Projectile
Projectile is a body or object thrown in a given direction with an initial velocity given by means of a projecting mechanism. It carries two motions independently, namely : a) constant horizontal velocity and b) vertically downward acceleration. In physics and ballistics, a projectile is any body projected through space. In military terminology, a missile discharged from small arms or from artillery weapons or a self-propelled weapon such as a rocket or a torpedo, or guided missiles. The terms projectile, shell, and missile are loosely interchangeable, but in modern military usage projectile is preferable as a more precise term.
Trajectory is the path trace by a projectile through space under the action of given forces such as thrust, wind, and gravity .
Maximum height reach ( summit ) is the greatest vertical distance reach by a projectile as measured from the horizontal projection plane.
Time of flight is the time required for the projectile to return to the same level from which it was fired.
Range is the horizontal distance from the point of projection to the point where the projectile returns again to the projection plane.
Galileo’s law of falling bodies led to an understanding of the motion of projectiles. Galileo could look at the fall of an arrow or cannonball and see it as made up of two independent motions: The vertical component was uniformly accelerated and conformed to his law of falling bodies; the horizontal motion imparted to the body by the bowman or gunner was at constant speed. When the horizontal and vertical components were combined, the resultant path was a parabola. This seemingly abstract geometrical account had practical consequences for efficient gunnery.
In a similar vein Galileo investigated mechanics and the strength of materials. In his studies of pendulums he discovered that the swing of a given pendulum takes the same time no matter how large its arc. Others soon pointed out that this was true only if the swing did not become too large.
1938: Physics
Study of the Atom.
The year 1938 continued to be a close follower of its predecessors in the investigation of the structure of the atom. The chemistry textbooks of 35 to 40 years ago proclaimed the atom to be a solid, indivisible particle. In fact its very name, atom, came from the Greek word meaning indivisible.
When Sir J. J. Thomson and his school began the study of the discharge of electricity through gases, and it began to appear that such conduction could not occur without a dismemberment of the atoms and molecules composing the gases, there was much shaking of heads on the part of those who knew the atom was not divisible; but when the late Lord Rutherford, then Professor Ernest Rutherford and a former student of Sir J. J. Thomson, put forth his theory that it was possible to transmute one element into another, the imagination of man had gotten quite beyond his control — or that was the firm conviction of those who still believed in the immutability of the atom.
Throughout all the years in which extensive investigations have been going on concerning the architectural design of the atom, there has been one common procedure; and that is to disrupt the atom and study its component parts amid the particles composing the wreckage. The atom is divisible into a number of smaller particles, and 1938 has witnessed a real advance in our knowledge of what these particles are which constitute the atom.
In order to make clear just what our procedure in studying atomic structure has been, we could refer to the study of an orange. As this fruit comes to us from the store, it appears a solid, spherical-shaped mass with a yellowish color. We could measure its diameter, we could obtain its mass in grams; but not until we removed its skin, pulled apart its sections and dissected them under the microscope, would we begin to understand something about the nature and structure of an orange. Only as we destroy the orange do we begin to know something about it. It is just this procedure which has been pursued for the most part in studying the atom. Bombard it with high speed projectiles until we have blown it into bits, and then see if these parts possess characteristics distinct from the original and are not just minute fragments like the original.
Belief in the value of research on the design and structure of the atom has led not only commercial organizations but large educational institutions into spending huge sums in the installation of various devices for disrupting the atom.
Atom Smashing.
Essentially these outfits for bombarding and thereby disrupting atoms consist, first, in producing unusually large potential differences or, what amounts to the same thing, intense electric fields by which electrically charged particles may be hurled with enormous speeds.
These high-speed projectiles are then directed toward groups of certain definite atoms, and by means of special devices the products of the dismemberment of the atoms may be studied.
There have been at least three main lines along which investigators have moved in developing means for producing high speeds in electrically charged particles and thereby obtaining high-speed projectiles for bombarding the various atoms of the universe:
(1) Development of the old-fashioned electrostatic generators, such as the Toepler-Holtz and the Wimshurst machines. This development has been largely due to the work of Van de Graaff and his associates. There is just being finished at the Massachusetts Institute of Technology such a machine under the personal supervision of Van de Graaff, from which it is hoped that a potential difference of 15,000,000 volts may be obtained.
(2) High potential differences have also been established by means of step-up transformers connected in series. The General Electric Company at Pittsfield, Mass. and the California Institute of Technology in Pasadena have experimented successfully along this line and have produced potential differences of over 1,000,000 volts.
(3) Perhaps the most popular form of device for speeding up charged particles of matter to serve as projectiles for disrupting atoms is the cyclotron, developed by E. O. Lawrence, of the University of California in Berkeley. In this atom-smashing instrument of Lawrence's the charged particles are started on their paths in a magnetic field which is normal to the path of the projectiles. The magnetic field causes the charged particles to move in an ever-widening spiral, and in cyclic steps the particles are given a boost in their speed by the application of a strong electric field. The final speed will depend upon the number of times the charged particles are accelerated, just as the height to which a child goes in a swing will depend upon how many times and how hard a bystander can give the swinging child a push at just the proper time. This form of electric projectile thrower has been developed in over a dozen educational institutions in the United States.
