/sci/ - Science & Math

Wouldn't it be this one?

>>4301034

>2012
>portals on moving surfaces

Portals can not be placed on moving surfaces.

>>4301034

Relativity makes no fucking sense with portals.

A.

If drop the opening of a cup over a coin, what happens? Does the coin fly upwards?

Please god not this thread again PLEASE GOD NO

>>4301049

In General relativity viewing the cup as flying downwards and the penny as flying upwards are both equally valid.

The answer is extremely simple, since you asked specifically what would happen "in Portal" (as in the game).

The portal would disappear. I've done it myself, shoot a portal at a surface and if that surface ever moves the portal vanishes.

However, if this were to occur IRL, then it would obviously be option B.
I give you two reasons:
1) In the reference frame of the orange portal, the orange portal can claim that it is at rest and the block is speeding towards it. Therefore it will send this information to the blue portal.

2) Consider looking into the blue portal, you see the block speeding up toward you, so when the block comes through it will continue with that speed on its way out.

B. obvisouly.

Think about if the bottom plateform was the one moving upwards at a fast speed, it would be the same thing. momemtum is conserved between portals.

It's B

If you were standing, looking at the blue portal, the box would be rushing towards you at some speed, and momentum from the point of view of the portal is conserved.

>>4301034
The serious and absolute true answer is A.

The reason is as follows:
You must remember that the two portal are the same area in space connected at two different areas in space, which is an inherently non-intuitive concept to grasp. The best way to think of it is like this - An open door is like two portals in the same location.
If you were standing still and a wall with an open door zoomed right in front of you then suddenly stopped, you don't suddenly go flying through the open door.
The only difference is that in this case the orientation is change enacting the force of gravity. So I guess a more apt analogy would be for a trap door, but that gives the false impression that the cube would fly out at the same speed you would fall.

Hopefully that explains it, shit is very non-intuitive and is a great testament to Valve's game design.

>>4301034
Portals (as implemented in the game Portal) break the "laws" of conservation of momentum and energy. Whatever happens is completely up to the game designers and bears no relation to any real-world physical laws unless they want it to.

That said, I'd guess A.

>>4301079
Sorry, you're wrong. This situation is not different at all than if the earth were rushing up to meet the portal. The box will move either way. From the refrence of the orange portal, the box has energy, and that energy gets put into the box when it passes the membrane.

>>4301060
Yes, and?

>>4301066
>>4301076
Momentum is conserved between a cup and a coin. That doesn't mean the coin flies upward.

>>4301087
The energy of the moving platform is completely transferred into the second (stationary) platform.

Again, coin and cup analogy.

>download the portal 2 mod kit
>try it out
hurr

In portal 2 portals can be on moving surfaces

youthink that's hard?
put a portal on a flat panel (lets say plywood with conversion gel) and then put it through a portal on the wall

>>4301042
in portal 2 you place portals on a moving surface
remember when you had to cut the neurotoxin lines?

>>4301079

>2012
>thinking that what happens in a game has any real-world basis

>>4301141
yeah really
while we're at it, let's discuss how the physics in mirror's edge are totally realistic and have no innacuracies

B would be true were it to be in space. The cube would keep flying forward. However, we are on earth and we have gravity acting on it. The cube would plop because the cube is in equilibrium, therefore, the only force acting on the cube is gravity, with a force of 9.81 N. When the cube goes through the other portal, gravity is acting in two directions with equal force. The angle of the output platform is 40° measured from the picture, and if we assume that the cube will come out at 45°, then we can add the two vectors and then we get the cube exiting the portal with a force of 7.51 N at 337.5°. If we knew the weight of the cube and the size of the output platform, we could determine how far it would go, but until then, it would plop. Unless I missed something, case closed.

>>4301315

wrong, it's A, is /sci/ really this stupid?

>>4301328
he has a whiteboard he must be right!

The game says 'The product of mass times velocity is conserved between portals'. So A

>>4301527
"Speedy thing goes in, speedy thing comes out" -GlaDOS

>>4301620
But the thing is stationary

Can we get a genius like terry tao on this?

>>4301315
Bullshit considering acceleration in Newtons
Force of moving surface has nothing to do with momentum of cube also use a fucking parallelogramm when summing the forces
You sir a a disgrace to that whiteboard

>>4301315
>>4301315
Bullshit considering acceleration in Newtons
Force of moving surface has nothing to do with momentum of cube also use a fucking parallelogramm when summing the forces
You sir a a disgrace to that whiteboard

B, because no matter the thing being stationary in the first place, it has momentum after it has exited the blue portal.

