Just playing around with some ideas for magical traps, when I considered conjured water.
CrAq can create water, whether it is room temperature, scalding hot, or ice, with no requisites. Any wetting effect disappears when the water's Duration finishes.
I could flood a library for Diameter duration, until I recalled that ink can run and vellum wrinkle.
But flooding a deep chamber - while grogs might float up to air, what about Magi protected by Parma Magica?
There is nothing to breath underwater, but could the magus float up?
Disregarding that after Diameter duration, the floaters will fall.
If you assume that Archimedes' principle (a body immersed in a fluid receives an upward push equal to the weight of the displaced fluid) is what makes stuff float in ArM5 physics, then no, magi protected by the Parma Magica would not automatically float, because any pressure exerted by the magically created water would be blocked by the Parma.
Even under fuzzier Aristotelian physics, where the four elements have a tendency to seek their natural vertical ordering (earth at the botttom, then water, then air, then fire), you could say the reason why a light object floats is that water "forcefully" tries to get to its proper position below the object, so again the magus would not naturally float, for the same reason he would not be crushed by a magically created rock created just over the top of his head.
Then again, you could attribute the floating upwards of a light object in water not to the water trying to get below it, but by the object naturally rising to its proper place. In this case, the magus would float. I like this interpretation less, but it's probably slightly more in line with what Aristotle thought. Aristotle attributed the tendency of a sealed, heavy container to float upwards when submerged in water to the upwards push of the air within the container (Aristotle rejected the possibility of a void) that not only won over the natural tendency of the container to move downwards, but also won the natural tendency of the water to stay were it was. And you could make the case that a body of a man is mostly water, with a little earth (the bones) but a fair amount of air trapped in the lungs (that's why when you have taken a large brearth you float more easily). The latter trying to move to its proper place is what tends to pull the man up, and Parma certainly does not resist it.
A magus can walk over created wood or earth just fine. There's no immediate reason to think one wouldn't be able to swim on created water or to think a magus wouldn't float.
Lazy answer: friction is a modern concept that doesn’t exist in the medieval paradigm. What exists is “you must exert effort to move". So, if the magus is making effort, yes, he moves.
More complex answer, but also even lazier: MR as stated in the rules is impossible to implement without having some very, very weird corner cases, regardless of you using a medieval or modern understanding of physics/metaphysics. In the end, you will only be able to "perfectly" solve it with judicious use of handwavium (that's the lazy part: don’t bother to fix it, just handwave as required).
Isn't that related to the question whether you can ride a magically created horse (The Wizard's Mount)? Sure, you can say you need a non-magical saddle and bridle, but I prefer the handwaving.
I also want to make it all work, but objectively speaking, that is trying to conciliate three very different (and incompatible) ideas: 1) magic exists, 2) despite of that, the world works basically the same and 3) except when it doesn't (mostly in your favor).
Eg.:
• If the maga falls onto a magical bridge, she is stopped by the bridge, and takes falling damage.
• If the magical bridge falls onto the maga, it is stopped, but does her no damage.
This makes absolutely no sense, and there's no way to conciliate it with how physics work. You can say "it's magic" but magic doesn't work in a consistent way either, so you don't have a general rule. You have a handful of exceptions which you must reference in a case by case basis.
Einauge brings a good point. If you have MR and it isolates you from magically created things you can't (in theory) ride a magically created horse.
Or rather, going by the bridge example above: you can ride it, but it can't ride you. You can punch it, but it can't punch you. If it kicks you upwards, you don't even move, and it's kick doesn't hurt you. But if it gently moves you up, yeah, that works.
I see why you think that, but be sure that I say "handwavium" intending no disrespect.
If you want, say that MR follows the Two Laws of Magic Resistance:
When MR is involved things work in the way that benefits the person with MR the most
When MR is involved things work as closely as possible as they would without MR, unless that would conflict with the First Law.
(The only problem with those laws is that 1) I don't really like them; 2) they aren't clear cut; and 3) I can think of a handful of cases in which I'd not follow them.)
