Momentum, a function of mass and velocity, is conserved between portals. In layman's terms, speedy thing goes in, speedy thing comes out.

GLaDOS: "Look, you're wasting your time. And, believe me, you don't have a whole lot left to waste. What's your point, anyway? Survival? Well, then, the last thing you want to do is hurt me. I have your brain scanned and permanently backed-up in case something terrible happens to you, which it's just about to. Don't believe me? Here, I'll put you on:"
Strange voice: Hellooooooooo!
GLaDOS: "That's you! That's how dumb you sound! You've been wrong about every single thing you've ever done, including this thing. You're not smart. You're not a scientist. You're not a doctor. You're not even a full-time employee! Where did your life go so wrong?"

We are pleased that you made it through the final challenge where we pretended we were going to murder you. We are very very happy for your success. We are throwing a party in honor of your tremendous success. Place the device on the ground, then lie on your stomach with your arms at your sides. A party associate will arrive shortly to collect you for your party. Make no further attempt to leave the testing area. Assume the Party Escort Submission Position, or you will miss the party.

Let's be honest. Neither one of us knows what that thing does. Just put it in the corner and I'll deal with it later.

This is your fault. I'm going to kill you. And all the cake is gone. You don't even care, do you?