Anyone who answers A, either doesn't know about momentum or doesn't believe that it exists.

>>4301844
But wouldn't all the momentum transfer to the platform?
I choose A

>>4301847
Okay, you don't know how momentum works.

>>4301852
Portals aren't compatible with momentum conservation, so your argument doesn't apply.

>>4301864
Erhm, how so?

The cup and coin analogy is not comparable to what happens here. Cups don't distort space, portals do.

>>4301864
And there weren't any moving portals, so your objection doesn't apply.

Even though the object is in rest in the reference frame of the orange side, in the reference frame of the blue side, it is moving. If you answer A, you say that an object that is transiting a portal doesn't have momentum. At all. For the whole time that any part of it is within the portal.

And that's stupid.

Portals are impossible, thus it can only be answered by the authors at Valve.

>>4301878

Why not think outside of the box and conjeture about them. This way if portals ever get to exist we get infinite nobel prizes and $$$

>>4301879
>2013 Nobel prize fro Physics goes to /sci/ for determining physics of imagination
I like it.

>>4301872
why SHOULD it have momentum? The portal has the momentum, not it.

>>4301872
You're missing the bigger point.

Portals as shown in the videogame Portal can't exist in the real world. They contradict basic physical laws. So neither answer is 'correct' because the entire scenario is meaningless. It's like asking "Does apple + happiness equal 3 or 4?"

Even if you ignore that, portal behavior and thus the answer is solely up to the game designers. And since they've never included such a situation in the game, the question is still meaningless and no answer is right. Alternatively, any answer is right. But neither is more correct than the other, regardless of your personal preference.

>>4301896
Because it's in movement from the reference frame of the blue portal. You can look it like this: even if the parts that had exited the portal DIDN'T have momentum, there's still more matter coming out of the portal, pushing the preceeding parts out of the way, imparting momentum on them.

If you can put portals on moving surfaces in Portal 2, why not just make a mod that does this and see?

>ENGINEER

>>4301905
>gedankendexperiment is pointless
Oh, do tell. It still has the point of being an exercise in non-linear thinking of physical phenomena.

>>4301034
B is correct, the block must be leaving the blue portal at the same speed in which the orange portal is moving down onto it.

>>4301042
>>4301045
Portals can move in Portal 2, but not in Portal 1.
http://www.youtube.com/watch?v=S1AIR6m8uLk

Assuming that the portal does nothing except transport particles from one point to another
I'd go for A
B implies you have given some energy to the box when infact nothing has touched it

Look, I know basically none of the posters thus far have even a college degree. I suggest you give it a try, read this text, and then try your hand at the question again. I would put this on a sophomore mechanics exam.

>>4301916
Regarding my picture, if you consider what happens between stages 2 and 3, the block is emerging out of the blue portal at the same speed in which the orange portal takes to fully envelop it.
If this speed is very fast, then the block must start to emerge very fast as well.
It makes no sense for the block to suddenly lose speed for no reason, so I suggest that it would shoot out of the blue portal at a high speed and then keep on going until gravity brings it down.

>>4301915

Then with respect to an static observer, the cube gained momentum out of nowhere.

>>4301315
your vectors aren't even in the right direction, dumbass.

>>4301034
B)

This is very basic physics.
Also, you can mod portal and to these kind of experiments yourself.

\thread

It always annoyed me that with Portal it's always described as conservation of momentum. Sure, momentum in indeed conserved but it seems to be such an unnecessary way to put it.

Since the portal transports an object we assume it's the same object going in one portal and out the other, so obviously the mass has not changed. So why not just say that velocity is conserved?

>>4301963
>Since the portal transports an object we assume it's the same object going in one portal and out the other, so obviously the mass has not changed. So why not just say that velocity is conserved?

Velocity has a direction (same for momentum), so that wouldn't be correct either. A technically correct statement would involve conservation of momentum with regards to a coordinate transformation.

>>4301079
^^

A

if the box was falling, or being pushed, it would be B

>>4301956

The perhaps a moving object should lose the same momentum that the object going through it exits with.

>>4301956
The momentum is transferred from the momentum that is on the downward moving plunger that has the orange portal.
The speed will be equal, but the momentum will actually be different. (The plunger and surface will probably weigh more than the block)
Portals can not exist in real life, infinite energy machines would be possible if they could exist.
B is logically a better solution if they could exist.

