Ragebait answers only
Momentum is conserved when passing through a portal. In the reference frame of the out portal, the object has no momentum, so it plops out.
Which is it Lemmy?
Submitted 8 hours ago by ThatWeirdGuy1001@lemmy.world to [deleted]
https://lemmy.world/pictrs/image/eb98a8db-3d71-4628-a7b2-2e19f3d5e965.png
Comments
crapwittyname@feddit.uk 18 minutes ago
surewhynotlem@lemmy.world 1 hour ago
I_Has_A_Hat@lemmy.world 16 minutes ago
In that situation both portals (the inside and the outside window frame) have equal, but opposite velocities and therefore cancel out any momentum from the person.
When dealing with OP’s scenario, only one portal is moving.
ramble81@lemmy.zip 33 minutes ago
Now what happens if you put it on a treadmill?
thedeadwalking4242@lemmy.world 20 minutes ago
Has to be, for the particles to clear the second portals entry they have to be moving at a relative velocity to the first portal at the same rate the first portal is moving
Peffse@lemmy.world 8 hours ago
ThatWeirdGuy1001@lemmy.world 6 hours ago
Stop stealing all my upvotes
HopeOfTheGunblade@lemmy.blahaj.zone 7 hours ago
Perfectly understood the assignment, 10/10, no notes.
Fedizen@lemmy.world 8 hours ago
There’d be no cube but yes
ButteredBread@sh.itjust.works 7 hours ago
why
victorz@lemmy.world 6 hours ago
In the first moment of exit, it might look like this though
megopie@lemmy.blahaj.zone 2 hours ago
The question is, Is it the velocity of the object relative to the portal that matters or the absolute velocity of the object?
In the later case, we must ask how the portals at different angles don’t send objects flying off at the speed of the milkyway’s travel through the universe.
The answer that requires the least assumptions and justifications is B.
tomi000@lemmy.world 1 hour ago
Exactly. What would that ‘other frame of reference’ be and how would the portal know about it?
Prunebutt@slrpnk.net 1 hour ago
What would that ‘other frame of reference’ be
How bout the other portal?
Simulation6@sopuli.xyz 3 hours ago
If the downward moving portal hits the platform and there is some elastic rebound, then the object will bounce outward slightly.
Rage bait answer: Portals can’t exist, it is from a child’s game.
chuckleslord@lemmy.world 48 minutes ago
Better rage bait answer: portals can’t move relative to each other. If they do, the one that is moving ‘pops’ and disappears from the surface. So this scenario is impossible inside the parameters of the game, so it’s not worth thinking about.
(The devs realized a lot of stuff could get horribly broken if portals could move, so they can’t move)
Demivan@lemmy.world 19 minutes ago
Except that they can move in that one level with gas and lazers )
TotallynotJessica@lemmy.world 8 hours ago
You can’t place a portal on a moving surface in either game, so neither.
lime@feddit.nu 7 hours ago
you can in one specific spot in portal 2!
Sonotsugipaa@lemmy.dbzer0.com 7 hours ago
Two, if you count… the big one at the end
Mika@piefed.ca 5 hours ago
So now we just need to build this test in there and we are set
ThatWeirdGuy1001@lemmy.world 8 hours ago
So the cube is just smashed?
It’s a practical answer which is slightly upsetting because I wanted more discourse.
kkj@lemmy.dbzer0.com 1 hour ago
No, cubes cannot be destroyed by crushing. The portal disappears when the surface moves and then the cube stops the surface’s descent.
viertesauge@feddit.org 8 hours ago
what about the portal on the moon?
alsimoneau@lemmy.ca 3 hours ago
It’s unstable and will dissipate on its own when the relative displacement between them will be too great.
luthis@lemmy.nz 8 hours ago
Well, everything is moving or not moving, relatively
gnufuu@lemmy.ca 7 hours ago
Skill issue
rethnor@lemmy.zip 6 hours ago
You’d make a horrible scientist.
kkj@lemmy.dbzer0.com 1 hour ago
Scientists know when to use existing knowledge.
