# Why Shock wave propagation is faster

From The Blast Wave

A fraction of a second after a nuclear explosion, the heat from the fireball causes a high-pressure wave to develop and move outward producing the blast effect. The front of the blast wave, i.e., the shock front, travels rapidly away from the fireball, a moving wall of highly compressed air.

From Wikipedia

A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium (solid, liquid, gas or plasma) or in some cases in the absence of a material medium, through a field such as the electromagnetic field. Shock waves are characterized by an abrupt, nearly discontinuous change in the characteristics of the medium. Across a shock there is always an extremely rapid rise in pressure, temperature and density of the flow. A shock wave travels through most media at a higher speed than an ordinary wave.

How shock wave differs from ordinary wave, and how it can travel faster then ordinary wave in same medium. In first ex, In nuclear explosion why shock wave is traveling faster then fireball, when both are in same medium. Can someone please explain the phenomenon in detail.

• The question (to Wikipedia, not you) is: What is "ordinary wave"? Wave of what? Of sound? If the shock wave consists of subatomic particles including photons, than the reason it's so fast seems obvious. May 30 '14 at 7:58
• @brightmagus I think Wikipedia's point is that a pressure wave gets renamed "shock" when the levels are high enough that there are discontinuities in the pressure-space curve and/or the material itself (e.g. atmosphere) is grossly nonhomogeneous. May 30 '14 at 11:55
• possible duplicate of How can shock waves travel faster than sound? Jul 10 '15 at 15:05

## 2 Answers

You can push the air faster than the speed of sound. If you do that, you will get a shock wave. A shock wave in this sense is a "wall" of supersonic-moving particles. You can definitely achieve this if you push on the air hard enough. A nuclear explosion is definitely "hard enough" :)

A shock wave will collide with "normal" stationary air, and give some of the energy to it. As the energy spreads to larger and larger volumes of air, the shock wave decays into a "normal" sound wave pulse. But before that, the wall of highly compressed air will travel at supersonic speed.

A nuclear fireball is the region where the air and the debris from the explosion is so hot that it glows. In the first moments of the explosion, the shock wave compresses the air so hard, that it heats up and glows. As the shock wave loses energy, it will lose it's glow, first "redshifting" and then disappearing almost completely. This phenomenon can be seen in this video: https://www.youtube.com/watch?v=KQp1ox-SdRI

As per my answer here. The air on the inside of the explosion is also moving "faster then the speed of sound" so relative to that air, the shock wave is traveling at subsonic speeds. Thus the shock wave travels at a weighted average of the velocity that sound would travel in the mediums on either side of it.