>>4301944
the block never has any momentum though
similarly there is no potential field to give it that momentum
the block will quickly emerge but will not out

if you take a carboard tube and thrust it down over a stone, does the stone come shooting out of the top?
by your logic it does

>>4301972
>>4301972

this

>>4301967

Implied talking about just the magnitude of course.

Similar thing in the Quantum Whatever game (can't remember the name, you can walk into the fluffy dimension where everything is lighter). During a presentation the player picked up a safe while in the "fluffy dimension" and threw it towards a pane of glass, the switches to the normal world while it's mid air, making it regain it's mass and break the glass.

The present goes "Momentum is conserved" when obviously, if it was conserved the safe would slow to a halt when it gained all that mass from nowhere.

Biochemist here.
Its A.

>>4301971

Not really, the punger either free falls or needs mechanical power to move it downwards if its fixed somewhere, in any case the sum of momentums is not conserved. The sum of forces acting on the cube is 0 and the interaction with a moving portal does not create any kind of potential. So it just pulls out and gravity does the rest but it does not shoot off

Assume two premises:

- Momentum, a function of mass and velocity, is conserved through portals. In layman's terms, speedy thing goes in, speedy thing comes out
- An object entering a portal has the same dimensions as one leaving (if it's a 1m long stick going it, it's a 1m long stick coming out)

Now, we have two facts: The MOMENTUM in and out is the same
The SHAPE in and out is the same

So therefore the object MAINTAINS the speed imparted by entering the portal
So therefore B

>>4301972

>if you take a carboard tube and thrust it down over a stone, does the stone come shooting out of the top?

Depends on what you mean with "shooting". Imagine the cardboard tube is infinitely thin (just a circle basically) and observe the stone from the point of the tube. Of course the stone exists the tubes top at the same speed (and momentum) as it entered it, which in turn is the same speed as the tube was slammed down with.

>>4301992
>The SHAPE in and out is the same

that's really the most important part. the distance from the top of the cube to the bottom is constant (through a portal or not). thus when the cube is midway into the portal, it's clear on the advancing-portal case that the cube has velocity relative to the portal, requiring by conservation of the cube size that the exiting portion of the cube be travelling at the same velocity normal to the exit portal.

>>4301992

>So therefore the object MAINTAINS the speed imparted by entering the portal

which is 0

>>4302004
For it to be 0, the cube would have to be moving relative to the portal at 0m/s. Which would mean the platform wasn't moving.

A, imagine the orange portal is a vagina, and the cube your dick, if a girl slams on your thights your dick doesn't get pulled in after she has stopped moving. the dick/box has no forward vector
>the portals can't be placed on moving surfaces guys don't belong here.

>>4301972
It has mass, and it is emerging from the blue portal, therefore it must have speed.
If we deduce that it must have speed and it must have mass, then it must have momentum.
My picture shows that between panels 2 and 3, the speed of the falling plunger must be identical to the speed of the emerging block, because the block can not exist in 2 places at once (either in whole or in part), and part of the block can not be in a state of none existence (ie, you can not have 1/3 of the block emerging through the blue portal, with 2/3 of the block already placed through the orange portal, leaving 1/3 unaccounted for)
After panel 3, the ''magic/portal' part of the situation has already happened, and all we have is a block, in 3D space that is not being affected by portals, but we have deduced that it already must have momentum.
In panel 4, there is no reason for this momentum to suddenly disappear, and the block has fully left the blue portal. (but only very recently, it is still right next to it, and travelling away from it)

It will start to slow down due to gravity eventually, but initially it must have high speed.

>>4301992
it's A, i'm absolutely sure. the box has no vector and no momentum, it's not speedy.

Let's assume a fixed orange portal and that it's the platform with the cube on it that's quickly smashing into the surface the orange portal is on. Imagine we then stand by the blue portal, looking down. We'd see the platform with the cube approach at high, hit the portal surface and shoot the cube into the air.

Now let's use the situation in the picture instead, with the portal moving and the cube standing still. Again, as we stand by the blue portal looking down, we see the platform with the cube speeding towards us. The exact same thing as if it was the platform moving, not the portal.

These situations are equivalent and should therefore lead to the same result. With the platform moving it's trivial to show (and vital to the game mechanics) that the portal will exit with the speed it entered, thus the same happens when the moving portal slams into the cube.

