That has no inertia, or acts without the use of inertia(adjective)
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Use "inertialess" in a sentence
"The coolest scene is when the battleship fires a shell at the invader ship (the cannon is wrapped with inertialess coils)."
"In House of Suns, I played with the idea of dispensing with airlocks and suits entirely, by inventing (with the aid of inertialess fields) something called "whisking": what if you could be shot between two ships sufficiently quickly that you were only exposed to vacuum for a few milliseconds?"
"Those inertialess ships I'm sure an inertialess ship wouldn't behave like those did."