TotallynotJessica@lemmy.world 41 minutes ago
It said ragebait answers only ¯\_(ツ)_/¯
If it is a question of real life physics, then B would be correct
Kolanaki@pawb.social 4 hours ago
The object’s momentum is preserved. Movement of the entrance portal does nothing.
tomi000@lemmy.world 1 hour ago
Whats the difference between an object moving and everything else moving? For an observer on the sun every one of us has huge momentum which would be preserved when entering a portal. Momentum is always relative to some frame of reference. Momentum ‘toward’ the portal would be conserved and mirrored through the other portal.
Master@sh.itjust.works 35 minutes ago
It would move really fast through the portal then slide down from stand still. So yea B is correct.
ThatWeirdGuy1001@lemmy.world 4 hours ago
I brought this up on another comment that’s for some reason now deleted but, if the universe moves around you, would you not perceive it as modernity?
Derpenheim@lemmy.zip 3 hours ago
There is no such thing as an absolute inertial frame of reference. If you are in free fall on earth, you would experience the same sensation as being in space. The only things that would tell you otherwise are the air resistance hitting you and what you can see.
So no, you would not be able to tell the universe is moving around you unless something else in the universe hit you.
berber@feddit.org 4 hours ago
well, in my opinion it is a bit of a paradox. i think you are partially correct.
your door analogy may be right from the perspective of the platform on which the cube is. however, one flaw is that the whole space is moving, not just the door. as soon as the cube enters that door, it will be inside that space, and if you consider the cube part of it, moving that space “downward”, then due the cube being held by the platform, you could consider the cube moving “upward” in the space behind the orange portal. if you consider preserving momentum there, then as soon as the apparatus stops moving, the cube should not stop its momentum and move further up in the space.
or look at it like this: if you are behind the orange portal, meaning you are looking at the blue portal, you will see a cube moving towards you. then as soon as the platform hits the wall of the orange portal, the preserved momentum should make the cube “fly”.
the change of perspective leads to two different conclusions, therefore i think it is a paradox.
the problem sort of is that there are two “spacetimes” that are kind of being changed. one could interpret it as both of these effects “pulling” against each other since when the cube has partially passed the portal, one part of the cube is “in one of the spacetimes”, ant the other part in the other. ultimately though, the cube then would actually fly because in the limit (infinitesinally close to when the platform hits the portal wall) the cube is completely on the other side of the portal, so from the view of that spacetime, the cube does get pushed and should fly.
i don’t even remember if it is possible in the video game for walls with portals that move. a video game would probablt have some kind of “center of mass”, like a single vector of coordinates where describing the position of the object. there is no notion of “partially” being somewhere (except for collision boxes, but i don’t mean that). so also from that view the box would fly. however, in most video games (with or without portals), if a platform moves up and suddenly stops, the object won’t travel further in that direction anyway, especially not ulwards (even sideways usually not).
Elgenzay@lemmy.ml 4 hours ago
As for the portals moving in the game, portals are destroyed if the panel they’re on is moved except for one specific part, where you can put a portal on a panel moving laterally to get a laser to cut the tubes connecting to the neurotoxin tank. That part needed to be explicitly programmed to allow it, and it’s not possible without cheating to physically reach that portal.
MrNesser@lemmy.world 3 hours ago
More like a hole in the wall with you standing perfectly still while the wall moves past you.
Sonotsugipaa@lemmy.dbzer0.com 8 hours ago
According to a gravitationally accurate simulation a guy did for Youtube, the correct answer is_
… ah, ragebait answers only. Well, mission accomplished, probably >:3
Equinox1289@sh.itjust.works 8 hours ago
For anyone who doesn’t know, Optozorax on YouTube has made a finite element analysis simulation of gravity ontop of his portal simulation so we can get an answer to the question. The answer is both. Both happen depending on how fast the portal is moving.
ButteredBread@sh.itjust.works 7 hours ago
wait is that the alt text? My client doesn’t evem display alt text… I think.
Sonotsugipaa@lemmy.dbzer0.com 7 hours ago
I’m not sure if it’s the alt text or simply the post’s body, but it reads “Ragebait answers only”
Tar_alcaran@sh.itjust.works 8 hours ago
Option B. Portals already break a couple of laws of physics and can reverse entropy, but I like to think you should look at the speed of an object relative to the portal.
toiletobserver@lemmy.world 7 hours ago
Speedy thing go in, speedy thing come out. Clearly explained in game.
rethnor@lemmy.zip 6 hours ago
But I’m this case the cube has no speed, so it wouldn’t just suddenly start moving.
min@lemmy.sdf.org 2 hours ago
What if the blue portal is at the bottom of the Marianas Trench?
