So if i cast an ongoing area of effect spell can i center it on something, so if the something moves it goes with it? Like casting physical barrier on a vehicle. Another example would be Silence. Can i cast it on myself to help me be more stealthy? Or do i have to target a specific place and it never move?

Long Answer: {{ Insert long-winded discussion of the relativity of magic and spellcasting. }}

Simple Answer: GM fiat.

Why do people on this board keep talking about GMO like its a bad thing?

Anyway, if the rules were clear, comprehensive and easy to understand, what would we have to argue about?

actually, no.

So i could Physical Barrier a slow moving vehicle?

Why do you insist on assaulting us with facts?

theoretically, yes. but it would have to be *really* slow, or the barrier would have to be way bigger than the vehicle, if you don't do a little houseruling. probably your best bet in such cases would be to simply use the armor spell on a vehicle.

Thanks, the replies on this board are amazing fast and very informative.

Actually, yes.

Your quote refers to manually moving an area of effect spell. It does not address, say, casting an area of effect spell on a cruise ship, in an airplane, inside a moving structure, or outside a moving structure. That requries GM fiat. Which is not a bad thing, despite people wanting to read it that way for whatever bizarre reason. It just means it's the GM's call because the conditions and scenarios are many and varied and there is no single right answer because of how relative the question can be. Especially since the Earth itself is spinning on its own axis, orbiting a star, spirling around a galaxy, and moving through the universe. And God knows what's beyond that.

how about this from the FAQ at shadowrun4.com

Ok, so on further reading, I'm discovering things I didn't know feel free to disregard my above post.

Its not definitive on the subject, but theres two takes you can go with for it. 1) Like a ward, the spell is centered relative to the vehicle/moving body and moves with it.

2) it doesn't, and is fixed to the earth (since all spells are it seems).

I'd probably go case by case with it, and say if the spell effect is smaller than the vehicle, then it can travel with the vehicle. Such as a clean air spell while you're going up in a trapped elevator. Whereas physical barrier around a vehicle, I would say moves relative to the earth, and the vehicle would have to go slowly.

Well technicaly the rules don't say anything about the RATE at which a spell moves, just that it must remain within LOS, so if you are continually moving the barrier spell as a complex action I suppose you could keep it up with your vehicle continualy.

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But thats a rules lawyer answer which is dumb and I hate. A better question a GM should ask himself is 'how would allowing moving barriers and AOE effect the balance in my game?'

A complicated and multi-faceted question. Some situations are obvious. When casting a barrier within a cruise ship or plane or other moving vehicle the barrier of course will move with the vehicle by default. But casting armor around a moving vehicle could be abusive certainly. The silence example is less so, but would be intruding in on the area of the Stealth spell, so is disallowed as well.

Building on these examples, I have generated a house rule/interpretation which fits with game logic even:
Magic is semantic in nature. Differences which may seem only semantic to us are important to it. Thus a barrier spell only works when it is performing the function of a barrier, that is to create a static impediment against objects. When a barrier spell is cast around a moving vehicle, it begins to function more like armor and so the spell fails. Similarly the spell silence is designed to dampen noise within an area, not to cover up the sounds you make when you move. So that what it does. In both these cases the spells area may still be moved as a complex action, but only while they are still performing there intended purpuse. Arbitrary? Maybe. But magic is like that.

Basically I think you should look at each spell and ask yourself, what is this spell purpose? When performing that purpose it succeeds, when expanded outside of it, it fails.