Text 8. Newton’s first law of motion – inertia 


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Text 8. Newton’s first law of motion – inertia



(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) More than 2000 years ago, ancient Greek scientists were familiar with some of the ideas in physics that we study today. They had a good understanding of some of the properties of light, but they were confused about motion. One of the first to study motion seriously was Aristotle, the most outstanding philosopher-scientist of his time in ancient Greece. Aristotle attempted to clarify motion by classification.

2) Aristotle divided motion into two main classes: natural motion and violent motion. We shall briefly consider each, not as study material, but only as a background to present-day ideas about motion. Aristotle asserted that natural motion proceeds from the "nature" of an object, dependent on what combination of the four elements (earth, water, air, and fire) the object contains. In his view, every object in the universe has a proper place, determined by this "nature"; any object not in its proper place will "strive" to get there. Being of earth, an unsupported lump of clay properly falls to the ground; being of the air, an unimpeded puff of smoke properly rises; being a mixture of earth and air but predominantly earth, a feather properly falls to the ground, but not as rapidly as a lump of clay. He stated that heavier objects would strive harder. Hence, argued Aristotle, objects should fall at speeds proportional to their weights; the heavier the object, the faster it should fall.

3) Natural motion could be either straight up or straight down, as in the case of all things on Earth, or it could be circular, as in the case of celestial objects. Unlike up-and-down motion, circular motion has no beginning or end, repeating itself without deviation. Aristotle believed that different rules apply in the heavens, and he asserted that celestial bodies are perfect spheres made of a perfect and unchanging substance, which he called quintessence (the fifth essence, the other four being earth, water, air, and fire). The only celestial object with any detectable variation on its face was the Moon. Medieval Christians, still under the sway of Aristotle's teaching, explained that lunar imperfections were due to the closeness of the Moon and its contamination by the corrupted Earth.

4) Violent motion, Aristotle's other class of motion, resulted from pushing or pulling forces. Violent motion was imposed motion. A person pushing a cart or lifting a heavy weight imposed motion, as did someone hurling a stone or winning a tug of war. The wind imposed motion on ships. Floodwaters imposed it on boulders and tree trunks. The essential thing about violent motion was that it was externally caused and was imparted to objects; they moved not of themselves, not by their "nature," but because of pushes or pulls.

5) The concept of violent motion had its difficulties, for the pushes and pulls responsible for it were not always evident. For example, a bowstring moved an arrow until the arrow left the bow; after that, further explanation of the arrow's motion seemed to require some other pushing agent. Aristotle imagined, therefore, that a parting of the air by the moving arrow resulted in a squeezing effect on the rear of the arrow as the air rushed back to prevent a vacuum from forming. The arrow was propelled through the air as a bar of soap is propelled in the bathtub when you squeeze one end of it.

6) To sum up, Aristotle taught that all motions are due to the nature of the moving object, or due to a sustained push or pull. Provided that an object is in its proper place, it will not move unless subjected to a force. Except for celestial objects, the normal state is one of rest.

7) Aristotle's statements about motion were a beginning in scientific thought, and, although he did not consider them to be the final words on the subject, his followers for nearly 2000 years regarded his views as beyond question. Implicit in the thinking of ancient, medieval, and early Renaissance times was the notion that the normal state of objects is one of rest. Since it was evident to most thinkers until the sixteenth century that Earth must be in its proper place, and since a force capable of moving Earth was inconceivable, it seemed quite clear to them that Earth does not move.


 

Text 9. Lightning

(from The Feynman lectures on physics mainly electromagnetism and matter, by Richard P.Feynman, Addison-Wesley Publishing company Inc., Reading, 1964)

1) The first evidence of what happens in a lightning stroke was obtained in photographs taken with a camera held by hand and moved back and forth with the shutter open—while pointed toward a place where lightning was expected. The first photographs obtained this way showed clearly that lightning strokes are usually multiple discharges along the same path.

