I believe the main Ars Magica book states that inertia is not part of the Medieval Paradigm, so that a magically moved object resisted by parma will hit with no force and do no damage.
If there is no inertia, it would seem that you cannot magically fling something and have it continue to impact without the magical effect still being active (and therefore resisted). Without inertia, when the magic ended the item would stop moving.
However, if I recall correctly the spell "Wielding the Invisible Sling" doesn't describe the need to Penetrate to affect the target.
So is the spell incorrect (it must Penetrate) or am I missing something?
IIRC things move under the effect of an Impetus, and are kept moving by the impetus imparted by air moving in behind them (?!?!?!)
Not an explanation that makes any sense in modern eyes - don't try to analyse it that way. However, it means that spells can variously hurl things and "let go" or keep them under a continuing (magical) influence.
The former are Aimed, but their motion is no longer magical; the latter (inc. WtIS) do not requie an Aiming roll because the magic guides them there all the way - and their motion is resisted.
No spells really describe the need to penetrate - simply because they all are understood to have to penetrate! Only a few targeted spell dont have to - such as creating a hole under someones feet et cetera.
Just because there is no concept of inertia it doesnt exclude a concept of motion. Just as a person throwinig a rock is in fact able to do it without having to carry and run with the rock all the way to the destination, nor does magic. The throwing of a rock, by a person, is in fact effectuated by his will and actions and those are the force that carries the rock to its destination rather than inertia.
Thus the magic doesn't just throw the object -or launch or catapult it to use other phrases- it is a force that carries the object constantly. The moment the magic stops the object would fall straight to the ground.
I have to disagree with you Caribet, but this will become an increasingly scholastic debate
I disagree that magic can create motion that can continue beyond the 'duration' of the magic. An object stops dead in its track the moment the magic subsides - so launching something with magic and then releasing it will not let it continue its otherwise natural path (in terms of science). Thus an object can only be propelled against a target if done so by magic, and thus it is resisted.
Aimed spells are for targeting the enviroment of the real target (the target of your action as opposed to the technical target of the spell's effect) and thereby harming or influencing the target indirectly - the aiming is needed partly because you'll often need to get as close to the MR as possible without trickering it and partly because you need a measure of dependence on the natural laws of ME.
The only way you'd be able to hit something without it being resisted is by using Rego to place an object directly above their heads and then releasing the magic - the direction of the fall cannot be influenced only the object's directly and natural fall toward earth will steer it (in lack of the word gravity). And as a sidenote the object would still have to be non-magical or penetrating to damage someone with MR.
But I'll have to admit to having strayed a bit on this in my neverending quest to make up some reasonable house rules on wards. The case being that I'd really like to somewhat distinguish between personal and ring/circle wards; at one time my idea was to only let ring/circle wards stop projectiles, and not let the personal ones - but it was inconsistent so my opinion on house ruling wards is still inconclusive.
My first thought when reading this was to suggest that you rule wards only affect projectiles that come into contact with them. A ward with Personal Range would not be very effective in that case, because it would have to hit the person before it is warded away, and would thus still do its damage.
Since the a personal range ward would still extend at least to the casters skin, it would suffice to stop any attack from doing more than pricking the skin... (enough to deliver a contact poison perhaps, but not much more)
Exactly the reason that I in the end I didn't find it consistent - especially since the target of the spell is individual and an individual is more than just the skin. It also includes what your wear etc.
As another mindbugling sidenote - if not throwing an object but flinging it by the use of a bow or a catapult... What force is then the 'propellant'? The force of the medium or the force of the creature (against which you are warded) using that medium...
For now my thoughts are not so much in direction of discriminating whether missiles or thrown objects hit or not. I still tend to end up descriminitating between the two types of wards in terms of Penetration, probably house ruling that Personal wards have to penetrated whereas circle/rings don't. This makes circle wards more powerful, which adds to their use and to the setting flavour. The argument could be the same as the ones mentioned here; that since circle wards stops the object(/Power/Creature) before the reaching the protected, wheres as personal wards only does so as they reach the protected. Making any sense?