Speedy Projectiles

When I wrote earlier about viscosity and speedy bullets, Guest flogged me for my explanation of penetrating power. I thought it was about the speed of sound, and he said "pressure" and made me figure out why. That's all in the comments to the linked post.

I knew that speed of sound had to be involved though, so I needed to think that through as well. Here what I came up with while trying to go to sleep the other night.

It is pressure that melts the solid (by compressing the material until the intermolecular forces cease to bind, but the speed of sound is important too. Excess pressure is radiated away at the speed of sound, in the form of sound waves, but when the disturbance (hypervelocity projectile, say) is moving faster than the speed of sound, pressure does not have time to be radiated away. Thus, excess pressure accumulates ahead of the projectile and does its melting work until the projectile slows below sound speed and permits it to radiate away.

All that is momentum based reasoning. Energy can be important too, especially for larger projectiles. It can do its work by heating the target above vaporization temperature.

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