The Van de Graaff machine commends itself for the steadily acting potential difference and also for the magnitude of the energy available in such machines. The advantage of Lawrence's cyclotron is the ability of the apparatus to impart high speed to charged particles without having to use such high potential differences as do the other devices.
In all of these atom-smashing devices one is impressed quite as much by the technical difficulties which had to be overcome to make them successful atomic catapults as by the brilliant results which have come from their use.
There is a fallacy in following too closely the analogy of destroying the orange in order to find out of what it is composed. It does mean its complete destruction so far as the orange is concerned. In the disintegration of the atom (atom smashing), however, this process is accompanied by one of creation. It is a real case of the transmutation of one element into another. In the death of one atom is the life of another.
Projectiles Used.
Thus far little has been said about the nature of the projectiles used in bombarding the atoms in order to disrupt them. There are five projectiles commonly used:
(1) Gamma rays, or x-rays of high frequency and therefore of short wave-lengths. These rays are the least effective of all those used in atomic bombardment.
(2) Protons, or the nuclei of hydrogen atoms (Protuim). These may be obtained from high-voltage discharge tubes operated by a Van de Graaff machine or from a cyclotron. The Proton is a positively-charged particle.
(3) Deuterons, or the nuclei of heavy hydrogen atoms (Deuteron). They may be obtained in a fashion similar to the protons.
(4) Alpha particles, or nuclei of helium atoms, may be obtained in the same way as protons and deuterons.
(5) Neutrons, or the particles having the mass of a proton but not carrying an electric charge. These are obtained from nuclear reactions when atoms are bombarded with the other projectiles just mentioned.
The negatron and the positron, particles which possess 1/1835 part of the mass of the proton, may also be given high speed in an electric field because the first carries a negative electric charge, and the second an equal positive charge. In both cases the charges are equal to the positive charge carried by the proton. Although able to acquire high speed, neither the positron nor the negatron seems to carry sufficient energy to batter the atoms into their component parts and is, therefore, not an effective agent in the disintegration processes of the atom. The negatron, or negatively charged particle, when joined to the proton forms a hydrogen atom, and together they possess a total mass of M11=1.662 x 10-24 grams.
The heavy atom of hydrogen is produced by the addition of a neutron to the ordinary hydrogen atom. Consequently, the nucleus of the heavy atom of hydrogen, the deuteron, consists of one proton and one neutron. It is particularly effective as a projectile for disrupting atoms.
Acquaintance with the alpha particle has extended over a comparatively long period of time, being one of the products of radioactive disintegration with which Rutherford worked. The helium nucleus consists of two neutrons and two protons. In the neutral atom of helium, two negatrons are added.
Component Elements of the Atoms.
Having described some of the high-speed projectiles of the physics laboratory, we may now ask ourselves what are the elements into which we decompose an atom? Are there components common to all atoms? The description of the two hydrogen atoms and of the helium atom at once throws light on atomic structure as possessing common factors.
In the very structure of the helium atom, for instance, we see that the projectiles used are themselves components of the atom. How far have we progressed in a complete analysis of the atoms? All we can say at present is that we have confidence in the existence of some of the constituents of the atoms and possess doubts about others. Very early in the studies of discharges of electricity through gases and of radioactivity, we became acquainted with the negatron and the proton as elements of the atom. We felt as our work progressed that all atoms could be reduced to these two building blocks, and the elements seemed then to be simply different aggregations of these two particles.
In the summer of 1932, however, this simple picture of the atoms was rudely shattered by an experiment performed at the California Institute of Technology by C. D. Anderson. He found unmistakable evidence of a third element within the atom, to which he gave the name positron, a particle possessing the mass of the negatron, 1/1835 part of the mass of the proton, and carrying an equal positive charge.
Then came a series of brilliant discoveries in Germany, France, and England, wherein it was found that when beryllium atoms were bombarded by alpha particles from polonium there were, amid the wreckage, particles which possessed the mass of the proton but were devoid of electrical charges. These particles were called neutrons.
Quite recently new evidence has been produced in cosmic-ray studies tending to show that another particle may exist in the atom; in fact, evidence is deduced that there are two of the same mass, but of equal and opposite charges of electricity. Anderson and Neddermeyer of the California Institute of Technology have given photographic evidence for these particles. At first they were called the X particle with a preponderating tendency in the United States to call this unknown particle the baryton from the Greek meaning a heavy electron. The Danish physicist Bohr would call it the Yukon in honor of Yukawa, the Japanese physicist who first postulated the existence of such a particle. The X particle has a mass about 240 times that of the negatron. Furthermore, Heitler of Bristol University, Bristol, England, has postulated another particle whose mass is the same as the X particle but also devoid of an electrical charge.
