The Physics of Superheroes: Spectacular Second Edition by James Kakalios Read Free Book Online Page A

(“atoms” for you specialists) an object contains. The mass of the atoms in an object is what gives it its “inertia,” a fancy term to describe its resistance to change when a force is applied. In outer space, an object’s mass is the same as it is on the Earth’s surface, because the number and type of atoms it contains does not change. An object in outer space may be “weightless,” in that it is subject to a negligible attractive force from nearby planets, but it still resists changes in motion, due to its mass. A space-walking astronaut in deep space cannot just pick up and toss a space station around (assuming she had a platform on which to stand), even though the station and everyone on it is “weightless.” The mass of the space station is so large that the force the astronaut’s muscles can apply produces only a negligible acceleration.
    For objects on the Earth’s surface (or that of any other planet, for that matter), the acceleration due to gravity is represented by the letter g (we’ll discuss this more in a moment). The force that gravity exerts on the object of mass, m, is then referred to as its Weight. That is, Weight = (mass) × (acceleration due to gravity) or W = mg , which is just a restatement of F = ma when a = g . Mass is an intrinsic property of any object, and is measured in kilograms in the metric system, while Weight represents the force exerted on the object due to gravity, and is measured in pounds in the United States. In Europe, Weight is commonly expressed in units of kilograms, which is not strictly correct, but easier to say than “kilo gram-meter/sec 2 ,” the unit of force in the metric system (also known as a “Newton”). When a weight in the metric system is compared to one in the United States, the conversion ratio is 1 kilogram is equivalent to 2.2 pounds. I say “equivalent” and not “equal” because a pound is a unit of force, while kilograms measure mass. An object will weigh less than 2.2 pounds on the moon and more than 2.2 pounds on Jupiter, but its mass will always be 1 kilogram. When calculating forces in the metric system, we’ll stick with kg-meter/sec 2 rather than “Newtons,” in order to remind ourselves that any force can always be described by F = ma .
    To recap, Superman’s mass at any given moment is a constant, because it reflects how many atoms are in his body. His weight, however, is a function of the gravitational attraction between him and whatever large mass he is standing on. Superman has a larger weight on the surface of Jupiter, or a lesser weight on the moon, compared with his weight on Earth, but his mass remains unchanged. The gravitational attraction of a planet or moon decreases the farther away one moves from the planet, though technically it is never exactly zero unless one were infinitely far from the planet. It is tempting to equate mass with weight, and easy to do so when dealing only with objects on Earth for which the acceleration due to gravity is always the same. As we will soon be comparing Superman’s weight on Krypton to that on Earth, we will resist this temptation.
    Finally, the third law of motion simply makes explicit the commonsense notion that when you press on something, that thing presses back on you. This is sometimes expressed as “For every action, there is an equal and opposite reaction.” You can only support yourself by leaning on the wall if the wall resists you—that is, pushes back with an equal and opposite force. If the force were not exactly equal and in the opposite direction, then there would be a net nonzero force, which would lead to an acceleration and you crashing through the wall. When the astronaut mentioned above pushes on the space station, the force her muscles exert provides a very small acceleration to the station, but the station pushes back on her, and her acceleration is much larger (since her mass is much smaller).
    Imagine Superman and the Hulk holding bathroom scales against