>>4302007

>if a girl slams on your thights your dick doesn't get pulled in after she has stopped moving

Sure, but that's because the vagina is continuous. With the portal, only the entry portal stops moving, the exit doesn't stop moving because it was already standing still.

>>4302010
It would have no vector and no momentum if there was no change in the moving platform and its location as time progressed. Because the platform is moving down, there is a difference between them. The speedy thing is speedy relative to the portal.

>>4302010

Relative to the portal it enters it does.

>>4302014
nope. it's A. unless the orange portal has a "bottom" of energy that hits and pushes the cube.

>>4302022
That was vague, sorry

>If there was no change in the location of the moving platform and the relative location of the cube as time progressed

B, if portals could be on moving surfaces.

However, don't think too hard about this shit. Portals violate conservation of energy AND momentum. Do not expect them to make sense.

>>4302023
uh, nope. it doesn't get touched by anything. nothing hits it, nothing gives it momentum. imagine the blue portal is actually at the top of the orange one, like, if it was a very thin piece of paper keeping the 2 separated, it would just pass through and sit there.

>>4302028
>B, if portals could be on moving surfaces.
Someone hasn't played portal 2. They can.

>>4302024

And where do you propose the velocity we observe the cube having when looking down the blue portal disappears to once the object has completely passed through the portal.

>>4302010
>hurr there is an absolute reference frame and I'm it

>>4302028
this is what B sayers really believe.
A.

>>4302033
You sure bro? I don't remember that. If you move the surface, doesn't it kill the portal?

It's been a while.

>>4302029
Imagine the cube is hovering in space, nothing round it. The piece of paper moves towards it at 10m/s. The piece of paper moves along it, at 10m/s. The piece of paper moves away from it on the other side at what speed?

So what speed is it exiting the paper?

>>4302029

But it's not just a thin piece of paper. It's a thin piece of paper with independently movable openings. If it was just a paper ring you wouldn't be able to move the entrance so the cube goes through it without at the same time moving the exit.

So,

1. Motion through portals dont change the mechanical properties of objects, during passing time

2. The box at rest is not experiencing ny force whilst the portal eats it

3. mv conserved immediately after -> emerges at rest?

>>4302037
Yep. Just before you meet GlaDOS for the second time, you put portals on moving platforms to laser cut tubes going to the deadly neurotoxin generator.

>>4302047
Oh yeah, that's true.

You never get to send objects through though.

For those of you who do not think that the cube has momentum and speed after it has emerged, I am sure you agree that the blue portal is static.
If the blue portal is static and the cube is also static, how can it possibly emerge?
For two objects to separate from each other they must have different speeds.

If you therefore agree with me that the blue portal is static and the cube does have speed, how can you possibly argue that the speed suddenly disappears for no reason after the cube has fully emerged?

>>4302039
the paper keeps moving at 10 m/s, the cube didn't move. it's basic logic, no impact, no vector, no energy applied directly, no speed. and it's not that the portal moving away slowed it down because it was shooting it at the same speed in the opposite direction, it simply doesn't have a vector.

>>4302049

You send loads of photons through it.

>>4302049
You could probably test it in-engine.

But it would work just fine, I bet.

>>4302052
No, don't do that.

Relative to the paper. Relative to the paper what speed does the portal exit the paper. Relative to portal B what speed does the object exit portal B.

>>4302037
see >>4301915
You are correct that in portal 1, a portal disappears if its surface begins to move. The laws change in portal 2.

>>4302052

Before you start talking about "having vectors" you should get basic mechanics down. Velocity only exists as a relation between two objects.

>>4302051
i already said this, unless the orange portal has an energetic/magnetic/magic field pushing the cube it's A.

>>4302057
Ignore this, I typo'd

Relative to the paper, what speed does the cube exit the paper. Relative to the portal, what speed does the cube exit the portal.

>>4302063
Please look at >>4302039

>>4302057
no speed. it's at rest.

>>4302051

Imagine you stand besides the blue portal and look down at the cube. From this point, the cube indeed has speed, as it keeps moving closer and closer to the portal and eventually through it.

/sci/ = the most painful 4chan board for anyone that actually works in /sci/.

two properties: conservation of mass, conservation of momentum. we only depart from reality here on the concept of momentum, where portal-logic dictates that momentum is invariant of direction (that is, changing direction of momentum requires no force applied.)

conservation of mass (equivalently, conservation of the rigid body shape) gives the top boxed equation. simple enough, anything going into the left portal has to come out the right.

conservation of momentum gives the second equation. important thing missed by many here is the basic principle of galilean invariance (left side sketch). the falling plunger on static cube is equivalent to a moving cube hitting a static plunger. if you don't understand this concept, i suggest you get off the internet and study.

