball reaches it's apex and starts falling it will have negative velocity in terms of your grid, whereas acceleration will be a constant -g assuming you only track the ball once it's finished accelerating upwards, and Y would only be negative if the ball ended up at a lower point than where it started. I'm also operating under the assumption that the velocity and acceleration hitting 0 doesn't count. On the other hand it could be acceleration since depending on our frame of reference the acceleration becomes positive on the way down since it's adding to velocity instead of subtracting, actually that's probably what they are looking for. This is why my physics professor insisted on labeling opposing forces in degrees instead of dealing with positive and negative.

10-04-2012, 10:25 AM   #19
archenemyfan

Join Date: Apr 2010
Quote:
 Originally Posted by JustRooster I was waiting for someone to say it.

isn't acceleration a vector quantity aswell?

10-04-2012, 10:28 AM   #20
Metallica1554
UG's Foo Foo Cuddly Poops

Join Date: Jan 2009
Location: ---- Lake Bodom ---- Join Date: May 2001
Quote:
 Originally Posted by archenemyfan isn't acceleration a vector quantity aswell?

Yes, but in this model where a ball is thrown up, the acceleration is due to the force of gravity, which is assumed to be constant and always negative (as the ball is always attacted back to the ground)
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