Monthly Archives: June 2012


Of course, immediately after I finish a video I realize that I came to an incorrect conclusion. For the fun of it, I’ll post what I was working on below. See if you can figure out where I was wrong (answer after the video).

“We could be wrong…”

If you break down the equations, you’ll actually discover that mass cancels out when calculating the initial velocity of the projectile. In other words, mass doesn’t actually matter. This experiment explores the way in which the Source engine handles an aerial faith plate’s acceleration and velocity more than it does mass. Mass easily could have been arbitrarily high or low and it wouldn’t have affected our results. Unlike what I said in the video, this experiment does not show that mass is an internally consistent variable. Whoops. However, this video does a good job of showing how well Source handles projectile motion, so I’m posting it for now. Expect a follow up with objects of varying mass.

Console Commands:

sv_cheats 1 (only needs to be done once)
noclip (lets you fly around the room)
phys_timescale 0 (freezes objects in the game)
phys_timescale 1 (lets objects move normally again)
impulse 200 (removes/replaces portal gun)


teach with portals and wired’s geekmom

Welcome to everyone from Teach with Portals and Wired’s GeekMom! It’s great to have you here! If you get any ideas or have any questions while you look around the site, I’d love to hear from you. Feel free to email me (cameron *dot* w *dot* pittman *at* gmail *dot* com) or just comment right here.

I’m working on another video right now. Check back later this evening to learn about W=Fd and means of testing the game’s internal consistency.

Edit: And it looks like EnGaming is linking to the Wired article. Hello EnGaming!

the ideal gas law

Portal 2 can help students study more than just mechanics.

Thanks again to Yasser Malaika from Valve for sharing this awesome demonstration of the ideal gas law made using the Puzzle Maker. If you watched the Games for Change presentation last week, this should look pretty familiar (see 22:30).

There’s a lot more to explore with this concept. Expect to see more of it in the future.

Tagged , ,

physics, portal, and reddit

Check out the lively discussion on reddit about this picture:

Of course, there are a few caveats to any discussion about the physics of portals. Portals don’t actually exist and even in the game world they can’t move (except for one special example). So, not only could this scenario not happen in real life, it couldn’t even happen inside the game. That being said, it’s still a fun thought experiment.

I would tend to argue that B is the correct answer and it completely comes down to relative velocities. The cube has a velocity relative to the orange portal, which means it also has a velocity relative to the blue portal. So when the cube pops out of the blue portal, it should have that same relative velocity.

Some of the comments arguing for A use people jumping through hula hoops or door frames in their response. The space on either side of a hula hoop or door frame maintains the same relative velocity; the space around each portal has a different relative velocity in the scenario depicted above.

I like this argument but I don’t agree with it. Reddit user Falconhaxx uses the means of transmission between the two portals to argue for A. Basically, the cube can either instantaneously appear at the blue portal or move through in infinitesimal slices. Either it pops from one portal to the other or it gradually moves through. In the case that it instantaneously appears at the other portal, option A is correct. Rather than doorways, think of the portals as transporters. Whatever state an object is in when it hits the first transporter will be carried over as it instantly teleports to the other. So, if the cube has no velocity when it reaches the first teleporter, it would have no velocity at the second transporter.

On the other hand, if portals gradually move objects through (which is the stance I tend to take), then the cube would be ripped apart by changing forces. As pieces of the cube go through the portal, each feels an infinite force differential which would rip it apart. Think of dividing by 0. The forces on the cube change over no distance at all (gravity is pointing one way when the cube goes into the blue portal but gravity is pointing a different direction when it comes out), which is more than any piece of matter can handle. As a result, the cube would be sliced into infinitesimally thin slices as slice after slice of it moves from one side of the portal to the other (I bet you didn’t know you could use a variant of the word ‘slice’ four times in one sentence).

Of course, the latter argument could be used for any object going through portals. If the game played by those rules, Portal would hold the record for being the world’s first deli meat slicer simulator. Sure, the game breaks a few laws of physics (that sounds like a good topic for a post…), but it wouldn’t be fun if it didn’t.

In the end, relative velocities make much more sense. If the cube has a velocity relative to the entry portal, it should have a velocity relative to the exit portal.

Think I’m wrong? Comment and let me know!

Edit: This guy has a good point too. Does the relative momentum of the blue portal to the cube matter?

terminal velocity part 1

Portals slow you down! Go figure.

A more in depth analysis:

Portals automatically drop your velocity to about 7.8 u/s (or 1000 game units/s) as you go through them, which is why your terminal velocity drops when you go through portals more often. Every time you hit a portal, your speed drops to 7.8 u/s. If you have less distance to fall after your speed drops, your speed has less time to increase. In the longer drops, you have more time to pick up speed, meaning your terminal velocity, as measured in this experiment, will be higher.

This video is meant to describe a way to quickly and easily run experiments in a classroom. Without knowing that portals affect your velocity ahead of time, this experiment would give students a strong indication that something weird is going on and the relationship between portals and velocity is worth investigating.

Console Commands:

sv_cheats 1 (must be done first but only has to be done once)
host_timescale 0.001 (slows time to 1/1000 speed)
host_timescale 1 (brings time back to normal)

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