I have a speedster character whose top speed is Mach 2. She holds back a lot of the time, but at full tilt she can accelerate to that speed - or slow to a stop from that speed - in exactly one second. Some quick Googling and back-of-the-envelope calculations tell me that this equates to an acceleration of 2,250 ft/s2 (686 m/s2), or about 70G, which is far more than an ordinary human can withstand.
I've therefore decided that my speedster will be passively immune to any physical effects that she would otherwise experience when accelerating or running at those speeds. These immunities would apply even when she isn't actually running. It's not a magical protection like the Speed Force, but a set of latent physical attributes.
I already know that she's going to be immune to G-forces of up to about 75G (735.5 m/s2) - anything below that will feel like 1G to her. She also has superhuman reflexes, to avoid faceplanting into the nearest wall as soon as she starts running.
My question is: What other physical attributes does she need to have in order to be able to survive such intense speed/acceleration? Is there anything else I've overlooked that she needs to be resistant/immune to, and is her existing G-force tolerance sufficient? Answers should ideally mention the implications of these increased tolerances.
- She wears goggles to protect her eyes while running, so that's not a concern.
- She's not immune to psychological effects, so a sudden deceleration might not hurt her but it would still disorient her.