10 July 2020- What is Physics?- Part 1

INTRODUCTION 

Without the science of physics and the work of physicists, our modern ways of living would not exist. Instead of having brilliant, steady electric light, we would have to read by the light of candles, oil lamps, or at best, flickering gaslight. We might have buildings several stories high, but there could be no hope of erecting an Empire State Building. We could not possibly bridge the Hudson River or the Golden Gate much less build a jet plane, use a cell phone, or watch a television show. The personal computer would be unimaginable.

All other natural sciences depend upon physics for the foundations of their knowledge. Physics holds this key position because it is concerned with the most fundamental aspects of matter and energy and how they interact to make the physical universe work. For example, modern physics has discovered how atoms are made up of smaller particles. It has also revealed how these particles interact to join atoms into molecules and larger masses of matter. Chemists use this knowledge to guide them in their work in studying all existing chemical compounds and in making new ones.

Biologists and medical researchers in turn use both physics and chemistry in studying living tissues and in developing new drugs and treatments. Furthermore their electrical equipment, microscopes, X-rays, and many other aids and the use of radioactivity were developed originally by physicists.

Physicists have also led in bringing people to think in scientific ways. What we call the scientific method had its real beginnings some four centuries ago in many fields of knowledge. The most impressive of the early triumphs came in physics and in its application to astronomy for studying the motions of the Sun, Moon, planets, and stars.

Galileo made the first real contributions, in the late 16th and early 17th century. He discovered the natural laws that govern falling bodies and the swinging of the pendulum. Shortly after this, Johannes Kepler established the three laws that explain all the motions of the planets. Finally, in the late 17th century Isaac Newton explained these results by establishing the law of gravitation. This law applies invariably to all matter in the universe—whether it is as small as a grain of sand or as large as the Sun. This triumph of explaining a vast range of phenomena with a single law inspired workers in all fields of knowledge to trust scientific methods.

This revolution in understanding was greatly aided by concurrent advances in technology. Instruments such as clocks, barometers (which measure the pressure of the atmosphere), and telescopes were invented and improved. For example, Galileo, Kepler, and Newton made contributions to the development of telescopes and thus gave astronomy a powerful instrument with which to work.

There is no exact distinction between physics and other natural sciences because all sciences overlap. In general, however, physics deals with phenomena that pertain to all classes of matter and energy. Physicists try to discover the most basic laws of nature, which underlie and often explain those of other fields of science.

One major branch of physics, mechanics, deals with the states of matter—solids, liquids, and gases—and with their motions. The pioneer achievements of Galileo, Kepler, and Newton dealt with solid masses of matter in motion. Such studies are a part of the subdivision of mechanics called dynamics, the study of matter in motion. This wide-ranging topic includes not only the motions of stars and baseballs but also those of gyroscopes, of the water pumped by a fire engine (hydrodynamics), and of the air passing over the wings and through the jet engine of an airplane (aerodynamics).

The other great subdivision of mechanics is statics, the study of matter at rest. Statics deals with the balancing of forces with appropriate resistances to keep matter at rest. The design of buildings and of bridges are examples of problems in statics.

Other divisions of physics are based on the different kinds of energy that interact with matter. They deal with electricity and magnetism, heat, light, and sound. From these branches of physics have come clues that have revealed how atoms are constructed and how they react to various kinds of energy. This knowledge is often called the basis of modern physics. Among the many subdivisions of modern physics are electronics and nuclear physics.

Physics is closely related to engineering. A person who uses physical principles in solving everyday problems is often called an engineer. For example, electricity is one of the subdivisions of physics; one who uses the natural laws of electricity to help in designing an electric generator is an electrical engineer.

SCIENTIFIC METHODS USED IN PHYSICS

Physics attempts to describe and explain the physical universe. Physicists therefore try to discover one or more laws—invariable principles of nature at work—that will describe a large class of phenomena. Newton’s law of gravitation is a fine example. Another is the law of reflection—“the angle of reflection is equal to the angle of incidence.”

Physicists express these laws in mathematical form, which can serve later as a basis for measurements and calculations. For example, Newton’s law of gravitation states that the force of gravitational attraction (F) between two separate objects depends on the amount of mass (m) of each one and the distance (R) between them. The masses must be multiplied together, and the pull diminishes according to the square of the distance. If the distance is doubled, for example, the gravitational pull is only one fourth as great. The whole law can be stated in a short formula: F  =  Gm₁m₂/R², where G is the universal gravitational constant.

