Can The Flash Really Run Up The Side Of Buildings?

Newton’s Third Law Of Motion

For people who aren’t into superheroes, let me bring you up to speed with a thing or two about one popular superhero named the Flash.

The Flash can, however, move his feet back and forth against the surface of the building, which would give the impression that he is, in fact, running. What he would actually be doing here is travelling a distance equivalent to the height of the building in the time between consecutive steps. That seems totally impractical, doesn’t it?

It seems impractical because it is indeed impossible for a normal fella with regular abilities. But not for the Flash.

When the Flash runs on a plain surface, he pushes on the ground at an angle with the surface of the road; consequently, the force that the road exerts back on him is also at an angle with the surface. The net result is that he accelerates in both the horizontal and vertical direction. The larger the horizontal velocity, the further he advances before gravity overcomes the (relatively) small vertical velocity and pulls him back to the ground, making him take another step to keep moving ahead.

The Act Of Running Up The Side Of Buildings

People who run really fast (including the Flash) can have both their feet airborne between steps, thanks to their higher-than-normal vertical velocity. If the Flash bounces about 2 cm vertically with each step, then he remains airborne for around one-eighth of a second. Even if the Flash runs at a speed of 3,600 mph, he could travel around 660 feet before he had to take another step. And that’s an insanely underestimated assumption about his speed; according to the comics, the Flash travels faster than light itself!

Anyone can scale buildings just by running up their sides, provided they’re fast enough. Since the Flash is faster than the fastest thing we humans know of (i.e., light), he can undoubtedly run up the side of a skyscraper. The physics checks out on this one.

Kudos, DC comics!

Note: This article is inspired by “The Physics of Superheroes” – a book authored by James Kakalios – a University of Minnesota professor who turned to comics and superheroes for his Physics lectures!

How Fast Does The Flash Actually Run?

This is the question everyone really wants answered, and the honest reply is that there isn’t a single number. In the comics, the Flash’s top speed has been written as faster than light, which already breaks the speed limit of the universe and lands him squarely in fiction. But the more interesting question is the one our wall-running trick raises: how fast would anyone have to move for the physics to start working in their favor?

Think back to the worked example above. Running up a wall is really a problem of friction. To stick to a vertical surface, you need a large force pressing you into the wall, because friction (the force holding you up) is only ever a fraction of that pressing force. The only way to generate a big enough inward push by running is to slam into the surface at enormous speed. Physicist Rhett Allain, who writes the Dot Physics column and has consulted for MythBusters, has worked through this kind of wall-running scenario and shown that the speeds involved are far beyond anything a human sprinter could reach. A person tops out near 12 m/s (about 27 mph); the Flash blows past that by orders of magnitude, which is exactly why the trick is his and not ours. For a closer look at the human ceiling, see our piece on the fastest a human can run and our companion article on how fast the Flash can run.

Could The Flash Run On Water?

If running up a wall is just a friction problem, running across water is a timing problem, and nature has already solved it. The basilisk lizard (nicknamed the “Jesus Christ lizard”) genuinely sprints across ponds. It works because each foot slap punches a temporary air-filled cavity into the surface and pushes water downward; the upward reaction force holds the animal up for the split second before its foot would otherwise sink. The lizard simply takes the next step before gravity wins, taking up to 20 steps per second. A 2004 study in PNAS measured these slap-and-stroke forces directly and found the support comes mostly from the first, downward half of each step.

So why can’t we do it? It comes down to power. In a classic 1996 Nature paper, Glasheen and McMahon calculated that a human would need to slap their feet down at roughly 30 m/s (about 108 km/h, or 67 mph) to support their body weight on water, demanding around 15 times more muscle power than a person can produce. We are simply too heavy and too weak for our size. A 2012 study even confirmed humans can only run in place on water at about 22% of Earth’s gravity (roughly lunar gravity) while wearing small fins. The Flash, of course, clears that 30 m/s threshold without breaking a sweat, so by the exact same logic that lets him scale a skyscraper, he could comfortably dash across a lake. If you want the everyday version of this question, we cover why ordinary humans can’t walk on water separately.