>>4302066
OK, so you don't know what relative means.

I'll put it nicely: At this point, you don't have the mental capacity to understand. see http://en.wikipedia.org/wiki/Frame_of_reference

>>4302063
choosing A, implies that after stage 3 in this picture >>4301944
The speed that the portal must have had between stages 2 and 3 suddenly disappears, and the cube just falls away from the portal due to gravity.

You have no logical explanation for why speed should suddenly disappear as soon as the object is fully disconnected from the portal/portal-rift.

Sure, it might just be an "impossible" situation, but trying to solve it really does show who understands mechanics and who has not.

1. Portals cannot work on moving surfaces
2. Even if they could, they can't transmit momentum...

>>4302065
10 m/s, aka 0 m/s for an observer
the only reason you'd think it was any different is because you are imaging the portals wrong, with the paper exemple is like there is no portal, and the paper has a hole. if you threw a cube at 10 m/s at a portal approaching you at 5 m/s on a static exit the cube would still keep its 10 m/s speed, there is no energy transfer, that's how portals are supposed to work, imagine a hula hoop approaching you at 5m/s, you throw a basketball through at 10 m/s, on the other side it goes out at 10m/s, ignoring the hula hoop's speed

>>4302072
i suggest you read this, and try to focus bro.
>>4302078

>>4302068
Correct, that is in favour of people like me who choose B, and is in a similar line of thinking to >>4301060

It does not matter whether the orange portal moves towards the cube, or the cube moves towards the orange portal, the effect is the same.
The only thing that matters is the difference is speed, which must also be the difference in speed between the blue portal and the block after it has emerged.

This is correct >>4302069

Easy:
v = Δs/Δt
lets say the height of the cubic is s = 1m

Since the cubic is entering the portal with the speed of the moving platform, it will leave the portal with the speed of the platform.

The speed must be the same because the Δs of the cubics height entering the portal in a specific time is the same Δs leaving the other portal. With numeric values: 10cm of the cubic enter the portal in 0.1sec, therefore 10cm of the cubic leave the portal in 0.1sec. The v of the platform and the v of the leaving cubic are identical.

>>4302076
Yes they can, see >>4301915

>>4302078

If both portals where moving (in oposite direction, i.e. the exit portal moving "backwards" at the same speed the entry portal i moving) what you are saying would be true, it's be exactly like a hula hoop. But portals are not hula hoops where the exit and entry are fixed to each other, rather they can move independently. It's because one of them is moving and the other is not (or rather, that they are moving at different speeds) that causes it to behave differently from your nice hula hoop example.

>>4302069

for the invariance to be rigorous, wouldnt the static portal acquire and speed relative to the falling portal?

>>4302075
The best part being that both groups think the other group doesn't.

>>4302076
1. http://www.youtube.com/watch?feature=player_detailpage&v=KEwjQV_QxPY#t=332s

2.http://www.youtube.com/watch?v=MjeMOhb9enI

>>4302080
>This is correct >>4302069
i actually need to flip a sign on the coordinate or V0, but yeah. FML for being a 4th year grad student and still looking at sci.

>>4302085
>for the invariance to be rigorous, wouldnt the static portal acquire and speed relative to the falling portal?
good point/question.
yes, you're correct, and then you have a negative velocity V0 on the outgoing portal. But then you end up with a 2V0 exit speed of the cube, and still a total of V0 exit speed relative to the static portal.

>>4302095
but really i'm just looking at this as two separate systems only connected by the hamiltonian of the total system. it's really not complicated if you apply basic rigor to the problem.

"thought experiments" are of questionable use
a) when you're considering a system that explicitly disobeys basic laws of physics, and
b) when you have no physical intuition to start with because you're a /sci/tard that couldn't pass a standard statics exam.

ITT:
retards think it's B
retards think that because the portal is moving the box has momentum
retards think that the red portal has an intertial frame of reference

>>4302078
>10 m/s,

Yes
>0 m/s for an observer
No. There is no observer, there are only frames. In the frame of the cube the cube doesn't move, which is obvious. Regardless of the frame, the cube and the paper move apart at the speed they moved together, which is what B describes

>>4302114
They are not retards just for having a different opinion than you do about what would happen in a situation in which imaginary physics is in play.

