Since inertia is the law that states "anything in motion will stay in motion, unless acted upon by an opposing force", I interpret Deku ignoring inertia meaning he can ignore any counter force, from drag or friction, to even an enemy, which I believe would be dura neg (unless said enemy is universal or above). That begs the question of why Deku wasn't completely tearing through Shigaraki every time he hit him with Gearshift activated, but I think since Deku can seemingly control when he's ignoring inertia, he didn't because he wanted to save Shigaraki. Probably not enough evidence for this to actually be the case, but just something fun to think about.