You could probably make Arresto Momentum relatively safe by having it choose between the caster’s frame of reference and the earth’s (at the caster’s position) based on whichever results in the spell doing less work. It’s still dangerous, obviously casting it on someone else’s moving vehicle would be a bad move, but it avoids making anything go “zoom” in a privileged frame of reference.
You could probably make Arresto Momentum relatively safe by having it choose between the caster’s frame of reference and the earth’s (at the caster’s position) based on whichever results in the spell doing less work. It’s still dangerous, obviously casting it on someone else’s moving vehicle would be a bad move, but it avoids making anything go “zoom” in a privileged frame of reference.
Eh, the spell probably just reads your mind and does whatever you actually want :-)