# Portal physics scenario

1. Jun 25, 2012

### kidcrumb

I did not create this little picture but it is interesting to say the least. It is a physics question or scenario, i don't know how to describe it but would love the input of the physics forum community. Which do you think would happen? A or B?

Background info. This is from a game called portal. Pretty simple. What goes into the orange portal comes out the blue portal. The question here in the picture is would the velocity of the downward portal transfer to the cube if the cube is not moving. That may not even be a right way to describe it but look up some youtube videos if you need more information on how they work.

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2. Jun 25, 2012

### Muphrid

I consider the "orange" and "blue" portals to be really the same object--they're points in space where the space is really stitched together a bit unusually. Cut two holes out of a piece of cloth and sew them together, and that's what you have here.

To that end, I expect all principles of relative motion to apply. As far as the portal is concerned, the box is moving and will continue to move even as it passes through with the same momentum (relative to the orientation of the portal, that is).

Too much analysis of this would get to the question of whether wormholes can actually exist, so as far as videogame physics goes, I go with B.

3. Jun 26, 2012

### A.T.

It's a tricky one. Usually the portals are static or inertial. So the rule is simple:
relative exit velocity = relative entrance velocity

But here the moving portal will experience a strong (de)acceleration upon impact, just as the box entered it. Or let's assume the portal already starts to (de)accelerate before impact. Then the relative entrance velocity decreases while the box is going through. This would mean the relative exit velocity decreases too, and the box will come out stretched.

If the box cannot strech, then the initially faster part, that comes out first, will be slowed down by the slower part, exiting later. So the net velocity will be somewhere between 0 and the initial high exit velocity.

Last edited: Jun 26, 2012
4. Jun 26, 2012

Staff Emeritus
The game rules disallow (with one exception, which is probably an oversight) moving portals, so I don't think this is defined.

5. Jun 26, 2012

### A.T.

Maybe it's planned for the next part, and the developers are asking for help here.

Constant velocity portals would be no problem. I assume the entry velocity of the center of mass is taken. So the answer would be B.

6. Jun 26, 2012

### A.T.

Here is more:

http://bbsimg.ngfiles.com/1/21876000/ngbbs4caa540391271.jpg [Broken]

Last edited by a moderator: May 6, 2017
7. Jun 26, 2012

### Staff: Mentor

A.T.: The first one is easy, you get crushed by yourself.
The second one is hard to describe, but I think it could work, unless parts of the table/portal borders hit itself. You would see parts of the blue portal exiting the center of the blue portal and could not move it inwards by more than 50%.

As far as I know, the portal game conserves velocity. The downside is a violation of energy conservation. As you can place the portals at different height, it is impossible to define a potential energy and gravity is not a conservative force in this system.
A tunnel (of negligible length) could fix this and accelerate objects moving "down" or "up". This would give the stretching A.T. described, if the box is too weak or the tunnel too short.

8. Jun 26, 2012