NeatNit@discuss.tchncs.de 4 hours ago
The answer is definitely B (see other comments), but I think the more interesting question is, how does it affect the portals themselves and the surfaces they’re attached to?
When the portals are attached to a solid unmovable wall (as in the games), we can pretty much ignore the forces acting upon the wall as they will just be automatically cancelled out by the normal force. But the same does not apply for portals attached to detached panels. Let’s say that instead of a piston pushing down, the portal is attached a free-falling panel with the portal facing down.
We are inclined to assume that as the portal falls onto the cube, it feels no force, as if it fell through air or through a vacuum. But is that really the case? Maybe instead, the falling portal experiences resistance from the cube? Or even, actually, an accelerating force?
Let’s assume conservation of momentum at each end of the portal. That is, in the general case: if the portal (which we assume has no mass) is attached to an object with mass M and velocity V, and “swallows” a cube or other object with mass m and velocity v, and at the end of this interaction the portal-surface object with mass M has velocity V’, then we expect:
M × V’ = (M × V) + (m × v)
Mass is not conserved because the cube was transferred to the other portal.
Solve for V’:
V’ = V + (m/M) v
This is a strange result. The cube’s momentum is imparted into the portal, but the result would be different depending on our reference frame. From a reference frame tracking the cube pre-interaction, v = 0 (by definition) and so V’ = V, the portal-surface object has no change in velocity. But in any other reference frame, it does.
Physics can’t be affected by reference frame (thanks, relativity) so this analysis must be wrong.
… I’ll keep thinking on this.
NeatNit@discuss.tchncs.de 3 hours ago
Okay, after thinking about this a while, I’ve decided that conservation of momentum just doesn’t work, unless the portal actually absorbs the mass of every object passing through it, which would probably cause its own set of contradictions.
Instead, I think forces that act through the portal are applied to the portal. What do I mean by this? Well, take this classic case of a cube floating halfway through two floor portals:
____ ____ | | | | ___++++++++++______----------___
(sorry, can’t do better than ASCII art right now)
The system has reached equilibrium, and nothing is moving. Both halves of the cube are being pulled down by gravity, but cancel each other out by applying a normal force on each other right at the portal interface. What I’m claiming is that this normal force is applied to the portals, which ultimately transfer it to the floor. If this entire floor was a scale, it would measure the calibrated weight of the Weighted Storage Cube.
But this is the static analysis. What happens when objects are in motion? To be continued…
NeatNit@discuss.tchncs.de 42 minutes ago
So we might think: a cube falling through a floor portal and flying out another floor portal right next to it, kinda looks like how a bouncy ball bounces off the floor, and should apply forces on the floor similar to that - an elastic collision. Unfortunately, this brings us right back to conservation of momentum, which didn’t work out. But maybe I just did it wrong? Maybe, if done more carefully, conservation of momentum can work without being dependent on the reference frame?
I genuinely don’t know yet. Time to bring out the calculus.
We have:
M = the mass attached to the portal (e.g. the floor)
V = the velocity of M
V’ = dV/dt = the acceleration of M (assuming no other forces)
F = MV’ = the force applied on the portal
m’ = dm/dt = the rate of mass entering the portal
v = the velocity of the mass entering the portal
If we assume conservation of momentum (our old nemesis), the force applied on the portal should be the same as the momentum taken from the mass entering the portal:
F = m’v
But now we’re back to the case from before, where the choice of reference frame changes the outcome! We’re not having that. So I hereby decree:
The force applied upon a portal by a mass entering it must be calculated by conservation of momentum in the intertial reference frame of the portal itself at the instant under examination.
This gives us, effectively: F = m’(v-V)
Now THAT’S something we can work with!
Note that this formula also works for the exit portal. In that case, m’ is negative.
A curious result follows.
For the entry portal, m’ is positive, and (v-V) is a vector pointing into the portal. (This must be the case, otherwise the mass would be exiting the portal). Therefore, the force would be pushing the portal into the wall.
