/sci/ - Science & Math

[Anonymous No.5188164]

[Anonymous No.5188169]

>>5188164

How would momentum be transferred to the block in case B?

[Anonymous No.5188172]

>>5188169
its not which is why B is the case. In order to stop B you'd need to transfer momentum.

[Anonymous No.5188176]

>>5188164
All that would happen is the brick would appear out of portal A very quickly, but with no momentum to keep moving it would just fall.

[Anonymous No.5188181]

Relative to the portal, it would have speed, but none on its own. So it would just go plop.

Y'know, portals are actually a great way to demonstrate conservation of energy. We need to get Portal in schools.

[Anonymous No.5188187]

>>5188176
how does it appear quickly without momentum?

[Anonymous No.5188184]

[Anonymous No.5188191]

>>5188176
This. Block has no momentum.

[Anonymous No.5188193]

>>5188181
the concept of a portal is a discontinuity in space. By Noether's theorem continuous symmetry of the Hamiltonian if and ONLY if conservation of energy.

[Anonymous No.5188195]

>>5188187

Portals.

>>5188172

Don't you mean "which is why A is the case"?

[Anonymous No.5188200]

>>5188191
then how does it emerge from the exit?

Ignore the whole going in part. Let that be on the other side of the world, you have know way of knowing anything about that. Watch the exit portal, and what happens. First a sliver of one side comes out, then a little more, then a little more. Its obvious that if you take an atom on the top side it will have a net velocity as it emerges, otherwise the whole object pancakes. So why does that velocity disappear?

To see it more clearly, let the portal not stop when it gets down past the cube, let it keep going such that the platform the cube sits on is small enough to fit in the portal. Its obvious that cube + platform+column will exit the exit with some constant velocity as given by the motion of the entrance portal. Why would that velocity go away if the entrance portal stopped?

[Anonymous No.5188205]

A
momentum is a vectorial quantity and therefore cannot be transfered into other directions

[Anonymous No.5188215]

>>5188195
>Don't you mean "which is why A is the case"?
No. Its not a teleporter. Things pass through it in part. if the box passes halfway, then the half that already exited gets displaced by the next half. It has momentum, it must. So since there's no mechanism to transfer momentum it can't slow down instantly.

[Anonymous No.5188219]

>>5188200
the portal makes the space not simply connected, it changes what the "next" point is, no other property of matter.

[Anonymous No.5188221]

>>5188205
momentum conservation does not apply in a world with spatial discontinuities. This is a mathematical truism.

[Anonymous No.5188222]

Just did this map in source in portal 2

B happened. I took the speed lines suggest "super high speed".

[Anonymous No.5188224]

>>5188219
Doesn't matter, just looking at the exit portal, the box will move.

[Anonymous No.5188238]

>>5188224
No it shouldn't.

Think about it this way, portals are just teleportation devices.

So if you are traveling 5 mph and teleport, where ever you end up you'll still be going 5 mph.

Now if you travel into a portal at 5 mph, then where ever you come out you will still be going 5 mph.

If you are holding perfectly still and teleport, then where ever you end up you will still be holding still.

If you are holding perfectly still and a portal hits you, then where ever you come out you will still be holding still.

[Anonymous No.5188250]

>>5188224
Believe momentum is conserved relative to the object moving through the portal.

[Anonymous No.5188252]

>>5188238
>So if you are traveling 5 mph and teleport, where ever you end up you'll still be going 5 mph.
I don't know I've never done it. Seriously, this abstraction is less well defined than the portals.
>Now if you travel into a portal at 5 mph, then where ever you come out you will still be going 5 mph.
If the entrance and exit are not moving relative to one another then yes, if one is moving relative to the other than no.
>If you are holding perfectly still and a portal hits you, then where ever you come out you will still be holding still.
Nope.

[Anonymous No.5188258]

>>5188250
I don't know what you're arguing for.

[Anonymous No.5188272]

>>5188164

It entirely depends on how you think a portal works.

Since we have no portals, much less different versions of them, you can't answer the question.

[Anonymous No.5188278]

If it were B, then
1) moving a portal over a stationary object causing the object to move would mean
2) moving an object into a stationary portal would cause the portal to move

But we KNOW (2) is false.

Hence (B) is not correct.

[Anonymous No.5188283]

>>5188272

It's already defined how the portals in portal work.

Speedy thing goes in, speedy thing comes out. But because the box has no momentum it can't be speedy when it comes out.

[Anonymous No.5188297]

>>5188222

Only thing I've seen happen in game is the portal stop when making contact with the unmoving box.

[Anonymous No.5188322]

>>5188283
why do you say the box has no momentum? Everything even sitting still is spinning through space at high speeds. Its all a matter of how define the rest frame. The box moves relative to the portal. It enters speedy, so it must exit speedy.

[Anonymous No.5188324]

>>5188278
Not this poster, but also, if B were correct, then lowering the top portal to about the halfway point on the box, then moving that portal laterally would cause the object to be split in half, yeah?

