### Author Topic: Portal Physics  (Read 13736 times)

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#### harold

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##### Portal Physics
« on: October 13, 2013, 06:22:03 pm »

(spoiler alert: if you try this in the game, the cube refuses to go through at all)

I'm going for B, because:
1) Option A implies the cube keeps whatever momentum the cube had in relation to the "ground". That means that:
1a) if you take a cube and a portal on a fast train and then slowly push the cube through, it comes shooting out the other side at high speed. Ok, then what happens when the cube is halfway through? The back end is moving into the portal at a slower rate than the front end is leaving the exit portal. So the cube is ripped apart.
1b) in the setup in the picture, the front half would not be moving at all, so it wouldn't actually be going through at all. Of course that means you couldn't have stuck in the front half either.
2) the cube has a relative velocity with the entry portal, it must have the same relative velocity with the exit portal
3) the argument that throwing a portal over an object is like throwing a hoop over it isn't really true - a hoop has the "exit" moving at the same speed as the "entry", and indeed the cube "exits" with the same relative velocity as it enters, and that leaves the cube stationary just as it would if the blue portal was pointed upwards and falling at the same speed as the yellow portal is falling (in that case the cube would move through space but gain no momentum).
4) from the perspective of the yellow portal (and choosing an other inertial reference frame like that is OK), the cube is moving and some speed and it will keep that speed at the other side.

Thoughts?
« Last Edit: October 13, 2013, 06:27:17 pm by harold »
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#### zeldaking

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##### Re: Portal Physics
« Reply #1 on: October 13, 2013, 06:30:58 pm »
I would say, flip this diagram upside down (the left side only) the cube now looks like it is falling into the portal. Its all on perspective. The portal moving into the cube is the same as the cube falling into the portal, thus the cube should keep its relative speed to the yellow portal and shoot out as in section b.
« Last Edit: October 13, 2013, 06:31:15 pm by zeldaking »

#### Runer112

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##### Re: Portal Physics
« Reply #2 on: October 13, 2013, 07:12:53 pm »
I thought it was an established rule that portals can't exist on moving surfaces?

But disregarding that, I believe option A would occur, because portals are simply spacial links and do not impart any kinetic energy on objects passing through. Perhaps this slightly modified scenario will help demonstrate this:

If you were standing next to the blue portal looking in, as the orange portal fell, it would appear as if the cube had velocity. However, it would also appear as if the ground below it had velocity. As the orange portal fell until cube was about halfway through the portal, suddenly the portal would stop moving due to hitting the raised ground around the cube. What would happen?

I think we can safely rule out the cube being spontaneously sliced in half, with the half that didn't go through the portal staying still and the half that did go through the portal flying off with the same velocity that the orange portal had before stopping. I also believe that it wouldn't really make much sense for the cube to fly off with half the velocity, as from the perspective on the orange portal's side, the cube would appear to be sucked away by nothing. It seems to me that the only logical outcome in this scenario is that the cube stays right where it was, and I believe this should be the case no matter what percentage of the cube goes through the portal.
« Last Edit: October 13, 2013, 07:14:53 pm by Runer112 »

#### harold

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##### Re: Portal Physics
« Reply #3 on: October 13, 2013, 07:15:28 pm »
Why is being sucked away not OK? (with half the speed of the yellow portal because only half the mass got the momentum)
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#### Roondak

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##### Re: Portal Physics
« Reply #4 on: October 13, 2013, 07:48:09 pm »
I agree that the portal would transfer no momentum, however as out portal is at 45 degrees, the box would tumble out.

#### aeTIos

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##### Re: Portal Physics
« Reply #5 on: October 14, 2013, 01:10:31 am »
Indeed, portals can't exist on moving objects, thus ruling out this scenario.
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#### AssemblyBandit

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##### Re: Portal Physics
« Reply #6 on: October 14, 2013, 01:25:51 am »
Going with Runer112's 'A' diagram. No matter how fast the portal is traveling, the object itself has no momentum. The faster the portal is moving, the faster the object peeps out the other side. Whats cool is that if you placed the other portal at a near 90 degree angle and adjusted how far the moving portal was held above it, the object itself would start tumbling in place. Which is the same reason why portals can't exist, infinite energy.

Awesome game by the way!
« Last Edit: October 14, 2013, 01:26:37 am by AssemblyBandit »

#### Eiyeron

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##### Re: Portal Physics
« Reply #7 on: October 14, 2013, 02:58:06 am »
ANd that's why you cannot place portals on a moving panel! :p

#### harold

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##### Re: Portal Physics
« Reply #8 on: October 14, 2013, 03:45:18 am »
There is a moving portal in Portal 2, and you can set a var in Portal 1 to make it possible in custom maps. The game reacts to this situation by not letting the cube through.

Say the yellow portal is moving at speed v (relative to the ground and therefore the cube). When the cube is starting to go through, the part that comes out the blue portal must have speed v (at an angle and upwards though) because that's the speed at which it is being pushed through.
The cube has speed and therefore momentum.
« Last Edit: October 14, 2013, 03:47:47 am by harold »
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#### willrandship

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##### Re: Portal Physics
« Reply #9 on: October 14, 2013, 03:47:11 am »
If portals cannot be on moving planes, then why can they be on the earth as it hurtles through the universe?

If it's relative to the gun, then why can the gun move?

If it's relative to each other, then why can the portals be on a planet with separate linear velocities due to curvature and rotation?

Lots of holes. I agree with the transfer of momentum.

#### Keoni29

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##### Re: Portal Physics
« Reply #10 on: October 14, 2013, 03:51:03 am »
Yep. The other end of the portal is stationary thus the block will have momentum when it comes out the other side.
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#### AssemblyBandit

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##### Re: Portal Physics
« Reply #11 on: October 14, 2013, 04:08:33 am »
So if you reach up at a portal falling down and it stops, your arm gets ripped off?!!!

*This is pretty interesting, going to google it!
« Last Edit: October 14, 2013, 04:10:59 am by AssemblyBandit »

#### harold

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##### Re: Portal Physics
« Reply #12 on: October 14, 2013, 04:21:10 am »
If the portal is traveling fast enough, I suppose so yes..
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#### AssemblyBandit

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##### Re: Portal Physics
« Reply #13 on: October 14, 2013, 04:34:23 am »
Wow! Changing my answer then, if a portal falls onto a stack of blocks, the force from the blocks below pushing the others up would send them flying! Portals would suck you up or rip off limbs if not careful

#### willrandship

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##### Re: Portal Physics
« Reply #14 on: October 14, 2013, 04:57:08 am »
It would apply a force. A calculatable force.

Arm ripping would be dependent on joint strength relative to said force.

Keep in mind the only thing pushing back for the obtained momentum would be air friction. Think of sticking your arm out a car window. Two separate mathematical velocities: You in the car, and your arm going down the road at 55 MPH.
« Last Edit: October 14, 2013, 05:00:27 am by willrandship »