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Definition:As an object moves through a gas, the gas molecules are deflected around the object. If the speed of the object is much less than the speed of sound of the gas, the density of the gas remains constant and the flow of gas can be described by conserving momentum and energy. As the speed of the object increases towards the speed of sound, we must consider compressibility effects on the gas. The density of the gas varies locally as the gas is compressed by the object
Mach Number:
http://www.grc.nasa.gov/WWW/k-12/airplane/mach.html
Example:
Photos:
http://edgerton-digital-collections.org/?s=hee-sc-09001#hee-sc-09001
Why?
Formulas:
Mach Number;
http://www.grc.nasa.gov/WWW/k-12/airplane/mach.html
References:
DRAFT:
http://videos.howstuffworks.com/discovery/29379-assignment-discovery-shock-wave-video.htm