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human
[DC] 2 questions about The Flash

The GF and I watched a Justice League movie earlier and I started wondering the following:

1. Does The Flash have to counteract momentum when coming to a stop? Depictions I've seen, he runs in and instantly stops. Seemingly without momentum carrying him forward as he is slowing to a stop. It's as if there's no speeding up/slowing down. It's more like moving/not moving. 

2. Say The Flash is speeding along but trips. Does he wipe out like a NASCAR racer, unable to stop himself and becoming very hurt? or would he just tumble at the speed of a regular person and receive minimal damage?
gpt
OK, first thing's first. The Flash gets his power from the Speedforce. This force control all motion, and the Flash controls the Speedforce. At first, the Flash didn't know his full range of powers, but he now does and can use many more abilities than Superspeed. He can steal or lend speed to objects (making them into motionless statues or sending them careening off, useful in combat). The Speedforce controls momentum, which allows him to start and stop with not kickback. This allows him to pick up and carry people at superhuman speeds without injuring them.

Further, the Speedforce protects the world from the effects of his speed. However, this power is selective and customizable. He can chose to make a sonic boom if he wants, and he can chose to have force impact someone, which makes his punches effective.

One of his strongest powers is the Infinite Mass Punch. By neutralising the Speedforce in such a way, he can let his fist gain relativistic mass by going very fast (Einsteinian physics), which allows him to hit "with the force of a white dwarf star". This can knock out Superman.

To answer your questions:

1. The Speedforce neutralizes momentum, so he can stop or turn on a dime. No slowdown or speed up necessary, though he often does instinctively slowdown slightly first.

2. He could theoretically stop himself midcrash and hover in the air (again, Speedforce controls momentum), but he doesn't often do this. The Speedforce does, however, save him from feeling the full force of a crash. So, he doesn't really have control when he crashes (he tries to slow down, but he's not always able to do that in the excitement of the moment), but he's not hurt by it, either. The Speedforce also negates damage to the environment, so the world doesn't get destroyed when he moves lightspeed.