The Physics of Superheroes: Spectacular Second Edition (24 page)

Fig. 19.
A rare example from
Flash # 106
of the realistic effects of the Flash’s sudden deceleration. The shorter the stopping distance, the greater the force his boots must exert on the ground when braking.

The reason that 1kg-m eter
²
/sec
²
is equal to 0.24 calories is that in the mid-nineteenth century, physicists were confused about energy, a situation that has not greatly improved in the intervening years. A calorie was originally defined as a unit of heat, as heat was thought to be a separate quantity distinct from work and energy. Hence, one system of measurement for heat was developed, while a different unit was employed to measure kinetic and potential energy. The physicist who recognized that heat was simply another form of energy, and that mechanical work could be directly transformed into heat, was James Prescott Joule, in whose honor a standard unit of energy (1 Joule = 1 kg-meter
²
/sec
²
= 0.24 calories) is named. While physicists use Joules when quantifying kinetic or potential energy, we’ll stick with the more cumbersome kg-meter
²
/ sec
²
in order to emphasize the different factors that enter into any determination of energy.
³⁸

It should be noted that a physicist’s calorie is not the same as a nutritionist’s calorie. To a physicist, one calorie is defined as the amount of energy needed to raise the temperature of one gram of water by one degree Celsius. This is a perfectly valid, if arbitrary, way of defining energy in a laboratory setting. But this definition leads to the observation that a single soda cracker contains enough energy to raise the temperature of 24,000 grams of water by one degree. In other words, to a physicist, the energy content of one plain cracker is 24,000 calories! In order to avoid always dealing with these very large numbers, a food calorie is defined to be equal to 1,000 of these “physics calories.” Therefore the twenty-four food calories in a single cracker are actually equivalent to 24,000 calories using the physics laboratory definition of the term. Just as well—it’s bad enough to think of the roughly 500 food calories in a cheeseburger, but if we considered that it actually contained 500,000 physics calories, we might never eat anything ever again!

To convert the Flash’s kinetic energy of 75 trillion calories into food calories, we should divide his energy by 1,000. This helps, but he still expends 75 billion food calories running at 1 percent the speed of light. Put another way, he would need to eat 150 million cheeseburgers in order to run this fast (assuming 100 percent of the food’s energy is converted into kinetic energy).
³⁸
If he stops, his kinetic energy goes to zero, and in order to run this fast again, he needs to eat another 150 million burgers. At one point in Flash comics (during the mid 1980s) it was briefly acknowledged that he would need to eat nearly constantly (even chewing at superspeed) in order to sustain his high velocities. In the Golden Age, the Silver Age, and now in the Modern Age, conservation of energy is conveniently ignored. Nowadays the Flash’s kinetic energy is ascribed to his being able to tap into and extract velocity from the “Speed Force,” which is a fancy way of saying: Relax, it’s only a comic book.

The next in a seemingly never-ending series of questions we ask is: Why does a cheeseburger, or any food, provide energy for the Flash? It’s easy to identify kinetic energy when something is moving, and the potential energy due to gravity is also pretty straightforward, but there are many other forms of energy that require some thought as to which category—potential or kinetic—they belong. The energy the Flash gains by eating is not due to the kinetic energy of atoms shaking in his food (a hot meal has the same number of calories as a cold one) but from the potential energy locked in the chemical bonds in his food. As energy can never be created or destroyed, but only transformed from one state to another, let us follow the chain backward, to see where the stored potential energy in a cheeseburger comes from.
³⁹

In order to understand the potential energy stored in food, we have to consider some basic chemistry. When two atoms are brought close to each other, and the conditions are right, they will form a chemical bond, and a new unit, termed a “molecule” will be created. A molecule can be as small as two oxygen atoms linked together, becoming an oxygen molecule (O
₂
), or it can be as long and complex as the DNA that lies within every cell of your body. The question of whether and when two or more atoms will form a chemical bond, and the elucidation of these conditions, is the basis of all chemistry. All atoms have a positively charged nucleus around which a swarm of electrons hover. The chemical properties of an element are determined by the number of electrons it possesses, and how they manage to balance their mutual repulsion (as they are all negatively charged) with their attraction toward the positively charged nucleus. When an atom is brought very close to another atom, the most likely locations of the electrons from the two atoms overlap and, depending on their detailed nature, there will be either an attractive or repulsive force between the two atoms. If the force is attractive, the electrons create a chemical bond and the atoms form a molecule. If the force is repulsive, then we say that the two atoms do not chemically react. In order to determine whether the force is attractive or not involves sophisticated quantum mechanical calculations. (We’ll have much more to say about quantum mechanics in Section 3.) If the force is attractive, and one restrains the two atoms to keep them physically apart, then there is a potential energy between the atoms, since once this restraint is removed, the two atoms form a molecule. In this way, we say that the two atoms, once chemically joined, are in a lower-energy state, just as a brick’s gravitational potential energy is lower when placed on the ground. Work has to be done to lift the brick to a height h, just as energy has to be supplied to the molecule to break it apart into its constituent atoms.

