My question is how mechanics work with magic in ars magica. In particular - movement.
A Trebuchet throws a stone high in the air. A magus casts a spell to teleport it onto the ground in front of him. Does the stone still move in it's original direction? Does it fly into the air again? Or does it just sit quietly on the ground.
If a person is falling at high speed towards the ground, then you teleport them, do they arrive still? Or do they just slam into the ground sooner?
My understanding of the medieval paradigm is that the teleported stone or person will arrive motionless and still. Momentum does not exist in Mythic Europe; instead, the person falls because his body is seeking its natural place (below the air, above the earth), while the stone is moving because the air it's passing through is pushing it (which was caused by the previous bit of air pushing it, and so on back to the trebuchet). So once the maga casts a spell to teleport the person to the ground, his body is at its natural place, so there's no need for it to be moving. Once the maga teleports the stone, the air around its new location doesn't have any reason to push it in any direction, so it doesn't.
If you like, add in the following rationalization: a Rego spell to move something from point A to point B can move it slowly, quickly, or instantaneously. In the slow and quick cases, the operation of the spell includes rendering the target motionless at the end of the spell (unless the maga wants something different). Why should instantaneous movement be different?
I was going to write out a response, but then noticed just in time that gerg had beaten me to it and done so more clearly than I could have done anyway. But yes, momentum doesn't exist, so things just stop.
We do use "real" physics, and we have tried a few versions, basically either the teleport spell cancels out any momentum UNLESS adding an extra magnitude for avoiding to interfere with it OR, momentum is retained unless adding extra magnitudes(depending on the energy involved or on the speed involved) to have the spell cancel the momentum. The latter has a slightly more realistic feel to it, but the former is far more playfriendly and more "ingame coherent" and so, also totally preferable.
As difficult as it may be for a mind, normally attuned to modern real-world physics, Ars magica Natural Philosophiae does not feature momentum. Thus any teleport spell result in the target losing any speed it might have had at the begenning of the teleport.
So a line of sight ReCo teleport spell is good enough help out when falling, or suddenly having a flight spell fizzle.