If you're Barry Allen, the superhero-in-disguise and star of the CW's hit show "The Flash," you can run at blinding speeds to save lives and do battle with evil metahumans.
In two previous posts, which focused on the overlooked superpowers and super amount of calories that Allen (played by actor Grant Gustin) would need to do his super thing.
This time around, we're looking at a feat he accomplishes early on in the series: running up the side of a building.
For answers that attempt to marry science fact with science fiction, Tech Insider spoke to James Kakalios, a physicist at the University of Minnesota and author of "The Physics of Superheroes."
Kakalios quickly pointed out the key factor in human locomotion, vertical or otherwise, is friction. "We run because of friction," he said. More specifically, "the friction force ... is the weight of an object pressing against the surface of the ground."
Friction plants our feet firmly on the ground, giving our muscles something to push off from. Without that downward force (acting against the force from the ground, known as the "normal force"), vertical travel gets tricky if not impossible, says Kakalios.
"If you're going up a perfectly vertical surface, then none of your weight is parallel to the surface, so technically you have no friction," he said.
But that's not the end of The Flash's story. If you can build up enough speed, it can be possible.
"There are ways around this, like moving so fast that between the first step and last step, you're at the top of the building," Kakalios said.
This is similar to how non-super humans run up and flip off walls (above). "Here again, you need to handle a lot of acceleration. Changing direction, that rotation of 90 degrees requires an incredible acceleration."
And of course, Allen has enough super-acceleration on tap to make Usain Bolt look like a sofa being pushed across a carpet.
All things considered, The Flash should be able to build up enough momentum to run up the side of a building, even without relying on fictional physics.
How he survives a sudden horizontal stop as he moves upward — likely akin to a crashing supersonic jet — is another story.