We are finally (and I can almost hear you saying: Thank goodness!) in a position to answer the question of why the Flash needs to eat, or rather, why food provides the energy he needs to maintain his kinetic energy. When the Flash runs, he expends energy at the cellular level in order to flex his leg muscles. This cellular energy, in turn, came from the breakfast that Barry Allen ate. From where did the energy in the food arise? From plants, either directly consumed or through an intermediate processing step (such as when plants are eaten by animals and those animals are eaten by humans). This stored energy in food is simply potential energy on a molecular scale. Plants take several smaller molecular “building blocks” and process them, stacking them up into a subcellular “tower of blocks.” This molecular tower of complex sugars, once constructed, is fairly stable. The process of lifting and arranging a group of blocks into a tall tower raises all of the blocks’ potential energy (except for the bottom block).

Similarly, plants do Work when constructing sugars from simpler molecules, raising the potential energy of the final, synthesized molecule. The potential energy remains locked within the sugars until the mitochondria within our cells construct adenosine triphosphate (called ATP for short) and release the saved energy, just as the Work in building a tower of blocks is stored as the potential energy of the top blocks until the tower is knocked down, converting their potential energy into kinetic energy. The amount of energy released by the ATP in the Flash’s leg-muscle cells is greater than the energy needed to “knock the complex-sugar tower down,” though the gain to the Flash is much less than the plant cell’s effort in raising the tower in the first place.

Where did the plant cell obtain this energy? Through photosynthesis, whereby the energy in sunlight is absorbed by the plant cell and employed in complex-sugar construction. The light comes from the sun (don’t worry, we’re nearly at the end of the line), where it is generated as a by-product of the nuclear-fusion process, in which hydrogen nuclei are fused together through gravitational pressure to create helium nuclei. Ultimately, all of the chemical energy in food is transformed sunlight that was generated by the same nuclear-fusion process in the center of the sun that occurs during the detonation of a hydrogen bomb. In this way, the majority of energy on Earth is solar energy at its source, just as all the atoms on Earth, from the ATP molecules throughout the Flash’s body to the ring in which he stores his costume, were created in a solar crucible (in the massive star that preceded our own sun).

Ultimately, all of life is possible because the mass of a helium nucleus (containing two protons and two neutrons) is slightly less than that of two deuterium nuclei (deuterium is an isotope of hydrogen in which the single nuclear proton is joined by a neutron) combined in the center of a star. And by slightly less, I mean that the mass of a helium nucleus is 99.3 percent of two deuterium nuclei. This small mass difference leads to a large outpouring of energy, since from E = mc
²
the change in mass is multiplied by the speed of light squared.

And 99.3 percent really is the magic number. If the mass difference were 99.4 percent, then deuterium nuclei would not form, and hence fusion of helium could not proceed. In this case, stars would shine too dimly to synthesize elements, and no violent supernova explosions would occur to both generate heavier elements and expel them into the void, where they might form planets and people. On the other hand, if the mass difference were 99.2 percent, then too much energy would be given off from the fusion reaction. In this case, too many protons would combine directly to form helium nuclei in the early universe, and no nuclear fuel would remain when stars formed. The source of this amazing fine-tuning of the basic properties of nature is the subject of current investigation.

Frequently, the constraint in performing a task is not how much energy is required, but how fast that energy can be supplied and used. Consider the Flash in fig. 19, stopping in roughly fifteen feet from a speed of 500 miles per hour. The Work needed to decrease his large kinetic energy to zero was a little over a million pound-feet, which called for a force of more than eighty thousand pounds. If the Scarlet Speedster were to stop over a longer distance, say a mile rather than fifteen feet, then the force needed would be under 230 pounds, and it is doubtful that he would leave such deep ruts and scarring in the ground. Put another way, it requires less force to change his kinetic energy if he has a longer time to do so.

Power is defined in physics as the rate at which the energy of a system is changed (technically, by “changed,” I mean transformed from one form to another, such as the kinetic energy of the Flash being converted to mechanical work done by his boots on the ground). If I do Work on my automobile, pushing it from rest until it is moving at 60 mph, its final kinetic energy is the same if this process takes six seconds or six hours. The rate of change of the car’s kinetic energy is obviously much higher in the first case, and if you have a motor capable of accelerating an auto from 0 to 60 mph in six seconds, we say your car’s engine has more power than the engine that takes six hours. As Work, “kinetic energy” and “heat” are just different expressions for energy, the rate of change of energy, measured in units of kg-meter
²
/sec
²
per second, is defined as a Watt (after James Watt, an early pioneer of the study of thermodynamics). We often need a lot of energy in a short period of time, so one convenient unit is one thousand Watts, which is termed a kiloWatt. A Watt is a unit of power—that is, the rate at which energy is used. To keep track of how much energy one uses in, say an hour, we multiply the rate of usage by the time—the resulting expression is referred to as kiloWatt-hours and is how the electric company keeps track of how much electrical energy you have used. Large power plants have to supply electrical energy to a great many homes at any given moment. A plant that can provide a kiloWatt of power (which a typical residential house requires) for one million homes is rated as capable of producing a gigaWatt of power.