2) Later, the "Boys" camera, which has two lenses mounted 180° apart on a rapidly rotating disc, was d eveloped. The image made by each lens moves across the film —the picture is spread out in time. If, for instance, the stroke repeats, there will be two images side by side. By comparing the images of the two lenses, it is possible to work out the details of the time sequence of the flashes. Figure 9-14 shows a photograph taken with a "Boys" camera.

3) We will now describe the lightning. Again, we don't understand exactly how it works. We will give a qualitative description of what it looks like, but we won't go into any details of why it does what it appears to do. We will describe only the ordinary case of the cloud with a negative bottom over flat country. Its potential is much more negative than the earth underneath, so negative electrons will be accelerated toward the earth. What happens is the following. It all starts with a thing called a "step leader," which is not as bright as the stroke of lightning.

4) On the photographs one can see a little bright spot at the beginning that starts from the cloud and moves downward very rapidly—at a sixth of the speed of light! It goes only about 50 meters and stops. It pauses for about 50 microseconds, and then takes another step. It pauses again and then goes another step, and so on. It moves in a series of steps toward the ground, along a path like that shown in Fig. 9-15. In the leader there are negative charges from the cloud; the whole column is full of negative charge. Also, the air is becoming ionized by the rapidly moving charges that produce the leader, so the air becomes a conductor along the path traced out.

5) The moment the leader touches the ground, we have a conducting "wire" that runs all the way up to the cloud and is full of negative charge. Now, at last, the negative charge of the cloud can simply escape and run out. The electrons at the bottom of the leader are the first ones to realize this; they dump out, leaving positive charge behind that attracts more negative charge from higher up in the leader, which in its turn pours out, etc. So finally all the negative charge in a part of the cloud runs out along the column in a rapid and energetic way. So the lightning stroke you see runs upwards from the ground, as indicated in Fig. 9-16. In fact, this main stroke—by far the brightest Dart—is called the return…


 

Text 10. Light quanta

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) The classical physics that we have so far studied deals with two categories of the phenomena: particles and waves. According to our everyday experience, "particles" are tiny objects tike bullets. They have mass and they obey Newton's laws—they travel through space in straight lines unless a force acts upon them. Likewise, according to our everyday experience, "waves," like waves in the ocean, are phenomena that extend in space. When a wave travels through an opening or around a barrier, the wave diffracts and different parts of the wave interfere. Therefore, particles and waves are easy to distinguish from each other. In fact, they have properties that are mutually exclusive. Nonetheless, the question of how to classify light was a mystery for centuries.

2) One of the early theories about the nature of light is that of Plato, who lived in the fifth and fourth centuries BC. Plato thought that light consisted of streamers emitted by the eye. Euclid, who lived roughly a century later, also held this view. On the other hand, the Pythagoreans believed that light emanated from luminous bodies in the form of very fine particles, while Empedocles, a predecessor of Plato, taught that light is composed of high-speed waves of some sort. For more than 2000 years, the questions remained unanswered. Does tight consist of waves or particles?

3) In 1704, Isaac Newton described light as a stream of particles or corpuscles. He held this view despite his knowledge of what we now call polarization and despite his experiment with tight reflecting from glass plates, in which he noticed fringes of brightness and darkness (Newton's rings). He knew that his particles of light had to have certain wave properties too. Christian Huygens, a contemporary of Newton, advocated a wave theory of light.

4) With all this history as background, Thomas Young, in 1801, performed the "double-slit experiment," which seemed to prove, finally, that light is a wave phenomenon. This view was reinforced in 1862 by Maxwell's prediction that light carries energy in oscillating electric and magnetic fields. Twenty-five years later, Heinrich Hertz used sparking electric circuits to demonstrate the reality of electromagnetic waves (of radio frequency). In 1905, however, Albert Einstein published a Nobel Рrizе-winning paper that challenged the wave theory of light by arguing that light interacts with matter, not in continuous waves, as Maxwell envisioned, but in tiny packets of energy that we now call photons. This discovery didn't wipe out light waves. It revealed, instead, that light is both wave and particle.


 



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