For the exit portal, m’ is negative, and (v-V) points out of the portal, therefore the portal again would be pushed into the wall.
The result: whenever anything passes through a portal, both ends always get pushed in the same direction.
So, back to my thought experiment.
If a portal attached to a free-falling panel falls onto a cube, the cube’s entry into it would brake its fall to some extent. However, this wouldn’t be the only force in play.
As I said in my previous comment, forces applied between the two halves of an object going through a portal, get applied to the portal itself. In practice, this mostly means tension and compression forces. In the static analysis from before, it was compression.
As the panel is falling onto the cube, a part of the cube is already out the other side and moving away the portal. If the panel has slowed, this would apply a pulling force (tension) on the part of the cube that still hasn’t made it in. This tension force actually compels the portal, and the panel it’s attached to, to speed up towards the ground.
So, in total, would the panel be slowed, accelerated, or unaffected? Maybe the forces cancel out and it just continues on its normal freefall? I don’t know yet. Would need to do more math.
portifornia@piefed.social 6 hours ago
My take:
The cube’s mass is x (eg one gram) and its velocity is zero, therefore its momentum (m*v) is zero. Objects in motion tend to stay in motion, while objects at rest (e.g. momentum of zero), tend to stay at rest.
The thing which is moving, is the portal. At best, the event horizon of it reaches the leading-end then the trailing-end of the cube at the speed the portal is moving over the depth of the cube. If the portal moves faster, that distance is covered faster, and the full cube is entirely on the other end of the other portal faster [than a slower moving portal].
Since going through the portal is a frictionless and forceless action, the speed at which the portal fully envelops the cube would neither add nor remove momentum upon it. And if the cube did have its own momentum it would be maintained, but since it does not, the cube would not move until another force, like gravity, acted upon it.
So, A.
meekah@discuss.tchncs.de 4 hours ago
I don’t think this can work. Think through it in several steps rather than just one. Once the cube just started entering the portal, a part of it will already be on the other side, as close to the 2nd portal as possible. When the portal continues moving, more of the cube needs to be on the other side, so it “pushes” the first part of the cube further away. The cube imparts momentum onto itself.
Another way to look at it is using reference frames. It really doesn’t make a difference whether the cube is moving towards the portal or the portal towards the cube. It only matters what the difference of velocity is. The difference of velocity must be maintained on the other side of the portal, and since the 2nd portal is stationary, the cube must move.
portifornia@piefed.social 6 hours ago
Or, this, but mind blown –> The exit portal would be the thing that moves backwards to maintain the zero momentum of the cube 😅
markz@suppo.fi 4 hours ago
A doesn’t work.
So you’re going really fast through the portal until the point you’ve fully passed it, at which you somehow decelerate or stop?
luthis@lemmy.nz 7 hours ago
Speedy thing goes in, speedy thing comes out.
Cube had no speed going in, therefore no speed going out. The portal speed is irrelevant
FishFace@piefed.social 3 hours ago
The cube has a speed of about 30km/s around the sun. You want to measure speed relative to the blue portal, but why that one and not the orange one?
megopie@lemmy.blahaj.zone 2 hours ago
And if the blue portal is at a different angle than the orange portal, then… well, how fast is the milky way traveling through the galaxy relative to the universe?
untorquer@quokk.au 5 hours ago
But reference frame changes as the cube passes through the portal. The piston would feel resistance as its momentum is transferred to the cube to conserve it.
NeatNit@discuss.tchncs.de 4 hours ago
The “ragebait answers only” in the OP is the only thing stopping me from actually raging at everyone saying A. I am giving them all the benefit of the doubt, they’re deliberately saying the wrong answer.
Everyone saying B missed the assignment and answered accurately.
ThatWeirdGuy1001@lemmy.world 4 hours ago
Where do the facts begin and the rage ragebait ends? Who knows, that’s the fun.
kek_kecske_31@lemmy.world 8 hours ago
All “moving” makes sense only relative to a frame. As there is no absolute frame, for all discussions one need to choose one. It seems that any other frame than the one fixed to the enter portal is arbitrary. How do you distinguish a moving portal from everything else moving around it? Thus the only canonical answer seems to be you preserve the relative speed versus the enter portal as the relative speed to the exit portal.
