The physics answer to this is that the center of mass of any system will remain in uniform motion. So, for example, before explosion, the projectile's center of mass moves upwards in an arcing trajectory. Upon explosion, the center of mass of all the pieces is STILL continuing on the same trajectory! So, on average, you'll find a ball of burning pieces radiating from this center of mass.
Not all fireworks explode symmetrically. The shape of the burst depends on the design of the shell. If the explosive charge in the shell is in the center of uniform material, the burst will be symmetric. However, if the explosive charge is near one edge, the burst will be lateral. No doubt you have seen fireworks that spray sparks to one side. See http://en.wikipedia.org/wiki/Firework for a description of different burst patterns.
They are usually designed to do so. A commercial firework consists of a lifting charge of black powder, in a cylindrical package, which hoists a bursting charge, which is usually spherical but may be cylindrical. A slow fuse is ignited by the lifting charge, timed to set off the bursting charge when the projectile is near maximum height.
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The physics answer to this is that the center of mass of any system will remain in uniform motion. So, for example, before explosion, the projectile's center of mass moves upwards in an arcing trajectory. Upon explosion, the center of mass of all the pieces is STILL continuing on the same trajectory! So, on average, you'll find a ball of burning pieces radiating from this center of mass.
Not all fireworks explode symmetrically. The shape of the burst depends on the design of the shell. If the explosive charge in the shell is in the center of uniform material, the burst will be symmetric. However, if the explosive charge is near one edge, the burst will be lateral. No doubt you have seen fireworks that spray sparks to one side. See http://en.wikipedia.org/wiki/Firework for a description of different burst patterns.
They are usually designed to do so. A commercial firework consists of a lifting charge of black powder, in a cylindrical package, which hoists a bursting charge, which is usually spherical but may be cylindrical. A slow fuse is ignited by the lifting charge, timed to set off the bursting charge when the projectile is near maximum height.
they are designed to because it looks good.
i don't think they are very symetrical.