2 2 GENERAL SURVEY 2 GENERAL SURVEY 11 3 2.1 TERMINOLOGY AND SCOPE Max PLANCK, an eminent German physicist, appropriately said, “Theory and experiment, they belong together; the one The great book of Nature lies ever open before our eyes without the other remains unfruitful. We are fully justified in and the true philosophy is written in it… But we cannot applying KANT’s well-known words on the unity of concept read it unless we have first learned the language and the and intuition and saying: theories without experiments are characters in which it is written… It is written in mathe- empty, experiments without theory are blind. Therefore both matical language and the characters are triangles, circles and other geometric figures.1 − theory and experiment − call for proper respect with the same emphasis.” Galileo GALILEI Florence 1623 Mechanics became the most important and advanced branch of natural philosophy − a term used by Sir Isaac In physics, experiments have a larger power to per- NEWTON to denote investigations of laws that hold in the suade than reasoning.2 material world and the deduction of results that are not di- Blaise PASCAL rectly observable, and which is today covered by the field of Paris 1663 physics. According to the German philosopher Immanuel KANT, science is characterized by an ordered arrangement of ERCUSSION, concussion, collision, impact, explosion, gained knowledge, based on data and observed phenomena P implosion, detonation and shock waves are rapid me- as well as on similar cases and their critical testing through chanical phenomena that are related to each other. Since they the application of “creative inspiration.” Phenomena that ap- cannot be resolved by the naked eye, for a long time their peared similar were particularly puzzling to early naturalists, sudden and discontinuous nature was hidden behind a veil of such as vague hypotheses and suppositions. First experimental evi- the instantaneous, discontinuous character of the ve- dence on physical quantities such as motion (velocity, accel- locities observed during the percussion of tangible eration), force and thermodynamic state (density, pressure bodies and of air molecules at a steep shock front; and temperature) were not obtained until significant progress the wall-like, crested front of a tidal bore in a river and was achieved in the late 19th century in high-speed diagnos- the steep front of a shock wave in air; tics, visualization techniques and photographic recording. the bow wave generated by a body moving through Increasing individual knowledge of high-speed events led water (Kelvin envelope or Kelvin wake) and the head observers to recognize connections between different phe- wave (Mach cone) generated by a projectile flying su- nomena, which stimulated analyses of interrelations. This personically, or by a planet moving through the solar initiated a purposeful, more systematic research character- wind; ized by experimental testing of qualitative hypotheses, the irregular reflection properties of hydraulic jumps which, beginning in the mid-1940s, eventually led to com- and shock waves; and puter-supported quantitative modeling of new concepts. the propagation behavior of a shock wave and a deto- Mechanics developed in the 17th century from mere con- nation wave. templations of accounts of observations and experiences into Since the first scientific investigations of the nature of per- one of the main pillars of the physical sciences. In the late cussion of tangible bodies, an increasing number of new dis- 16th century, the Italian physicist and mathematician Galileo continuous high-rate phenomena have been observed, calling GALILEI reestablished mathematical rationalism over ARIS- for new definitions, explanations and classifications. To TOTLE’s logico-verbal approach and suggested that experi- some extent the technical terms used by early natural phi- mentation should be used primarily to check theoretical losophers to describe high-speed phenomena reflect their deductions, while the French philosopher Blaise PASCAL early knowledge and understanding. However, as we have favored the principle of empiricism and proposed the looked more deeply at the physical processes over decades experimental approach. Both conceptions, supplementing of increasing research activity, the meanings of some of the each other in a unique manner, established the principle terms have changed, and our insights into their complexity that was to become the guiding maxim of modern science. 3 M. PLANCK: Zum 25jährigen Jubiläum der von W. FRIEDRICH, P. KNIPPING and M. VON LAUE gemachten Entdeckung [Lecture held du- 1 G. GALILEI: Il saggiatore (“The Assayer”). Mascardi, Roma (1623). ring the meeting of the DPG in Berlin on 9th June 1937]. In: (hrsg. vom 2 “Dans la physique, les expériences ont bien plus de force pour persuader Verband dt. Physikalischer Gesellschaften) Max PLANCK. Physikalische que les raisonnements.” See B. PASCAL: Traité de la pesanteur de la mas- Abhandlungen und Vorträge. Vieweg & Sohn, Braunschweig (1958), se de l’air. Desprez, Paris (1663), chap. I. Bd. III, pp. 364-367. 