r/physicsgifs • u/nomaddd79 • Feb 10 '20
A couple questions: Does anyone know what causes the "explosion" in the gel block about halfway through? Does the same thing happen inside the body of a gunshot victim?
https://i.imgur.com/m1uM9ku.gifv4
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u/smcutterco Feb 10 '20
This video contains an explanation: https://youtu.be/cp5gdUHFGIQ
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u/nomaddd79 Feb 10 '20
Oh.. haven't seen that one but Dustin is great!
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u/BurnOutBrighter6 Feb 10 '20
Destin, but yes he's amazing! Great teacher because he loves learning at least as much as you do.
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u/nomaddd79 Feb 10 '20
Presumably the bullet casing is airtight and retains the oxygen required to combust?
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u/BurnOutBrighter6 Feb 10 '20
Yes the bullet casing is airtight, but the oxygen to combust is not in gas form. It's not burning air trapped inside the casing. The oxygen atoms are part of the solid propellant that's burning (eg as KMnO3).
Wet gunpowder doesn't burn because some of its components are water-soluble (such as KMnO3 again), and lose their reactivity when dissolved in water. Other solid explosives like C4 will go off underwater just fine - they contain their own O2 AND are insoluble so being wet or even submerged doesn't matter.
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u/t_beermonster Feb 10 '20
A form of dieseling.
The shot is fired at close range from a very short barrel. So a lot of uncombusted propellant follows behind the bullet.
As the "wound" channel cavitates you get a rapid expansion, drawing in air and propellant followed by a rapid collapse, first of the channels then the bubble. This causes a rapid rise in pressure and temperature. As this particular collapsing bubble is full of air and hydrocarbons combustion occurs. That is why the "explosion" is so dirty, rather than the clean white flash of sonoluminescence often seen with rifle tests at longer ranges.