This formula can be used in turn to give answers to a host of problems. Newton used it to help explain Kepler’s laws. Later it was used to find the masses of other planets, stars, and galaxies. Today it is also used (with some corrections courtesy of Albert Einstein) to plan the trajectories of spacecraft.

Whenever possible, physicists try to discover laws and test them by experiments in which the variables involved can be controlled or measured accurately. For example, Michael Faraday discovered the laws of electrolysis by measuring how much material was transported by known amounts of electric current in an electrolytic cell. Robert A. Millikan determined the fundamental unit of electricity, the charge carried by one electron. He did so by making thousands of measurements upon microscopic droplets of oil that were kept dancing in a vacuum between oppositely charged metal plates.

Often when physicists cannot make a direct experiment, they can solve problems indirectly. An example is the way physicists learned the chemical composition of the Sun and other stars. Direct tests were impossible, because no human being could hope to approach close enough to a star to get a sample for chemical analysis. About a century ago, however, a way was found around this difficulty.

For a long time, physicists had known that light from a glowing substance, such as the matter in the Sun, can be separated into bands of colored lights, called the spectrum. In the case of sunlight, this spectrum is crossed by many fine dark lines, called Fraunhofer absorption lines. In 1859 Gustav Kirchhoff and Robert Bunsen found that these dark lines could be matched exactly by bright ones produced by heating chemical elements to glowing in a laboratory. Thus the lines show exactly which chemical elements are in the Sun. In fact, the element helium was found first in the Sun by its spectral lines and only later identified on Earth. This technique, called spectroscopy, made it possible to learn the chemical composition of a star by using a spectroscope attached to a telescope. Modern refinements also indicate a star’s temperature and the speed and direction of its motion.

Both conducting experiments and formulating theories thus play essential roles in the advancement of physics. Physical experiments result in measurements. Scientists compare these measurements with the outcome predicted by theory. A theory that reliably predicts the results of experiments is said to embody a law of physics. However, a law may need to be changed, limited, or replaced if a later experiment makes it necessary.

HOW KNOWLEDGE OF PHYSICS DEVELOPED

Many ancient cultures demonstrated curiosity about the world around them. Through patient observation, they found patterns in nature that could be used to make certain kinds of predictions. For example, the ancient Egyptians noticed that when the star Sirius began rising in the morning before the Sun, the Nile River would soon flood. This prediction was of immense practical importance since the flooding was the basis for their agriculture. However, without the tools and methods of physics, they had difficulty finding explanations for such patterns and were often content with the practical results they had obtained.

The ancient Greeks, on the other hand, were generally not content with only simple knowledge gained through practical experience. They sought deeper explanations. The ancient Greeks thought the world to be a rational place, with its “secrets” accessible to the powers of human intellect. This belief, along with the lack of good experimental methods, led many to try to explain nature through reason alone.

Naturally, different thinkers arrived at different opinions, and they argued passionately for them, often from such principles as beauty, symmetry, and simplicity. While experience has since shown such concepts to be indeed very useful in physics, ancient thinkers had no objective way to determine who was right. Some propositions, such as Democritus’ theory that matter is composed of elementary “atoms,” turned out to be basically correct. Often, though, the theories were later overthrown. (For example, Ptolemy’s theory that Earth was the center of the universe later had to be abandoned.) Nature had simply not yet been described accurately enough, or in sufficient detail, to make these attempted explanations meaningful.

Scientists began making progress almost 1,500 years later, in Galileo’s time, by attacking specific problems that could be tested with well-defined methods. They let the formulation of more-general theories wait until enough phenomena had been described in sufficient detail to warrant an explanation.

During the two centuries after Galileo’s time, tremendous progress was made. By the early 1800s, physicists had won considerable basic knowledge about the interactions of specific forms of energy, such as heat, and matter. Even then, however, each type of interaction had to be studied separately. More than a century passed before anybody could begin to gather the interactions together into a general theory of the physical universe.

A survey of physics can be made best by proceeding as the physicists did, considering first the separate, specific types of interaction between matter and energy. The most basic topic in the study of physics is mechanics, because it establishes fundamental measurements that enter into all interactions. Different units are needed for measuring various forms of energy such as light and electricity. However, physicists keep these units consistent with those used in mechanics. This practice helps to hold all the branches of physics together as one body of knowledge.

