Go Back   UG Community @ Ultimate-Guitar.Com > UG Community > The Pit
User Name  
Password
Search:

Reply
Old 10-04-2012, 09:33 AM   #1
archenemyfan
UG Addict
 
archenemyfan's Avatar
 
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!.
archenemyfan is offline   Reply With Quote
Old 10-04-2012, 09:37 AM   #2
Colgate Total
I <3 drugs
 
Colgate Total's Avatar
 
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
Colgate Total is offline   Reply With Quote
Old 10-04-2012, 09:39 AM   #3
Weaponized
add me on deviantart
 
Weaponized's Avatar
 
Join Date: Mar 2011
Location: [QUOTE=Weaponized]the fast lane[/QUOTE]
Acceleration duh
__________________
Quote:
Originally Posted by crazysam23_Atax
Is the officer going to ask for the butt, so he or she can check whether it is hot?


last.fm
Weaponized is offline   Reply With Quote
Old 10-04-2012, 09:40 AM   #4
Portuguese_boy
Registered User
 
Join Date: Dec 2005
Acceleration, you're right.
Portuguese_boy is offline   Reply With Quote
Old 10-04-2012, 09:41 AM   #5
whoomit
 
whoomit's Avatar
 
Join Date: Jul 2006
If I kick a ball straight up at 20 m/s, will it stay at 20m/s the entire journey?
__________________
whoomit is offline   Reply With Quote
Old 10-04-2012, 09:42 AM   #6
Weaponized
add me on deviantart
 
Weaponized's Avatar
 
Join Date: Mar 2011
Location: [QUOTE=Weaponized]the fast lane[/QUOTE]
Velocity duh
__________________
Quote:
Originally Posted by crazysam23_Atax
Is the officer going to ask for the butt, so he or she can check whether it is hot?


last.fm
Weaponized is offline   Reply With Quote
Old 10-04-2012, 09:42 AM   #7
archenemyfan
UG Addict
 
archenemyfan's Avatar
 
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?
archenemyfan is offline   Reply With Quote
Old 10-04-2012, 09:43 AM   #8
Kid_Thorazine
Registered Sadist
 
Kid_Thorazine's Avatar
 
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.
Kid_Thorazine is offline   Reply With Quote
Old 10-04-2012, 09:50 AM   #9
magnus_maximus
lord of the files
 
magnus_maximus's Avatar
 
Join Date: Dec 2006
Location: behind you
Quote:
Originally Posted by Colgate Total
Velocity is the speed; acceleration is the rate at which is speeds up or down.

Rate of change of displacement, nub.
__________________
captain cabinets
Quote:
Originally Posted by Xiaoxi
Well then I am the gayest pedophile.
magnus_maximus is offline   Reply With Quote
Old 10-04-2012, 09:54 AM   #10
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.
xyber56 is offline   Reply With Quote
Old 10-04-2012, 09:58 AM   #11
Telecaster7
Call me Leon
 
Telecaster7's Avatar
 
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)
Telecaster7 is offline   Reply With Quote
Old 10-04-2012, 09:59 AM   #12
Metallica1554
UG's Foo Foo Cuddly Poops
 
Metallica1554's Avatar
 
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
Metallica1554 is offline   Reply With Quote
Old 10-04-2012, 10:11 AM   #13
archenemyfan
UG Addict
 
archenemyfan's Avatar
 
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.
archenemyfan is offline   Reply With Quote
Old 10-04-2012, 10:11 AM   #14
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.
captsnow is offline   Reply With Quote
Old 10-04-2012, 10:11 AM   #15
JagerSlushy
duckin the 12 olol
 
JagerSlushy's Avatar
 
Join Date: Mar 2009
Location: Ask your girlfriend.
Velocity references speed and direction. Acceleration is a change in the rate of speed, which is always positive.
__________________
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 10:12 AM.
JagerSlushy is offline   Reply With Quote
Old 10-04-2012, 10:11 AM   #16
magnus_maximus
lord of the files
 
magnus_maximus's Avatar
 
Join Date: Dec 2006
Location: behind you
Quote:
Originally Posted by archenemyfan
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.

...no. It makes it more negative.
__________________
captain cabinets
Quote:
Originally Posted by Xiaoxi
Well then I am the gayest pedophile.
magnus_maximus is offline   Reply With Quote
Old 10-04-2012, 10:12 AM   #17
magnus_maximus
lord of the files
 
magnus_maximus's Avatar
 
Join Date: Dec 2006
Location: behind you
Quote:
Originally Posted by JagerSlushy
It's acceleration. Velocity doesn't reference direction, only speed.

You are wrong.
__________________
captain cabinets
Quote:
Originally Posted by Xiaoxi
Well then I am the gayest pedophile.
magnus_maximus is offline   Reply With Quote
Old 10-04-2012, 10:13 AM   #18
JagerSlushy
duckin the 12 olol
 
JagerSlushy's Avatar
 
Join Date: Mar 2009
Location: Ask your girlfriend.
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.
JagerSlushy is offline   Reply With Quote
Old 10-04-2012, 10:20 AM   #19
mulefish
UG Addict
 
mulefish's Avatar
 
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
mulefish is offline   Reply With Quote
Old 10-04-2012, 10:24 AM   #20
JustRooster
Organ Donor
 
JustRooster's Avatar
 
Join Date: Jan 2005
Location: Madison, Wisconsin
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.
__________________

Quote:
Originally Posted by dannyalcatraz
Love is blind, and probably missing a few other sense, too.

JustRooster is offline   Reply With Quote
Reply


Thread Tools Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump



All times are GMT -4. The time now is 04:49 AM.

Forum Archives / About / Terms of Use / Advertise / Contact / Ultimate-Guitar.Com © 2014
Powered by: vBulletin Version 3.0.9
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.