Seat belts and air bags act to slow you down by applying a smaller force over a longer time, and that’s a much safer method of applying the stopping force than hitting the steering wheel or windshield. The total change of motion with or without seat belts and air bags is exactly the same, but with modern safety technology the injury-causing force is not nearly so great.
The Third Law
To every action force there is an equal and opposite reaction force.
Even though this law is probably the most often quoted of the three, it is the least intuitive. It is obvious that a pitcher exerts a force on the ball, but less obvious that the ball pushes back on the pitcher’s hand with an equal and opposite force. When you stand up, your shoes apply a force to Earth just as large as the force Earth’s gravity exerts on you. When you try to open a screw-top bottle that is stuck, your left hand twists one way while the right hand is twisted the opposite way. You cannot touch your lover without feeling his or her touch in return.
The third law says that forces always come in equal and opposite pairs, but that the forces in the pairs act on (and therefore accelerate) different objects. You are pushing down on the chair in which you are sitting. The third law says that the chair is exerting an equal upward force on you. You really can learn Newton’s laws by the seat of your pants.
Newton’s third law also explains how a rocket can fly in space, even when there’s nothing to push against. It works like this: the rocket motor heats gases, which are accelerated out through the engine nozzle. The first law tells us that in order to accelerate gas, we must exert a force on it. That force must, of course, be exerted on the gas by the ship. The third law then tells us that an equal and opposite force must be exerted by the gas on the ship. That’s what makes the ship go. A rocket ship in space is similar to someone standing on roller skates and shooting a gun. Both recoil in one direction as they throw something out in the other.
GRAVITY
Newton’s laws tell us what happens when forces act on objects, but the laws tell us nothing about what those forces are. You’ll discover several different forces in subsequent chapters—some well understood, like electricity and magnetism, some still mysterious, like the so-called strong force. Newton himself described nature’s most familiar force—gravity.
Before Newton, there was a kind of schizophrenia evident in the way scientists thought about gravity. The force that held the planets in their orbits (which we can call celestial gravity) was held to be completely different from the force that makes things fall to the center of the Earth (terrestrial gravity). In the century before Newton, different people made enormous progress in studying these types of “gravity” separately.
Cannonballs
Terrestrial gravity was an obvious thing to study in an age when cathedrals could collapse and cannonballs could sink ships. What made the work of scientists in the seventeenth century different from what had gone before was the appearance, for the first time, of laboratory experiments—controlled studies of gravity’s effect on falling objects. The most famous of these experiments were performed by the Italian scientist Galileo Galilei (1564–1642). Galileo is best known for his trial on charges of suspicion of heresy for teaching the doctrine that the Earth moves around the sun (instead of vice versa), but in our view his most revolutionary contribution to science was his demonstration that carefully run experiments can yield profound insights into the nature of the universe. He is, in fact, often called the “father of experimental science.”
Galileo studied terrestrial gravitation not by asking about the nature of gravity, but by observing how objects behave when gravity acts on them. In particular, he did a series of experiments on balls rolling down inclined planes (the purpose of the incline being, in his words, to “dilute” gravity enough so that he could measure the time it took for the ball to roll with the primitive clocks available to him). By meticulously measuring the time it took the ball to travel various distances, he was able to find out how the speed of the ball changed in transit. His bottom line: Terrestrial gravity causes all objects to accelerate the same amount, regardless of their mass, and the rate of that acceleration is constant. These simple observations allowed Galileo and his contemporaries to understand (and predict) things like the fall of a stone or the arc of a cannonball. They are the basic facts that tell you everything you need to know about how unsupported objects behave at the surface of our planet.
Ironically, Galileo probably never performed his most famous “experiment”—dropping two balls of different masses from the leaning Tower of Pisa to show that all objects fall at the same speed. Had he actually done the experiment, the resistance of the air might have caused the heavier objects to fall slightly faster than lighter ones, thereby disproving the very thesis he is famous for establishing!
Planets
While Galileo was working out the effects of terrestrial gravity, European astronomers were making equally bold progress at understanding movements of the planets. The German astronomer Johannes Kepler (1571–1630), using data on planetary motions assembled by the Danish astronomer Tycho Brahe (1546–1601), succeeded in discovering how the planets move in their orbits. He found, for example, that the orbits of all planets (including the earth) are elliptical—not circular, as everyone before had assumed. Like Galileo, he summarized his studies of planetary motion in concise statements, known as Kepler’s laws of planetary motion.
Galileo and Kepler employed a number of similar methods in their research. Both men relied heavily on observational or experimental data. They were not, like many of their colleagues, armchair philosophers. If they wanted to know what the world was like, they actually went out and looked. Both men ended up by summarizing and codifying their results in a series of statements (or laws) written in mathematical form. These mathematical statements could be used by anyone to make predictions about the real world.
