Please help settle a debate between a group of friends. In Portal games, is exit velocity always normal to the portal surface being exited?

More importantly: please offer some kind of undeniable proof that will allow the debate among friends to end. I am nowhere near a machine with Portal installed for several days, but my memory of the series is that yes, exit velocity always is normal to the portal surface. Put more simply: if the exit portal is on the ground, no matter how you enter the entrance portal, you will come out of the exit portal moving vertically up. (Granted, you may be able to move out of that vertical column a split second later because of video game physics that allow forward motion to be initiated in midair, but if you do not press any movement controls after exit, you will rise/fall in a perfectly vertical column and end up re-entering the floor portal you just exited).

  • Added the test with the ball as you suggested
    – WizLiz
    Commented Jun 10, 2014 at 18:56
  • If you voted for my (now self-deleted) wrong answer, go vote for WizLiz's awesome answer below, which uses science to prove the angle matters in both games.
    – Jaydles
    Commented Jun 10, 2014 at 19:24
  • @Jaydles thanks regardless for your help. Both of you guys rock!
    – The111
    Commented Jun 10, 2014 at 20:27

1 Answer 1


No, angle of penetration matters in both games.

The only online article we could find covering this suggests that the angle matters in Portal 2, but did not in Portal 1, but after doing science, we can clearly demonstrate that the article's wrong - entry angle matters in both games.

Since you asked for undeniable proof, the following videos show you this mechanic in Portal 1.

4 tests on the lastest version of Portal 1

Tested on June the 6th, 2014 :

  • Using a ball instead of Chell : (If you are only going to watch one video, make it this one) I tested this on advanced chamber 17. The ball falls through the blue portal and gain momentum (while falling). During the fall I place an orange portal above the slanted surface, as soon as the ball comes out of the orange portal I place another orange portal on the slanted surface giving the ball an angle of penetration (45° I assume), the ball is then ejected out of the blue portal but not vertically, it's propulsed against the wall just in front of me. I repeated this operation several times with different portal placement in order to be sure that this was not due to a collision of some sort and it's not, the outcome should be quite easy to reproduce if you want to test it by yourself.
  • Portal placed vertically : If speed vector was normal to the wall, I should not pass through the emancipation grill
  • Portal placed horizontally : I use the Angle of entry in the orange portal to land way further than the blue one.

To finish, in Portal 2 angle matters aswell but it's way easier to show it since its part of the gameplay with lasers.

Taking advantage of Angles of penetration in game

1. On the playground (a map designed with the purpose of learning those mechanics)

Example on vertical surface : In this example you can clearly see that the angle of penetration is conserved at the exit, allowing to reach the plateform on the left when exiting the portal.

Example on horizontal surface : In this example you enter twice the blue portal and goes out of the orange portal at 2 different angles (and speed but that's not the point here) allowing you to reach the top plateform. If the exit velocity vector was normal to the surface you wouldn't be able to reach the top plateform at all.

2. In an official chamber

This mechanic is for example what allows to finish the room 15 in 1 step in the least step challenge. Once you get into the "loop", you can place a blue portal on the ground and reach the elevator thanks to the momentum and the angle of exit (given by the angle of penetration). Again here, if the exit velocity vector was normal to the ground, when the blue portal was placed Chell should just have jumped vertically. Obviously it's not the case since she reaches the elevator.

  • The second one is two different speeds, not angles.
    – OrangeDog
    Commented Jun 10, 2014 at 6:54
  • @OrangeDog if you watch closely, the first angle of exit in the second video does not allow to reach the top plateform, the speed is indeed a factor but the angles of exit are different
    – WizLiz
    Commented Jun 10, 2014 at 6:56
  • 1
    @WizLiz, Aperture would approve of your use of science here! I'm starting to think you may be right, but one thing's still throwing me - you can move in the air in Portal, so it's not clear to me that the entry angle is the only variable, as opposed to the fact that you come out facing that way and moving in that direction due to pushing the stick that way on exit. Again, not entirely sure - just trying to dissect the possibilities.
    – Jaydles
    Commented Jun 10, 2014 at 16:07
  • 2
    Here is one idea that would remove any doubt about player movement vs momentum. Drop an object (preferably a sphere if those exist) straight down, some significant distance. The object should eventually hit a portal on a surface 45-deg to its trajectory. The exit portal should be on a horizontal surface. Observe the exit trajectory of the sphere from the exit portal. Make sense?
    – The111
    Commented Jun 10, 2014 at 16:53
  • 1
    Well done. Didn't take the time to watch all your videos, but you implemented my suggested ball test to perfection! For science! :-)
    – The111
    Commented Jun 10, 2014 at 20:21

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