Gravity shows up a lot everywhere. We can calculate it using experiments in the game engine, but we run into some difficulties because of friction. And given that we can’t ascertain the effects of friction off-hand in a virtual environment, we have to look into the game engine itself to figure out the strength of gravity.
According to Valve’s wiki, gravity is automatically set to 600 u/s2. Their units, however, are different than the ones I’ve been using. As you may recall from previous videos, I’m calling each wall panel 1 unit. In reality, each wall panel is made of 128×128 game units.
Doing a little bit of conversion, we get
600u / 128u/panel = 4.6875 panels (which easily rounds to 4.7 panels). So, gravity is automatically set to 4.7 panels/s2.
But what’s a panel to us? How does that compare to the real world? Luckily, Valve went ahead and gave us a convenient conversion chart.
According to the chart, 128 units = 2.4 m.
4.7 panels * 2.4 m/panel = 11.25 m.
Inside Portal 2 (and every other game that runs on Source), g = 11.25 m/s2.
To put it in more scientific terms gSource = 1.15gEarth
It sounds weird, but it isn’t surprising. Other game engines have similar tricks, such as the Unreal Engine 2004 running time at 110% speed. Maybe it makes games seem more realistic? Not sure, but it’d be a good question for a psychologist.
Physics with Portals 
[…] we now need a way to convert between the units Tracker uses and panels, our favorite unit for distance in Portal 2. There is a viewpoint difference between objects at the top of the fall and objects at […]
[…] I convert game units to meters by using 128 u = 1 panel and 1 panel = 2.4 m (described in depth here) to find that the acceleration due to propulsion gel is 12 […]