It really, REALLY depends on which Flash we’re talking about. Even then it’s super inconsistent.
I’m Snyderverse, he’s faster than light and rewinds time. I think on the TV show he does this too. But then sometimes other people can react to him? Idk it’s pretty dumb. If we’re applying physics to Flash, if he’s even running at 25% the speed of light, and the air he’s hitting isn’t causing a wave of fusion reactions akin to a supersonic shock wave from a fighter jet, colliding with something solid isn’t going to be much different.
Somehow as he’s breaking through the light speed barrier gravity still affects him. But at those speeds he’s got enough momentum to leave the solar system if he runs off an incline and does a little hop I guess. He seems really stuck to the ground otherwise.
Comics are weird. Asking questions like, could the Flash run so fast he exits the universe can almost always be answered with “if the plot calls for it.”
He has the power of fast. He goes exactly as fast as he needs to. It’s magic, not science. They make it sound like science but it’s magic.
Kinda like how Tony Stark invents time travel in an afternoon, or Peter Parker affords an apartment in NYC of even only barely.