12 2 GENERAL SURVEY has created a wealth of new, related terms. The main techni- elastically, like hard elastic spheres (e.g., glass marbles, bil- cal terms used in the fields of percussion, collision, impact, liard balls). In contrast, inelastic percussion between moving explosions, detonations and shock waves are discussed be- bodies produces permanent deformation. While momentum low in more detail. is conserved here as well, kinetic energy is not. Very old ex- amples of inelastic percussion are the soft-hammer percus- sion method used in flint knapping {⇨Fig. 4.2–D(d)} and 2.1.1 PERCUSSION, CONCUSSION, IMPACT, the wooden mallet used in combination with a metal chisel AND COLLISION in stone masonry {⇨Fig. 4.2–E} and wood carving. Modern vehicle design attempts to largely absorb the kinetic energy Many early terms describing high-speed phenomena were in the case of collision accidents by using materials which derived from Latin, then the language used in most learned deform plastically. works: examples include the terms collision, percussion, ex- The word percussion [Germ. der Stoß] – used almost ex- plosion, and detonation, which are still in use today. How- clusively by French and English natural philosophers ever, reflecting the state of knowledge of each time period, throughout the 17th and 18th centuries, and also partly in the they only gradually evolved into their present-day meanings. 19th century – relates to the analysis of the physical process This slow process of arriving at clear definitions, starting in involved when bodies striking each other with some degree the 17th century, was not caused by poor communication. of force, while the resulting effect is described by words Surprisingly, many early naturalists used to exchange know- such as shock, blow, impact, knock etc. Percussion refers to ledge and ideas with their colleagues abroad at an intense solid bodies, more rarely to liquids, and to air.5 It is interest- level via correspondence or by traveling. Actually, in order ing to note here that the word “percussion” will prompt most to comprehensively characterize the essentials of high-speed English-speakers to initially think of music rather than phys- phenomena it is necessary to obtain a deep understanding of ics, ballistics and engineering. On the other hand, some en- how the phenomenon in question evolves over time, both cyclopedias (such as the 1974 edition of the Encyclopedia qualitatively and quantitatively. This learning process, which Americana) only refer to the medical meaning of the term only evolved slowly because of the insufficient temporal percussion {VON AUENBRUGGER ⇨1754}. resolution of early diagnostics, will be discussed in more de- In modern textbooks on mechanics, the classical theory of tail in Sect. 2.8. percussion is often referred to as the “theory of impact,” while in physics textbooks it is usually called the “theory of colli- 4 Percussion. The term percussion designates the action of sion.” Indeed, Edward J. ROUTH {⇨1860} used the term im- striking of one moving object against another with signifi- pact rather than percussion in his widely used textbook Dy- cant force. Since the birth of classical percussion in the 17th namics of a System of Rigid Bodies. Thomas J. MCCORMACK, century {HUYGENS ⇨1668/1669; WREN, WALLIS ⇨1669; who translated Ernst MACH’s book Die Mechanik in ih- Sir NEWTON ⇨1687; ⇨Fig. 2.16}, the term has been con- rer Entwicklung, historisch-kritisch dargestellt {E. MACH ventionally applied in reference to solid bodies, and in this ⇨1883} into English, followed ROUTH’s terminology and sense it has also been used throughout the evolution of (ter- translated Die Theorie des Stoßes into “The Theory of Im- minal) ballistics. The fundamental theory of percussion was pact.” However, the term center of percussion − coined in based on two spheres of the same material but different England in the late 17th century {WALLIS ⇨1670/1671} − has masses moving in a straight line and impacting either head- long been used by both mechanical engineers and physicists. on (central percussion) or at an angle (oblique percussion). The principle of percussion has been widely applied in Real percussion phenomena depend on the shapes of the military technology, but also in civil engineerinmg and impacting bodies, their masses, their elastic properties (rigid, medical diagnostics which created a number of percussion- perfectly elastic, elastic or inelastic), and their initial veloci- related terms (see Sect. 2.2.4). ties. In purely elastic percussion no permanent deformation takes place, and both momentum and kinetic energy are con- Concussion. The term concussion6 describes the action of served. In the early development of the kinetic theory of violently shaking or agitating, particularly in relation to the gases, percussion models generally assumed that the gas shock of impact. In the past it was also used to describe the molecules collide with one another or with the wall perfectly 5 J.A.