[Anonymous No.5188325]

This thread is getting old.

btw: everyone knows it's B

/thread

[Anonymous No.5188334]

>>5188322

But the space around it is also moving speedy so its inconsequential.

[Anonymous No.5188343]

>>5188325
> the_matrix.jpg
> it's b
obvious troll is obvious

[Anonymous No.5188346]

>>5188334
what if the exit portal was on the moon?

[Anonymous No.5188347]

>>5188322
The rest frame is clearly the POV of the picture itself. In it, the cube is stationary. It must stay so.

[Anonymous No.5188355]

>>5188346

Then the other portal would suck the cube up as it jettisons all the gas in the room into space.

[Anonymous No.5188360]

>>5188347
Nope.

[BKitteh !!gCuV52FdfK/ No.5188362]

http://en.wikipedia.org/wiki/Newton%27s_laws_of_motion

[Anonymous No.5188365]

>>5188362
don't really discuss discontinuities in space

[BKitteh !!gCuV52FdfK/ No.5188370]

>>5188365
True

[Anonymous No.5188384]

Remember the portal has no special properties, it literally makes a window from one place to another. It conserves momentum and not much else.

If you had a platform moving at several thousand miles an hour with a portal on either side of it and a person jumped up at the right time to travel through it then, assuming he doesn't get hit full on in the face by it, he'd just pass through it. He wouldn't be sent flying back at faster than the speed of sound.

[Anonymous No.5188386]

>>5188360
>>5188278

[Anonymous No.5188392]

>>5188365
there's no discontinuity, just take the fucking integral of mass over time on the blue side and multiply it by the fucking delta velocity of the orange side of the portal, what's so fucking hard to understand you assburger-riddled piece of shit, an object that is in motion will stay in motion until acted upon by a motherfucking force, and conservation of momentum, you piece of shit, read the fucking goddamn article and fuccking understand it you nigger

[Anonymous No.5188403]

Portals can't be placed on moving surfaces for exactly this reason, Valve couldn't figure out what it meant for momentum. The question itself is pointless as there is no real world reference for such a phenomenon

[Anonymous No.5188397]

>>5188384
THATS BECAUSE THE FUCKING FRAME OF REFERENCE DOES NOT CHANGE AND NO FORCES ARE oh my fucking god. you gotta be trolling.

[Anonymous No.5188422]

>>5188403

But isn't the source engine big on physics?

Surely all they had to do was make it so the portals could except objects while moving and then see what their engine made of it, like a normal physics model.

[Anonymous No.5188429]

>>5188422
accept*

[Anonymous No.5188437]

>>5188422
it wasn't, no

As I recall they encountered something similar to the kind of catastrophe which plagues research into wormholes (they have this habit of collapsing in various models without positing some kind of negative energy).

[Anonymous No.5188443]

>>5188422
Its as big on physics as a game can be

Physics accomodate to gameplay, you can steer in midair etc

It basically depends on the way the portal works,

[Anonymous No.5188444]

>>5188437

But the whole draw of the source engine was mad physics it could do, the gravity gun and realistic moving bits and pieces. That was what Valve focused on when they made HL2.

[Anonymous No.5188451]

>>5188444
> physics itself undecided about what happens with wormholes
> but source engine
Are you even listening?

[Anonymous No.5188460]

>>5188444
>mad physics
>just throwing shit around
> x= v0x*t, y= v0y*t-g*t²

its just a game engine, realism takes second place over gameplay

[Anonymous No.5188461]

Fuck off back to /x/ with this crap

[Anonymous No.5188465]

>>5188451
>>5188460

But didn't they hardwire it to make it so physical objects can't go through moving portals?

[Anonymous No.5188472]

>>5188465
I dont think they ever got to do moving portals in the first place

[BKitteh !!gCuV52FdfK/ No.5188477]

>>5188472
http://www.youtube.com/watch?v=yxNudEkQ7B4&feature=relmfu
http://www.youtube.com/watch?v=Vrtqinq9oHA&feature=relmfu

[Anonymous No.5188478]

A

[Anonymous No.5188511]

for the idiots

can I please rest in peace tonight.

[Anonymous No.5188517]

>>5188477
It looks like they implemented it somewhat, but it doesnt look to me like it was meant to look like that if it ever got into the game (I played through portal 2)

after doing a little bit of research
https://developer.valvesoftware.com/wiki/Portals_on_Moving_Surfaces
"
Portals on moving surfaces behave differently than static portals. They calculate physics much more loosely and can fling passing objects around at strange speeds and angles."

This makes the things even more unreliable they would be for just belonging in a game

[Anonymous No.5188534]

>>5188472
They implemented moving portals in Portal 2, but not very well:

http://www.youtube.com/watch?v=S85nudR6D-Y

[Anonymous No.5188566]

>>5188511
what if it suddenly stops halfway. it will rip apart right ?