B makes more sense to me, if you think A is more likely, then what is your answer to this?
>>4302051

Something happens between the portals. The momentum changed! see pic and assume no external forces

>>4301038
/thread

>>4302128
>They are not retards just for having a different opinion than you do
This isn't a matter of opinion. It's not up for debate. There is a wrong, and right.

>>4302137
And you happen to be wrong. Explain the error in >>4301038

>>4302138
that is what i drew. it's B. lrn2math.

>>4302133
That seems to be in favour of B.
The block clearly carries on moving for a few frames after it has fully emerged.

People who choose A can not explain how the emerging block would suddenly, for no reason, lose the speed that it must have gained while emerging.

>>4302141
OIC. I assumed from the fact you were arguing against Harriet that you were an Afag

>>4302142
Looks like everyone ITT agrees

Right, 0.999...=1. Do we agree?

>>4302128
the same reason that if you take a collapsable tube and you throw it down on top of a cube, the bottom is stationary and the top has a velocity, the cube emerges out of the top of it with some considerable velocity but the cube does not fly out of the top of the tube
you cannot use the reference frame of the red portal because it instantly stops when it hits the platform
from the frame of reference of the box;
the box has 0 total momentum
some other object moves past it (which has no effect on it other than parallel transportation)
space is homogenius
so the box still has 0 total momentum by the very definition of what momentum is

>>4302143
I was arguing against Harriet because she's a dumbass that thinks it's a question of intuition/feeling, and that others are entitled to a difference of opinion. It's a problem that is trivial to solve with the simplest application of basic rigor. I know that concept totally escapes 99% of /sci/tards because they think skimming wikipedia articles on particle physics somehow qualifies them to actually discuss anything, but that's not how science works.

>>4302143
He posted >>4302069
I agreed with it here >>4302080

>>4302146
At least 1 person is arguing for A; we do not all agree yet.

Yes, 0.999...=1.

I changed my mind and think it is B, however

'speedy thing comes in speedy thing goes out'

The value is conserved, however the direction of momentum is not, therefore its not constant >>4302130

>>4302142
it didn't gain any speed emerging

>>4302157
It must be moving to even be able to emerge from a static portal.

>>4302154
>I was arguing against Harriet because she's a dumbass that thinks it's a question of intuition/feeling
No, I was arguing logically about why B must be correct.
see >>4301944

>>4302153
this

>>4301034
>2012
>not knowing how to get the source code for portal and mod that shit

For fucks sake, the answer is "B"....DURRRR.

\thread

>>4302156
The portal has a frame of reference. Ignore the topological impossibility of that for a moment, think of the portal of one entity with a coherent frame surrounding it. Things moving in this frame that go through the portal have the same momentum entering as they do exiting, as they would in regular space.

>>4302157
Simply by admitting it emerged you have conceded that it had speed emerging, if it had no speed then it would not be emerging or receding.

>>4302166
look at the tube example, the cube can emerge from the top of the tube without having any momentum

>>4302078
here, think about this for a second, imagine the other side of the hoola was laying down with the exit up, relativity doesn't apply, it would come out at 10 m/s upwards, if it did, it would at 15 m/s, but you are supposed to think with portals, where would the extra 5 m/s energy of the cube come from?

>>4302170
You still don't understand frames of reference. I can't teach you how to drive when you don't know what a pedal is.

>>4302170

But it's been stated time and time again. In your tube example the exit it's emerging from is moving while the exit portal in this example is not moving.

>>4302166
>The portal has a frame of reference. Ignore the topological impossibility of that for a moment,
Just.... Apply some rigor to this. If the plunger is accelerating, then sure, you don't have an *inertial* frame of reference *on* the portal. An important part of analyzing mechanical systems like this is to choose coordinates/reference frames that make your analysis far easier. Attaching your coordinate system to the block, for example, makes it easy to accommodate an accelerating plunger.

I just assumed the plunger was at constant velocity, but it's easy to argue that to a first order approximation, it's velocity isn't changing much over the distance of the cube and is therefor considered inertial. Honestly, a sophomore mechanics class would/should require you to analyze the system including all gravitational effects. This is the power of rigor. If you do things right from the start, complex systems can be as simple as the simplest systems.

>>4302154
It's always sad to see people using a trip making a fool of themselves.

Portals like these don't work with real physics. Sure, if you go buy the rules you've assumed/stated in
>>4302069
then your answer is probably right. But I can make different assumptions about "portal-logic" and get a different answer. And there's nothing you can do about it.

