I'm currently working on a superhero story where a speedster (someone with the superpower of running really fast) loses his speed while moving at max speed: Mach 2. This loss happens without him being able to slow down, essentially pulling him from Mach 2 to 0 in an instant. One of the abilities that falls under his powers is immunity to G-forces from acceleration and deceleration. This loss happens while he's running out on the ocean. I imagine this is bad for him, but just how bad? And does some kind of shockwave hit the water once this loss hits him?

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    $\begingroup$ The speedsters of comic book lore are pretty famous for having not just bad physics, but often a complete disregard for it. For example, the speedsters typically do not suffer air drag, nor do they seem to mind that their accelerations cannot be achieved without the ground beneath them being capable of providing a reactionary force large enough. Which laws of physics does your speedster already disregard? We'll need to know that before we can discuss what would happen to him using the laws of physics. $\endgroup$
    – Cort Ammon
    Commented Sep 5, 2016 at 15:51
  • $\begingroup$ You don't need super powers to travel Mach 2; the super powers are just for actually being able to produce a force to accelerate (and, in particular, enough force to overcome wind resistance). And, of course, to survive the experience. $\endgroup$
    – user2781
    Commented Sep 5, 2016 at 17:25
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    $\begingroup$ "This loss happens without him being able to slow down, essentially pulling him from Mach 2 to 0 in an instant. " Either he can slow down or he can't. You can't say he can't slow down, then say he instantly slows down. As for Mach 2 effects on the human body... Pilots have successfully ejected at Mach 3. However at that speed, water is as hard as concrete. $\endgroup$
    – Aron
    Commented Sep 5, 2016 at 17:46

2 Answers 2


Instant stop = reduced to pulp.

Of course, it's more likely your speedster would still be going at Mach 2, but suddenly unable to move his legs fast enough to maintain that speed against air resistance.

With the water passing underneath at Mach 2, his next step will likely result in a torn-off foot and falling forward. The Mach 2 airflow from the front will then lift him, cause him to spin uncontrollably for a while, perhaps tear off clothing, hair and/or limbs. All that doesn't matter as the hero has died from massive hemorrhaging in his brain due to the G-forces involved in the spin.

Then he crashes into the water and turns into a cloud of fish food. There might be a small shock wave involved, but no more than he was already causing running over the water at Mach 2... Some animals involved in this production will have been harmed.

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    $\begingroup$ Hurray to inertia! $\endgroup$
    – Skye
    Commented Sep 5, 2016 at 13:52
  • $\begingroup$ He already has to have invulnerability to slicing and dicing to run so fast in the first place -- frictionless skin or something. Does that get turned off also along with G-force immunity? If so, I agree with "fish bait". If not, other answers may apply. @Jacobs Please give more details about how he survives at superspeed in the first place. $\endgroup$
    – SRM
    Commented Sep 5, 2016 at 14:59

Make the power shut down over a short period of time with some kind of hysteresis, like an inductor that loses its charge. You can arrange it to suit the narrative, losing active ability to run and then diminishing protection from his existing speed. The protection can diminish a little faster than his speed, so he suffers a crash that he feels but doesn’t injure him beyond what your plot needs.


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