The Physics of Superheroes: Spectacular Second Edition by James Kakalios Page A

water. That is, even if the water does behave like a solid under the rapid compression under his feet, would the Flash be able to obtain traction in order to run? One way he could accomplish this is by generating backward propagating vortices under his feet, thereby gaining a forward thrust under Newton’s third law. This mechanism was proposed as the means by which water-strider insects propel themselves along the water’s surface. Here again comics were ahead of the curve. The Flash’s ability to run across a body of water was likened to a rapidly skipping shell skimming over the water in Flash # 117 , more than thirty years before scientists understood the water strider’s method of locomotion. 19
    When it comes to air, there is a lot of space between neighbor ing molecules. At room temperature and pressure, for example, the distance between adjacent air molecules is about ten times larger than the diameter of an oxygen or nitrogen molecule. Moreover, each air molecule at room temperature is zipping around with an average speed of approximately 1,100 feet/sec or 750 mph (which is the speed of sound in air). When we run through air, we don’t build up a high-density region in front of us, because our speed is much less than the average air molecule’s velocity. Think about herding cattle: If the cows are running when you try to push one into the herd, the others will just run away. If they are walking very slowly, and you push at the same rate, the others don’t have time to get out of the way, and they pile up into a herd. One can, of course, move faster than the speed of sound (a feat first performed by Col. Chuck Yeager in 1947), but the expended effort is large. When trying to displace a volume of air faster than the air molecules are moving, a high-density region (that is, a shock front) would pile up in front of you.
    In fact, in “The Challenge of the Weather Wizard,” the Flash uses just such a shock front to knock out the Weather Wizard. Mark Mardon, a small-time crook, stole his deceased scientist brother’s “weather stick,” a device that enabled him to control the weather. Much like any other self respecting comic-book villain, once in possession of a weapon giving him mastery over the fundamental forces of nature, he immediately adopts a colorful costume, calls himself the “Weather Wizard,” and sets upon robbing banks and vandalizing police stations. The finale of the story, as shown in fig. 9, comes when, “with a tremendous surge of speed, the Flash slams toward his foe so fast that the air in front of him piles up into a wave-front and a split instant later strikes Mardon like a solid sheet of glass.” This is indeed a physically accurate consequence of the Flash’s supersonic velocity, and the variation in the compressibility of air at high velocities bedeviled fighter pilots in the 1940s attempting to break the “sound barrier.” Whenever the Flash runs at or faster than the speed of sound, the pressure waves he generates create a “sonic boom.” Just such a loud crash heralded the first appearance of the Flash in Showcase # 4 .

    Fig. 9. Panels from “The Challenge of the Weather Wizard” (Flash # 110) demonstrating that the faster one moves, the harder it is to get the air out of the way.
    Once the Flash has moved the air before him out of his path, he leaves a region of lower-density air in his wake. Compared with the surrounding air at normal density, this lower-density trail behind the Flash can be considered a partial vacuum. Air rushes in to fill any vacuum, and anything standing in the way of this rushing air behind the Flash will be pushed into the wake region. The faster he runs, the greater the pressure difference between the air behind him and the surrounding air, and the larger the force as this pressure imbalance is corrected. This effect is noticeable even for slower-moving objects, such as when a subway train enters a tunnel. The enclosed geometry of the tunnel