Newton's Third Law of Motion applies. This is often stated as 'for every action there is an equal and opposite reaction.'
is the price one pays; everything has an impact on something else. A magical spell for love here may cause hate over there. An allowance for the reaction might even be written into the spell, a safety valve of sorts.
Indeed, I would think all the laws of physics would apply to magic. Inertia, force, mass, would all be factors in its operation. And I wouldn't worry that the Newtonian laws are not exactly accurate — they are 'close enough' for our purposes.
We can apply this concept of action and reaction to all writing, actually. Anything that happens has consequences of some sort. Even if only internal.
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I'll admit that I've never written anything that explores such a world of magic. 'Spells,' as such, do not exist in my Donzalo/Malvern world; the seemingly magical stuff there involves moving through alternate worlds/dimensions. That's just work, involving the physical and mental strength of the sorcerer. Which is also a price.
In fantasy tales, one also finds the sort of magic that relies on agents. This is more true of older fantasy than new, I think. The summoning of demons or elementals to do ones dirty work has been a staple of magic pretty much from the beginning. These generally extract a price for their services, but beyond that they too would have to follow those rules of physics.