Imagination Unleashed
This being said what is to say that when spells are stopped there is no 'residue' force, such as when a knight had fire negating armor and is hit with a fireball which contains alot of physical momentum and force? Why can't he 'fall back' or stumble?
For the simple reason that fire, itself, is an illusion.
Flame as we see it is simply light given off by burning gas, and the distortion of such light due to superheated air that is moving in waves.
As such, a fireball does not have mass, simply heat. Any force that is put into it (other than perhaps telekinetic, in the event of a magical fireball) would be directly due to mass. As a magical fireball does not have a fuel source, such as would hucking a burning log at someone, there is no real mass to draw from other than perhaps gasses. Gasses do not have much mass. However, in the event of an explosion people can be thrown back and things can go kablooie? Why? Because extremely rapid expandiature of superheated gasses creates a shockwave of 'hard air', to use a loose term, which delivers a fast, and hard 'blow' of force. Like a wave breaking on the shore.
However, in the event of a fireball, traveling through the air at constant size rather than rapidly expanding, it is more or less a ball of superheated air... at which, the victim would experience, at most, a strong wind.
TANKUU-COMEAGIN.