Some years ago Bainbridge, of Harvard University, claimed experimental evidence for a particle with a mass equal to that of the negatron or the positron, but also without a charge. This discovery appears now without substantiation, but nevertheless from a theoretical point of view was helpful in explaining the effects found in the disintegration of atoms. At the present time there appear to be three classes of particles composing the atom.
From this list it appears that we have 6 definite particles as elements in the atoms which compose our universe. Just how they are joined together in the atom itself is quite beyond the most imaginative picture we can draw today, but surely progress is being made as we find out more and more about the parts which form the whole.
How the Atom Particles May Be Visualized.
How does the physicist know when he finds a new particle? No naked eye, no microscope however powerful can see these particles just described, and so special means must be devised whereby one may experimentally visualize what is going on.
These high-speed particles are said to be ionizers, that is, when such swift projectiles pass through a gas their impacts on the atoms or molecules of the gas break those atoms up into charged particles called ions. This ionization of the gas makes it electrically conducting, because there are free electrical charges in the space through which the high speed particles have moved.
Furthermore, these ions or charged particles in a conducting gas become the nuclei on which droplets of water will form when the point of saturation is reached and condensation occurs.
If now one focuses his attention on one single high-speed particle as it is fired through such a gas, it will leave a trail of these ions in its wake, on which, if proper conditions are imposed, visible droplets of water will form and indicate, when illuminated, the path along which the high speed particle has been shot.
Illumination of this series of droplets makes the trail of the particle readily visible to the naked eye and also possible of being photographed. In this way the paths of the high-speed particles may be mapped and, if such a particle hits an atom head on, various particles will be ejected from the atom at high speed which, in turn, will make their traces and which are peculiar to the particles making them. In this fashion the investigators have learned a great deal about the constituents of the atoms by their behavior in a so-called cloud chamber. From the direction, magnitude and character of these visible paths of the high-speed particles it is possible to infer a great deal regarding the size and quality of the parts which compose an atom. Such in large part has been the program of research in Pure Physics for 1938.
Applied Physics.
The past five years has seen another important development in the field of physics, viz., Applied Physics.
Heretofore, the moment a physicist applied the principles of physics to some particular problems, he ceased to be a physicist and became an engineer.
No better illustration of this new field can be found than in that branch of work called the physics of solids, particularly of metals. The inquiring metallurgist and the physicist have found that the methods of quantum mechanics are able to throw much light on the properties of solids, such as the interatomic, intermolecular forces, which give us a measure of adhesive and cohesive forces.
Problems of Hardness.
When a scientific association numbering over ten thousand members devotes a large part of the program of its annual meeting to the one subject, hardness, that particular subject must take on unusual significance. At the twentieth annual meeting of the American Society for Metals held in Detroit, Oct. 17 to 21, 1938, a large part of its program was devoted to the conditions which influence the hardness of metals, to the methods for measuring hardness and to allied topics on hardness. In fact, starting on the evening of Oct. 19 shortly after 7 P.M., a symposium was held on the general subject of hardness and was continued until after midnight. This program was of interest to physicists as attested by the number present at this meeting of metallurgists primarily. It is a subject in which physicists are interested because hardness is primarily a function of cohesive and adhesive forces.
What is hardness? Everyone knows when a body is hard and when another is soft. Fundamentally, a body is hard when it offers resistance to penetration, say by one's thumb or some stylus agreed upon. Actually, we know very little about hardness if we are thinking in terms of absolute units of measure. We have all sorts of devices and gadgets for measuring hardness. They give us a relative measure of resistance to penetration, but so many factors come into the measurements, that even these relative values have little significance.
In the final analysis we can at least say that these various so-called hardness testers really measure the suitability of a substance for a particular purpose. Thus, in the manufacture of automobiles, hardness criteria become important means for telling whether the quality of the various parts are suitable for the different purposes to which they are to be assigned. The hardness of the chocolate coating on candies tells us whether it will be suitable for shipping without too much breaking down and forming a soft conglomeration at its journey's end.
During 1938 considerable work has been done investigating the physical processes which go on when the indenter of a hardness tester penetrates the surface of the material whose hardness is to be measured.
Microphotographs of the material immediately around the point of contact of the indenter show that a plastic flow or slippage along crystal planes has occurred. The lines of slip are easily discernible in the photographs. The greater the slippage, the greater the indentation and, therefore, the softer the material. One may define the hardness of a solid as its resistance to slippage along slip planes. This involves cohesive and adhesive forces, as previously mentioned. The same processes come into the picture of the phenomenon of creep, and one must distinguish between creep which seems to last over indefinite periods such as one finds in metals at high temperatures and the temporary creep which one finds at ordinary temperature. Marble, glass, and the modern plastics like Bakelite and Leucite are examples of an indefinite period of slippage at slip planes. As the physicist studies these various physical phenomena will he come to know what hardness means? It is a problem of applied physics in which the physicist of today is profoundly interested.
There has been presented in these pages the two broad lines along which Physics is expanding today — the trails of pure and applied physics with illustrations of each.