Kepler’s laws of planetary motion and Galileo’s rules about falling bodies summarized the best scientific knowledge available in astronomy and physics, respectively, but they appeared to have nothing to do with each other. Each referred to a different sphere of reality. It took the genius of Isaac Newton to see that both men were, in fact, studying exactly the same thing.
The Apple and the Moon
According to Newton, he got his great idea while watching an apple fall in an orchard while he could see the moon in the sky. He knew the apple fell because a force acted on it (first law), but it struck him that the force pulling on the apple might well extend all the way out to the moon and pull on that object too. In fact, he knew that since the moon was constantly changing direction, a force had to be acting on it. It was this speculation, triggered by a simple everyday event, that led to the healing of the artificial distinction between the earthly and the heavenly, and that finally gave humanity both a new way to approach the world (science) and a new metaphor (the clockwork universe).
Newton knew that a dropped apple would fall straight down to Earth under the influence of terrestrial gravity. Throw an apple straight out and it follows a curved path as gravity pulls it down. Throw the apple harder and it lands farther away. Throw it very hard indeed and it could even circle Earth. Once it makes one circuit, it will continue around and make another (ignoring air resistance), and will in fact continue to do so forever. But of course this is just what the moon (or any satellite) does. The force that constantly acts on the moon—that keeps pulling it into a curved path instead of the straight line the first law says it should follow—is gravity, the same gravity that pulls down on the apple. With this insight, Newton abolished the centuries-old split between Earth and the heavens and showed that both were fit subjects for scientific study.
He went even further, deducing the exact mathematical formula for the gravitational force. Only three physical quantities determine gravitational force: the masses of the two objects and the distance between them. He stated his result in what we know as Newton’s law of universal gravitation:
> Between any two objects there is an attractive force proportional to the product of the two masses divided by the square of the distance between them.
This law has many interesting consequences. Obviously any large mass will exert a large gravitational force, but no special distinction is made between large masses and small ones. Earth pulls on the apple, but the apple also exerts a force on Earth. In fact, the two forces are the same size. We speak of apples falling to the ground because they are much less massive than Earth and so undergo a much greater acceleration due to the force exerted by the apple. As the apple falls 15 feet to the ground, Earth “falls” a distance about the diameter of an atomic nucleus toward the apple.
The law of gravity tells us that every object in the universe is exerting a gravitational force on you right now. Earth exerts the biggest, but the person next to you exerts a force as well, as do the most distant star and galaxy. In practice, however, the massive sun and nearby moon are the only heavenly bodies that can exert a bigger force on you than familiar nearby objects like buildings. This simple fact is one of several reasons why scientists have a hard time taking astrology seriously.
The Clockwork Universe
With the law of universal gravitation, Newton closed the circle on his work. He had the force—gravity—that operated everywhere, and he had the rules—the laws of motion—that governed the operation of all forces. Suddenly scientists saw the universe in a new way, ordered and predictable as never before. With Newton’s equations and the language of mathematics, scientists could describe and predict the behavior of all kinds of systems. In the centuries following Newton’s work, philosophers compared his vision of the universe to a clock. The visible phenomena in the world, like the hands of a clock, move in response to the actions of invisible gears—the natural laws. In the solar system the motions of the planets are governed by the law of universal gravitation and the laws of motion. The planets tick along, as regular as a clock. For the Newtonians, in fact, the universe resembled a clock in other ways: once set in motion by God, the universe followed an inevitable course. The future was completely and comfortably predictable.
This is a wonderful vision, but like all scientific ideas it had to be tested. The most dramatic test of Newton’s vision of the universe was made by his fellow Englishman Edmond Halley (1656–1742). Using Newton’s laws and historical records, Halley was able to work out the orbit of the comet that now bears his name and to predict its reappearance in the sky. When the comet was “recovered” on Christmas Day, 1758, the event powerfully underscored the idea of the clockwork universe. Not only could Newton’s scheme explain things that were already known, it could make reliable predictions about events that had yet to occur.
Today, with the advent of quantum mechanics and the field of complex chaotic systems, scientists’ ideas about the clockwork universe have changed. The universe is still, in the modern view, governed by simple laws, but these laws do not always allow us to make the kind of straightforward predictions about the future that Newton envisioned. Nevertheless, much of the Newtonian mind-set survives in modern science.
THE SCIENTIFIC METHOD
Newton’s development of the clockwork universe was the first, classic example of the scientific method in use. The method depends on a constant interplay of observation and theory; observations lead to new theories, which guide more experiments, which help to modify the existing theories.
