The word "shock" has developed many meanings and uses since it's origin in about the 1560's A.D. At that time, "shock" was a military term meaning a "violent encounter of armed forces or a pair of warriors." It derives from the French word "choc" (violent attack) and the Old French "choquer" (to strike against). Additional uses have arisen through history, such as: (1) a sudden or violent mental or emotional disturbance; (2) a medical state associated with a severe depression in vital processes such as blood pressure; (3) a convulsive contraction of muscles caused by discharge of electricity through an animal body; (4) a procedure in which a hot vegetable is plunged into ice water to stop its cooking. Or, as shown to the left, a shock of corn!
In this discussion and related ones that will follow, I use the term to describe "a sudden change in the properties of a physical system." The physical system might be a gas, such as air or the atmosphere of a star, in which case the thermodynamic variables such as pressure, temperature, density change suddenly over a narrow region, often only a few molecules in width. Or, the physical system might be water, in which case the depth of water changes suddenly. Or, in a commonly used analog, the system might be flowing traffic, or skiers on a slope (see image), where a sudden obstruction such as an accident (or a tree in the cartoon shown here) causes the velocity and density of vehicles to change suddenly.
Shock wave theory has developed in parallel with military needs. Quantitative analyses of shock waves are relatively young. Courant and Friedrichs wrote the classic book on supersonic flow in 1948--note the date relative to WWII and the use of bombs in that war. A still-popular volume "Physics of Shock Waves and High-Temperature Hydrodynamic Phemomena" was published by two Russians, Zel'dovich and Raizer, in 1963, and translated into English in 1966. This English translation is "only" 915 pages long!!
An example of an
air
shock occurs when an airplane reaches high velocity.*** An observer on
the ground experiences this as a sonic boom that quickly comes and goes.
The shock appears to be moving to an observer on the ground, but to the
pilot in the aircraft, it is stationary, attached to the rear of his
aircraft. In the image of a supersonic jet at the left, the white area
contains 
Something as small as a whip or as large as a jet aircraft (see image) can generate shock waves. Shock waves are also generated when meteors hit the earth. Large meteors burrow create huge craters (more later) but even smaller ones can be destructive. For example, in 1650 B.C., it is speculated that a meteorite decimated the town of Tall el-Hammam in Jordon (see here). The event here may be the basis for one of the early Genesis stories. The object that caused this destruction was probably ~50 meters in diameter, and released its energy of ~1 Megaton into the atmosphere in a burst in the atmosphere. According to the website cited above "...the flash of heat from the explosion would have caused hair and textiles to burst into flame, and melted metal and metal and brick. Moments later, a shock wave would have hit, causing winds that researchers estimate reached speeds of up to 1,200 km/h--knocking structures in the town flat and killing every living thing in the town."
In the next post, I'll talk about the structure of a shock wave and the expansion wave that follows it to bring conditions back toward the ambient condition of pre-shock pressure.
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***(Here is a good explanation of the history of humans trying to achieve ever higher and higher velocities.)
***A good Youtube tutorial on the formation of shock waves is here, and a good text review is here.
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