|MadSci Network: Physics|
The leading edge of a typical explosion is a shock wave, which travels at the speed of sound. The speed of sound in air is about 350 meters per second, and the speed of sound in water is about 1500 meters per second. In a vacuum there would be no shock wave of this type. However, we can't usually see the shock wave, we can only hear it. The dust or shrapnel or whatever coming out of the explosion may be traveling more slowly than the shock wave. The initial speed of the fragments depends very much on how the explosion was made. Perhaps this explains what you have seen in the movies. Or perhaps what you saw in the movies wasn't realistic.
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