ANCIENT BEGINNINGS OF MECHANICS

Primitive humans maintained their place in the world by learning to use stone tools, bows and arrows, and other mechanical devices. No one knows when they began to use sticks as levers or how and when they developed the wheel, but it is known that the physics of simple machines and the science of measurement developed early.

In about 2600 bc the Great Pyramid of Egypt was built with sides of very nearly the same length and corners that are very nearly right angles. This achievement required excellent measuring equipment and skill in using it. 

In about 330 bc Aristotle wrote the treatise Physics, which was the dominant authority for many centuries. Although many principles contained in this work have proved valid, some are wrong. Perhaps the most famous is the statement that heavier objects fall through a given distance in less time than lighter ones. It was almost two thousand years later before Galileo’s experiments proved this to be incorrect.

Another great scientist of ancient Greece was Archimedes. One of his most celebrated discoveries was the law of buoyancy, which explains why objects float or sink in fluids. He also made important contributions to knowledge of levers and pulleys.

GALILEO FOUNDS MODERN MECHANICS

Although a number of important discoveries in mechanics were made during the next 18 centuries, it was Galileo who opened the door to an entirely new world of physics. At the age of 19 he is said to have timed with his pulse the swings of a great chandelier in the cathedral at Pisa. He found that the swing always took the same time, even though the size of the swing became smaller and smaller. Galileo later developed the idea that a simple pendulum could be used for measuring time. Pendulum clocks were a great improvement over the sand and water clocks then in use.

Galileo studied the motions of falling bodies. He found that, in contradiction to Aristotle’s claim, heavy bodies fall at exactly the same speeds as lighter ones when air friction is discounted. (Friction is a force that slows movement between objects that are rubbing against one another, such as air molecules and a falling ball.) Galileo also studied accelerated motion by rolling balls down inclined planes. His experiments laid the foundation for modern mechanics.

Earlier scientists such as Roger Bacon also had insisted upon careful observation and experiment as the way to win knowledge, rather than depending upon mere appearances. Nonetheless Galileo is considered the father of the experimental, or scientific, method because he devised critical experiments that forced conviction even though the results contradicted earlier authorities.

NEWTON’S MONUMENTAL CONTRIBUTIONS

In the year Galileo died (1642), there was born in England one of the greatest scientists of all time, Isaac Newton. His experiments with light laid the foundation for the modern science of optics. He also built the world’s first reflecting telescope (a telescope that uses mirrors to focus light). His studies of falling bodies and of the solar system led to his celebrated law of universal gravitation (which was discussed above).

Newton developed a special mathematics for treating problems in mechanics. Thus he became one of the discoverers of calculus. The other was Gottfried Wilhelm Leibniz. Newton provided firm bases for expressing natural laws as mathematical formulas. His method started with locating objects in space with measurements made to three axes at right angles to each other through a chosen point called the origin. Such measurements are called Cartesian coordinates after René Descartes, who devised them.

Newton also discovered many of the basic laws of mechanics, including the three fundamental laws of motion, which he published in 1687. These laws describe the relations between the forces (pushes or pulls) acting on an object and the motion of the object.

The first law states that, if an object is at rest (not moving), it will remain at rest unless a force acts upon it. Likewise, if an object is moving in a straight line at a constant speed, it will keep moving in a straight line at that speed unless a force causes it to change its direction or its speed, or both. This principle had been discovered by Galileo and perfected by Descartes.

Newton’s second law is one of the most important laws in all of physics. It concerns the changes that a force can produce on an object’s motion. A force can change an object’s speed, the direction of its motion, or both. An object’s velocity expresses both how fast and in what direction the object is moving. Acceleration is the rate at which an object’s velocity changes. According to Newton’s second law, when a force acts on an object, it produces a change in the velocity of the object in the direction of the force. The magnitude (size or strength) of the acceleration is proportional to the magnitude of the force. For example, if a person kicks a ball, the ball will move in the direction it was kicked. The stronger the kick and the lighter (less massive) the ball, the greater the acceleration. The force (F) acting on an object is equal to the mass (m) of the object times its acceleration (a), or F = ma.

Newton’s third law states that when two objects interact, they apply forces to one another that are equal in strength and opposite in direction. In other words, for every action (or force), there is an equal and opposite reaction (force). When a person hits a ball with a tennis racket, the force of the racket causes the ball to experience a sudden change in motion. At the same time, the ball exerts an equal and opposite force on the racket. This force pushes backward on the racket, and the player feels the shock of the impact.

That's it for today. Tomorrow we will look at this more deeply. Until next time, stay curious, and stay sciencey!