In Newton’s case, some of the observations and experiments were recorded by Galileo, others by Kepler. In each case, the cycle of observation, theory, test-against-new-observations was repeated until the investigators achieved a complete understanding of the phenomenon being studied. Newton, as we pointed out, incorporated these understandings into his sweeping theory of motion, and then his new theory was used to make many predictions like the projected reappearance of Halley’s comet. Only after many such tests was the theory accepted by scientists.
The scientific method does not require researchers to be unbiased observers of nature. Scientists almost always have a theory in mind when they perform an experiment. But the method does require that scientists be willing to change their views about nature when the data demand it.
Newton provided a model for the development of modern science in many ways. He was the first to use the scientific method, and he was the first to show that scientific theories can develop by incorporation rather than revolution.
When Kepler published his laws of planetary motion, he swept aside the old ideas about the solar system. This was a revolutionary change—the old notions were seen to be wrong and were abandoned. When Newton published his work, however, he showed that all of Kepler’s laws could be derived from universal gravitation and the laws of motion. His work, then, incorporates Kepler’s and expands upon it, but does not invalidate it. In the same way, Newton was able to derive Galileo’s conclusions, in corporating them into the same theoretical framework that ac commodated the description of the planets. This has proved to be a common occurrence in science. When Albert Einstein produced the theory of general relativity, our current best theory of gravitation, it incorporated Newton, Kepler, and Galileo, and when some future theoretical physicist produces the final unified field theory, it will likely incorporate Einstein.
Despite what you read in sensationalistic headlines, true revolutions are rare in mature sciences.
SCIENTISTS
The universe seems far too complex to comprehend all at once, so the classic scientific approach is to examine well-defined pieces of our surroundings, one at a time. The universe can be divided into an infinite number of “systems,” which are nothing more than parcels of matter and energy Each parcel, which can contain almost anything from a single spinning subatomic particle to an entire galaxy, is fair game for scientific study. Astronomers probe stars and the solar system. Chemists investigate systems containing carefully selected groups of atoms. Geologists study minerals or mountain ranges. Biologists examine complex systems called cells or ants or forests. Each system can be something you hold in your hand, like a rock, or it can be an integral part of something else, like your body’s nervous system.
The scientific enterprise consists of thousands of specialized subdisciplines—the chemistry of fluorine, the turtles of Malaysia, the properties of young massive stars, the evolution of the AIDS virus, lasers, quarks, diamonds, slime mold—each with its own practitioners and jargon. These varied specialties differ primarily in the size and contents of the system under study. All systems, be they stars, bugs, or atoms, are governed by the same set of natural laws, but they are studied and described in very different ways.
Hundreds of thousands of Americans make their livings as scientists. Most of these women and men can be described with one of four broad labels: physicist, chemist, geologist, or biologist. Science is a seamless web of knowledge, but people like to create their niches. So each of the four main science branches (not to mention the hundreds of highly specialized “twigs”) has developed its own distinctive style and organization.
Physicists study matter and energy, forces and motions—the concepts central to all science. Physicists take pleasure in pointing out that theirs is the most fundamental science, because all other fields, from chemistry to cosmology, mineralogy to molecular biology, depend on a few basic physical principles. Physicists are the generalists among scientists, and fields as far apart as molecular biology and field ecology have benefited from an influx of physicists over the years. Nevertheless, parts of physics have turned into the most abstract of the sciences. Physics conventions are replete with discussions of ten-dimensional space, quarks, and unified field theories. For some reason many physicists, particularly those in universities, seem to enjoy appearing sloppy and disheveled—always the ones without ties at faculty meetings. If you want to make a physicist happy, tell him you thought he was the plumber.
The American Institute of Physics, based in the Washington, DC area, represents about 100,000 physical scientists,
including astronomers, crystallographers, and geophysicists, who are members of ten affiliated societies. The largest of these groups, the American Physical Society, boasts almost 50,000 hard-core phyicists on its membership rolls. These societies sponsor professional meetings, lobby for physics research and education, and publish prestigious research journals such as The Physical Review and Physics Today.
Chemists are pragmatists, studying atoms in combinations to discover new and useful chemicals. Most chemists, even those in academia, maintain close ties to industry; science and its applications are seldom far apart. Chemists hold more patents than any other kind of scientist, and they are frequently observed wearing business suits.
The American Chemical Society, headquartered in the nation’s capital, represents both research chemists and chemical engineers. This blend of science and industry, unique among the major science societies, gives the ACS more than 160,000 members, making it the largest U.S. science society (surpassing even the interdisciplinary American Association for the Advancement of Science in total membership). The American Chemical Society sponsors meetings, supports chemical education, and publishes numerous books and journals, including the weekly Chemical and Engineering News. As a lobbying organization, the ACS must walk a fine line between environmentalists and major chemical corporations, both of whom are represented among the membership.
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