HailHydra@infosec.pub 5 hours ago
empireOfLove2@lemmy.dbzer0.com 7 hours ago
A. Object momentum relative to the world is retained thru portals regardless of the speed of the portal surface or the portal itself. The portal is only transmitting matter from A to B, it does not impart energy of any kind.
FishFace@piefed.social 3 hours ago
If the portals don’t impart energy, you wouldn’t gain speed when you put one directly above another and drop through forever. The higher portal increases your potential energy.
What if the entire room were rigged to a piston, the orange portal were held still and the room forced upwards at speed. Would you see scenario B then?
If so, how do the portals know which to choose? Remember that neither portal is actually stationary: they’re both orbiting the sun at 30km/s, etc.
hperrin@lemmy.ca 8 hours ago
It went through the portal at speed, so it should exit the portal at speed. That’s my take.
Remember_the_tooth@lemmy.world 7 hours ago
If we could test it, I think it would be helpful to put portals on either side of a very light, plastic disk in an attempt to see if it works like a hula hoop or not. If we drop the hoop over the cube, would the hoop:
-
Just land on top of the cube because the hoop has far less energy than a much more massive cube traveling at the same velocity?
-
Drop around the cube, such that the cube is just sitting on top of the exit portal?
-
Launch the cube into the air with an initial velocity equal to that of the hoop when the cube entered the portal? If so, where does all the additional energy required to accelerate the cube come from if the hoop’s momentum is far less than the cube’s momentum at the same velocity?
-
ButteredBread@sh.itjust.works 8 hours ago
Ignoring the game it would in theory be B because it would need to leave the portar at the relative speed it entered it.
For A to happen it would need to leave slowly which is impossible because it entered quickly and keeps momentum.
YoFrodo@lemmy.world 8 hours ago
[deleted]ThatWeirdGuy1001@lemmy.world 4 hours ago
If the universe moved around you would other people not perceive you moving? Would that not be indistinguishable to movement?
ButteredBread@sh.itjust.works 8 hours ago
speed is relative.
Ignoring that fact tho. it would still exit at the speed the pressure moved. if you say it was static it should not exit at all sonce speed was 0.
Another way to see it is also how every chunk that enters the portal needs to be on the other side instantly. So it would need to exit at the speed the portal moves because if it didn’t not it wpuld not get to the other side instantly and then again, it should keep momentum.
luthis@lemmy.nz 7 hours ago
This is excellent rage bait
ButteredBread@sh.itjust.works 7 hours ago
wut
Th4tGuyII@fedia.io 6 hours ago
C. The moving portal platform breaks the black platform and the whole thing goes through the portal.
Jokes aside, B.
Devoid of any other external forces, in the cube's reference frame there's functionally no difference between it moving towards the portal or the portal moving towards it at a constant acceleration.
Ergo the cube should be pushed out the other side at the same momentum that the system had prior to the collision, and not be simply plopped out as in answer A.
Fedizen@lemmy.world 8 hours ago
The piston moves throught the portal and the portal hovers in the air.
Marty_TF@lemmy.zip 8 hours ago
a. while the portal travels down it collects air, increasing air pressure on the other side, meaning once it reaches the cube, the increased air pressure has mad the air solid so the cube cant fly through it.
NeatNit@discuss.tchncs.de 5 hours ago
Now there’s a high quality ragebait answer
ephrin@sh.itjust.works 8 hours ago
A
Hiro8811@lemmy.world 5 hours ago
What? That cube will definitely get crushed, no idea what’s up with A or B but that cube will get crushed and I’m looking forward to it
AppleTea@lemmy.zip 7 hours ago
B. this is because of Galilean relativity i think maybe
NaibofTabr@infosec.pub 7 hours ago
Assuming the black platform does not fit through the orange portal, and the moving panel with the orange portal stops on impact (which is what appears to be true because on the blue portal side we see the surface of the black panel), then the cube is no longer moving relative to the orange portal and therefore has no momentum. Once the cube is entirely on the blue side of the portal it will be affected by the gravity on that side and slide down as if on a ramp (A).
DarrinBrunner@lemmy.world 2 minutes ago
B. Because it’s relative.
Also, because Cave Johnson is your dad.