(For what it's worth, although I preferred A originally I agree with you.)

>>4302175
There is only one useful frame of reference you can use, that of the box
You cannot follow the portal because it stops when it hits the platform
>>4302177
Use a collapsable tube like someone above said then.

>>4302165
Somebody did, and they made a video
http://www.youtube.com/watch?v=S85nudR6D-Y
Go to 1:30
Apparently we are all wrong.

>>4302182
>It's always sad to see people using a trip making a fool of themselves.
How's that then? Apparently you've changed your mind. That's an impressive feat on the internet.
>Portals like these don't work with real physics.
no shit?
>Sure, if you go buy the rules you've assumed/stated in >>4302069
to my knowledge of the physics in the game portal, that is the ONLY change to classical physics used to create the effects. i stated it explicitly. if you think that's a wrong assumption, and there is some other kind of physical adjustments in the game, feel free to state them.

just please don't use stupid "thought experiments."

There are two answers:
Imaginary portal physics answer - B, because relative quantities.
Game Engine answer - A, because momentum is a property of game objects like cubes, not of the game reality.

It's A. there's no acceleration on the cube, no vector. it traverses the portal faster but it doesn't have a push to keep going after traversing. no. energy. no. possible. real life. physics. applicable.

ITT;
people haven't take their first lagrangian mechanics class

>>4302201
There are two answers:
Imaginary portal physics answer - A, because relative quantities.
>wtf does that even mean
Game Engine answer - A, because momentum is a property of game objects like cubes, not of the game reality.

ftfy

>>4301034
B is the correct answer.

1) From Chelle's reference frame, objects traveling via portal need not conserve momentum as space is non-homogenous in those regions.

2) From the reference frame of a moving object (Chelle or the cube) the magnitude of momentum is conserved because its motion in space is continous. (This is why when you drop a cube in a portal it keeps the magnitude of its momentum.) The momentum vector isn't conserved because space is still non-homogenous.

Basically, the continuity of space implies that the magnitude of momentum is conserved. While homogenity of space implies momentum is conserved.
This is all very basic physics, taught in undergrad.

To those still arguing for A, maybe this will make it easier to grasp.

>>4302194
this proves A, like i said at the beginning, if there was some kind of energy/magnetic/magic field it would be B.

>>4302213

>>4302222
No it does not, it shows that this experiment does not work on the game engine.
At around 3:20 when he decides to use himself rather than the block, he is clearly projected out, similar to B.
However, for some reason this is at an even faster speed than the platform is moving, for no reason.
The physics seems a bit off on this one, but if it did work, it would be B.

>>4302191

>Use a collapsable tube like someone above said then.

The ends a collapsible tube are in no way independent from each other the way two portals would be. The two ends of a collapsible tube are independent because they are not directly connected. The space "between" two portals is not a tunnel but rather an infinitesimally thin circle.

>>4302213
except there is no specification of how far away the other portal is, if you take the distance at infinity space once again becomes homogeneous, canonical momentum is conserved and the answer is A
since the position of the blue portal has no effect on the problem the answer must still be A if we move the blue portal back beside the red one

>>4302213
\thread

unexpectedly this turned out to be an interesting discussion

>>4302230
take the limit as the length of the tube goes to zero then

>>4302228
Actually maybe that makes sense. He was moving right before the portal touches him, so his speed of ejection might be the speed of the moving platform + his own moving speed.

I would prefer to see this done with a faster moving platform, to be able to see the object shoot out at high speed, but that video seems to confirm B, so I am happy with that.

>>4302242

If I do that we're back at the hula hoop, where the entrance and exit can not move independantly.

>>4302228
IT GETS PUSHED OUT BECAUSE THE ENGINE IS PUTTING PRESSURE, A VECTOR ON CHELL BY PUSHING, TOUCHING HER, WITH SAID MAGIC/MAGNETIC FIELD, IT IS A, HNNNGH.

limit as length goes to zero and shear goes to infinity then

bear in mind that if it's true for an infinitesimal displacment it'll be true for any arbitrary displacment

>>4302236
>>4302236
>if you take the distance at infinity space once again becomes homogeneous

LMFAO. Nope.
The "space" in portal HAS FUCKING HOLES IN IT DUMBFUCK! it is not globally homogeneous no matter how you look at it. THERE ARE FUCKING "HOLES"!

Some regions have local homogenity (regions without portals).

I think it's A.

