This article was originally posted as a reply to Nwkeys a few weeks back, but I thought it was interesting enough to clean it up a bit and re-post it here.
Twice now on Stargate Universe we've seen people coming through the gate from the Milky Way, and getting hurled across the room.
The "Thrown through the gate" thing is never explained, but my hunch is that it has to do with a difference in momentum between the "In" gate and the "Out" gate. This prompts an interesting (no, really) discussion in physics.
Imagine you've got a gate on earth. We're whipping 'round the sun at 800,000 miles a day. Mars is traveling at a mere 663,000 miles a day. (Yeah! It's slower! Less massive, and further out, hence slower). If you gate from earth to Mars, it's not just a question of hopping from one room to another, it's more like you've got two parallel trains running alongside each other, and one is traveling 1500 miles an hour faster than the other. If you jump out of one train and into the other, the difference in velocity would squash you flat and dead instantly.
Now let's assume you want to gate from earth to a planet orbiting the star Vega: Our solar system is moving towards Vega at 181 MILLION miles a day! If you hopped from a train moving 800,000 miles a day to one moving 181 MILLION, you wouldn't be smashed to death, you'd be smashed to subatomic particles!
We've seen people hopping all over the galaxy with no problem. Obviously, then, the gates must have some method of compensating for that, absorbing the massive relative momentum, but there must be some theoretical upper limit, and I'd guess, given how far the Destiny is from the Milky Way, the gate simply can't completely absorb all of it, so you're still coming through as if you'd been thrown from a slow-moving car. Whenever they do a "Big Jump" like that, we see lots of steam shooting out of the sides of the gate, implying it's venting a lot of heat or energy or whatever, and is close to an overload.
This isn't just a Stargate problem, either. Let's imagine you're in the USS Happyprise, a starfleet vessel orbiting Earth. Captain Egoboost decides to beam down to the surface to get some tail. But wait, he's traveling at 17,000 MPH when he's on the ship, and a mere 1050 MPH on the ground. Somehow the transporter has to swallow 1550 MPH of momentum, otherwise the only kind of action the captain's going to be seeing is the spatula used to separate his very flat corpse from the nearest wall.
Of course, being Trek, they pretty much ignore all science in favor of dialectical ideals anyway, but still.
Larry Niven wrote a really good essay on the hard-science problems of teleportation in the early 1970s called "Theory and Practice of Teleportation." It's a must-read if you can find it.