The Physics of Superheroes: Spectacular Second Edition by James Kakalios

effect is that he accelerates in both the vertical and horizontal direction. The vertical velocity gives him a bounce up off the ground, and the horizontal component propels him in the direction he is running. The greater the vertical velocity, the higher the bounce, while the larger the horizontal velocity, the farther he advances before gravity overcomes the small vertical velocity and brings his feet back to the ground, ready for another step. Very fast runners, which would certainly include the Flash, can have both feet up off the ground between steps. The faster they run, the longer their time “airborne” between steps. If the Flash bounces about 2 cm vertically with every step, then he is in the air for about one eighth of a second before gravity pulls him down for another step. But one eighth of a second is a long time for the Crimson Comet. If his horizontal velocity is 5,250 feet/sec or 3,600 mph, then the horizontal distance he travels between steps is more than 660 feet. This is approximately one eighth of a mile, which we used as the benchmark for the tall building that Superman leapt in Chapter 1. As long as the Flash maintains at least this minimum speed, he needn’t worry about losing his footing along the way, simply because he will scale the height of the building between steps.
    Before he can scale a skyscraper, the Flash has to radically alter his direction from the horizontal to the vertical. As will be discussed in a later chapter, any change in the direction of motion, whether it is Spider-Man swinging on his webbing or the Flash changing his path at the side of a building, is characterized by an acceleration that requires a corresponding force. Rotating his trajectory by ninety degrees up the building’s face entails a large force, provided by the friction between the Sultan of Speed’s boots and the ground. In addition to superspeed, the Flash’s “miracle exception” must therefore also extend to his being able to generate and tolerate accelerations that few superheroes not born on Krypton could withstand.
    Newton’s laws of motion can also explain how the Flash is able to run along the surface of the ocean, or any body of water, for that matter. Just as Gwen Stacy had to be concerned as she was about to strike the water while moving at her large, final velocity, the great speed of the Flash’s strides enables him to run across its surface. As one moves through any fluid, be it air, water, or motor oil, the fluid has to move out of your way. The denser the medium, the harder this is to accomplish. It requires more effort to walk through a swimming pool, pushing the water out of your way, than to walk through an empty pool (that is, one filled only with air), and it is harder still if the swimming pool is filled with molasses. The resistance of a fluid to flow is termed “viscosity,” which typically increases the denser the medium and the faster one tries to move through the fluid.
    The density of water is much greater than that of air—water molecules are in contact with one another, while there are large, open spaces between air molecules. It is even more difficult to move through water when traveling at high speeds. But for the Flash, when running on top of the water’s surface, this is a good thing. Just as someone is able to water-ski if he or she is towed at a large velocity, the Flash is able to run faster than the response time of the water molecules. As his foot strikes the water’s surface at speeds greater than 100 mph, the water acts more like a solid than a liquid beneath the Flash’s fleet feet (to test this, try rapidly slapping a pool of water), and therefore his oft-shown ability to run across bodies of water is indeed consistent with the laws of physics. In fact, at the speeds at which he typically runs, it is practically impossible for him to not run across the water’s surface. However, in order to acquire forward momentum, the Flash must push back against the