Think about it. The cube has no starting velocity. In addition, the moving portal does not affect its velocity in any way; it simply relocates the object.

By the time it has exited the blue portal, it will be slipping down the slope due to gravity.

Whether or not it would re-enter the blue portal or not is up for debate, but because the cube is not given any velocity (except for when it is affected by gravity as it slides down the slope) it will not fly.

>>4302236

>>4302264
The answer is B. You are wrong.

>>4302260
>hasn't into his first geometry class

>>4302256

Shear? Now you're just making things harder than they are.

>>4302214

Look at this, as the orange portal keeps moving down, will the platform sticking out of the blue portal extend indefinitely?

If you answer yes, the answer to the original question is clearly B and if you answer no you are wrong.

ITT'
idiots thinking it's B
idiots using words they don't understand to claim it's B

>>4302269
>using Euclidean geometry instead of Minokowski

>>4302272
ITT: The answer is B

\thread

>>4302271
idk what other word I can use to describe the displacement if one end from the central axis

>>4302264

>By the time it has exited the blue portal,

But admitting exiting takes implies it has a velocity. Just imagine standing besides the blue portal looking down into it as the orange portal moves towards the cube. Through the blue portal you will clearly see the cube moving towards you.

>>4302274
>implying they aren't entirely equivelant in this example
>implying relatavistic speeds

yep, you're just shitting out buzz words

>>4302283
you cannot take the reference frame of either portal because they are non inertial

>>4302277

But even if you "displace them" they are still physically one object. The portals are not a really long and narrow tube but an out right discontinuity in space.

>>4302284
The answer is B. I already explained in >>4302213. It is not my fault that you aren't educated enough to understand basic physics.

>>4302260
no

there's an extra pocket, when you take the distance to go to infinity you are no longer sticking two parts of the space together, you're injecting a bit of space behind the portal

so space is still homogeneous

>>4302291
nope.mkv

>>4302285

If you cannot do that then no frame of reference is valid for this problem because you will have to deal with the portals in any case.

>>4302296
you can take the blue portal away off to infinity and sit in the box's reference frame

the answer is A.
it is not by using terms like
>if you take the distance at infinity space once again becomes homogeneous
that you magically become right, not with science.

>>4302297

Yes, and then you're looking through the orange portal just as you take the reference frame of right outside the blue portal.

>>4302293
Are you fucking retarded? Like seriously? I would feel bad for picking on the mentally disabled.

Objects in the proximity of the portal, see space as non-homogenous. You cannot deny that. If you do any kind of observation in a portal reigion of space, you will observe a nonhomogenity of space. Space is not translationally invarient in that region, hence momentum is not fucking conserved.

Stop being fucking retarded.

http://en.wikipedia.org/wiki/Homogeneity_%28physics%29
http://en.wikipedia.org/wiki/Translational_invariance

>>4302306

This thread has literally made me dumber.

For those don't don't understand, the true answer is that neither would happen in the real world because portals break the laws of relativity.

"But what if they didn't?" You might ask. "What if, theoretically portals worked as described in the game Portal?" Well the answer is that because portals don't make sense in real life you would need to make up every theoretical physical rule involving portals, and Valve never made a rule for this situation so it is undefined.

And to those saying it's A or B because think of it from this reference frame or that reference frame or whatever, I'd just like you all to know that while you're probably an intelligent person, you're an idiot.

tl;dr
Which would happen in real life?
Neither, because portals don't exist.
What if they did?
Undefined.

>>4302373

I am glad there are not so many people that think like you because it'd be so destructive for advancement. We are applying what we know to describe the mechanics behind protals, thinking outside the box and seeing what happens. They do not exist? So what

>>4302373

Or you could just use the rules set up in the game, which clearly gives the answer B.

>>4302343
>>4302373
truth

If I had the time I would load up Hammer and make this experiment in the game.

It's A. Think as if you were in the position of the cube. You see that platform coming fast at you. You don't speed, you just stand still.

God dammit...

From one side of the portal/s it is A.
From the other side it is B

The correct answer is not both though, it is neither.

This thread: http://www.youtube.com/watch?v=i2o5wrUnMyA

>oh_look_its_this_thread_again.jpg
the answer is A, and a simple proof is that if you imagine the orange portal only partly covering the box, the B-faggots will say it is 'sucked' upwards by the portal, but there is no sucking force, portals dont do that, so if the box isnt moving, it stays static even if it passes through a portal.

tl;dr B-fags are retards.