[Anonymous No.5188616]

>>5188566
So then every atom that goes through it has 0 velocity right?

Why wouldn't it come out as a pancake then?

Oh what's that you say? So the next set of atoms pushes the last set out with the same speed with which they enter, but then as soon as the very last layer of atoms of the object exists the portal they all magically know to just STOP

[sage No.5188643]

>>5188566
50% of its mass would have velocity 1
50% of its mass would have velocity 0

0.5*1 + 0,5*0 = 0.5

it would have a momentum of 0.5

would go at half speed.

it would behave much like a magnet in a copper pipe.

[Anonymous No.5188653]

>>5188566
no. it would have half as much momentum. 1/2m*v instead of m*v, but its still a solid object unless the velocity is insanely large it would stay together with that momentum.

[Anonymous No.5188657]

I just came up with a biggass reply and the thread 404'd on /v/. Fuck the police, I'm posting it here.

Because the way people think right now makes both answers wrong.

It's just a theory about how you could make moving portals work in theory:

The portals create an exact copy of your universe and link them.

Let's start with non-moving portals to illustrate what I'm saying. You're sitting in a room as the observer with Chell (the character in the game) and her portal gun, and she opens up portal entry and exit. In my theory she created an exact copy of the universe that she can now go into. Which means that there is another Chell on the other side who's going to do the exact same thing as her.
This still works with the game itself, since you can see yourself if you place the portals right, and by seeing yourself you can easily imagine that you're looking at your duplicate in the universe you just created.
Both Chells will do the exact same thing in their universe, so if you see the Chell of your universe enter the portal (leave the room), you'll see the Chell from the other universe enter yours.

What most people would think at this point as the observer in that room is that the portals teleported your unique Chell form your unique universe from point A to point B. Which is the point of my theory: thinking like most people do, that it all takes place in one universe, allows neither answer to be true, so why not imagine portals DO create that universe? That would allow moving portal theories. Everyone would be happy.

That way, in this problem, the cube, would enter the orange portal, and you'd see the cube from the OTHER universe fly out of the blue portal since it conserves the momentum it had in your own frame of reference.

[sage No.5188673]

>>5188657
that has absolutely nothing to do with anything, unless the laws of physics of each universe are not compatible.

[Anonymous No.5188696]

>>5188673

But I feel like it has something to do with the OP

I made the reply B plausible, as well as thoughts about moving portals, which were impossible in the first place if you didn't think with two universes and made replying to this then solution-less problem pointless

Unless there's a major flaw I didn't think about

[not op No.5188718]

>>5188696
b is plausible because it's what happens because of physics, whether it's in the same universe or in multiple.

[Anonymous No.5188726]

[Anonymous No.5188730]

[Anonymous No.5188747]

>>5188718

It's the better answer but it's still flawed

[not op No.5188754]

>>5188747
how is it flawed, look at this gent's intuitive explanation if you can't grasp variables and physics and shit: >>5188730

[Anonymous No.5188781]

Ya, this idea of portals is greatly flawed.

To see that, shoot a portal on the roof and a portal on the ground
>infinite gravity
There would be infinite earths below you, but above you there are none.

So here is how the portal game should have gone:
>The portal will now open in 3, 2, 1
>insta-blackhole, end of universe
>game over

[Anonymous No.5188790]

None because this can't happen.

A is the most plausible but again, this can't happen so neither.

[Anonymous No.5188801]

>>5188754

for starters momentum (the vector, that's right) is only conserved in case B if the entrance and the exit are parallel

[Anonymous No.5188811]

B is the same logic as you suddenly gaining momentum when someone passed a hoola hoop through you really quickly.

But fucking portals man, they can make up any shit they want.

[Anonymous No.5188839]

>>>/v/

[Anonymous No.5188856]

>>5188811
>hoola hoops transport matter conserving linear momentum

[Anonymous No.5188937]

>>5188181
> portals are actually a great way to demonstrate conservation of energy

They're really not, pic related.

[Anonymous No.5188941]

>>5188937

kinetic energy only

[Anonymous No.5188943]

>>5188941

Still a big fat nope. You can accelerate indefinitely for free by placing two portals above each other. Therefore kinetic energy is not conserved.

[Anonymous No.5188946]

>>5188941

But that's not how conservation of energy is done in schools, ever.

[Anonymous No.5188975]

I think i haveth something. Look at it like this:
You have the world around the cube and it's stationary in comparison to the cube right? But the world inside the orange portal is moving rapidly and it is in that world the cube soon will be in so it should still be moving just as fast in comparison to that world once it has passed through the portal.

I hope i made sense otherwise read it again more slowly.

[Anonymous No.5188984]

>>5188943

Yes it is, it's a never ending cycle of Potential and kinetic. But due to drag and friction over time it violates conservation

[Anonymous No.5188997]

[Anonymous No.5189004]

>>5188984

That's why portals have to randomly start eating people and pooping out demons after awhile.