The recent PEPCON discussionk, along with a recent MythBusters
rerun, got me to thinking on this subject. The Mythbusters ep in
question is the one where they demonstrate that a Hollwood SFX explosive
can be set off by a high-powered rifle bullet (~1400mph, IIRC), but then
tried to set off a car-trunk load of the stuff by slamming a truck into
it at ~300 mph (using the University of New Mexico's rocket sled), and
failed. A quick BOTE suggests that the truck should have had more than
200x the impact energy of the bullet, so I got to thinking: just what
are the key factors in play here? Total impact energy obviously isn't
it. Is it energy *concentration* -- joules/unit volume, or even
joules/impact area? Or is it the impact *velocity*, as relative to the
shockwave propagation rate in the explosive? Or is this one of those
questions where the answer is "it depends"? :)