[Traceur]

some fantasy lore's describe magic as forcing your will over the fabric of the universe. in this case, there's absolutely no reason why your fireball should care for physics: if physics had any say in the matter the fireball wouldn't have being there in the first places. in this case the Newton of fantasy worlds was clearly a dimwitted no-talent mage, since magic is beyond the rules of the world.

but other fantasy lore's describe magic as knowledge about the mystical unknown aspects of the universe: there are unknown aspects to the fabric of the universe that the common man can not comprehend, but those who do can directly interact with those aspects, like the classic trick of summoning a ball of hot lava out of thin air hurtling away from you in the direction your facing. in this case, the fireball should have it's physics - it would be within the world and subject to it's laws.

a not uncommon compromise is that you do in fact force your will over the universe - but only to a limited extent, based on your talents and know-how.
the higher your skills, the less you need to care for the constraints of the physical world.

another type of compromise is that the two ways of doing things - often called sorcery & wizardry - exist side by side for different schools.