Backsplatter as bullet penetrates a water container
Jearl Walker www.flyingcircusofphysics.com
Dec 2009 When a bullet is shot through an empty vessel, such as a drinking glass, the vessel shatters into many pieces, most of which move in the direction of the bullet. The impact of the bullet forces them in that direction, giving them momentum (the product of mass and velocity) in that direction.

However, if the vessel is filled with either a liquid or a pliable material, something strange can happen. Material at the entrance point can move in the reverse direction, that is, back toward the weapon. Here are several video examples shown in slow motion.

http://www.youtube.com/watch?v=2U_CBZkuK-w bullet in slow motion

http://www.youtube.com/watch?v=XIayrDbVdJ0&NR=1 slow motion of bullet through various things

When a bullet enters, say, a glass of water, it shoves on the liquid so suddenly that a shock wave (or pressure wave) is sent throughout the interior, with sudden increases in pressure. That pressure increase can propel material out the entrance, back toward the weapon. Depending on the speed of the bullet, it can also blow material out the exit point even before the bullet reaches that point. If the container is not very strong, the pressure increase can cause the container to explode, sending material in nearly all directions.

Here is a grim, historical example. The suggestion that a second sniper was involved in the assassination of President John F. Kennedy was bolstered by the spray of brain debris. Some of the debris was back in the general direction of Lee Harvey Oswald, the gunman. So, the argument was, if the bullet from Oswald passed through Kennedy’s head, how could debris have been propelled back toward the weapon? That must mean there was a second sniper in front of Kennedy, supposedly hidden on the grassy knoll in front of his motorcade. I don’t mean to trivialize Kennedy’s death, but we now have slow-motion videos where we can easily see the reverse propulsion of materials. Thus, the spray of debris does not in itself require a second sniper.

If you are interested in the physics of Kennedy’s assassination, the best paper available is Louis W. Alvarez, "A physicist examines the Kennedy assassination film," American Journal of Physics, 44, No. 9, 813-827 (September 1976). It is a very convincing argument, using several pieces of evidence, including the audio recordings of the gunshots, that there was single gunman.

http://www.youtube.com/watch?v=AUSo6ehRCHI&feature=related 50 caliber bullets pretty much just explode a target

http://www.youtube.com/watch?v=vwTBmXT4-lE&feature=related bullet (from the right) through a glass of wine

References
Dots · through ··· indicate level of difficulty
Journal reference style: author, title, journal, volume, pages (date)
Book reference style: author, title, publisher, date, pages
· Jones, R. V., "Impotence and achievement in physics and technology," Nature, 207, 120-124 (1965)
· Jones, R. V., "Self-frustration," in A Random Walk in Science, R. L. Weber, editor, Crane, Russak & Company, 1973, pages 197-198
· Alvarez, L. W., "A physicist examines the Kennedy assassination film," American Journal of Physics, 44, No. 9, 813-827 (September 1976)
· Edgerton, H. E., and J. R. Killian Jr., Moments of Vision, MIT Press, 1979
· Settles, G. S., and S. S. McIntye, photo, New Scientist, 142, 39 (4 June 1994)
· Levy, M. L., D. Sullivan, R. Faccio, and R. G. Grossman, “The assassination of President John F. Kennedy: a neuroforensic analysis---Part 1: A neurosurgeon’s previously undocumented eyewitness account fo the events of November 22, 1963,” Neurosurgery, 53, No. 5, 1019-1025 (November 2003)
· Levy, M. L., D. Sullivan, R. Faccio, and R. G. Grossman, “A neuroforensic analysis of the wounds of President John F. Kennedy: Part 2---A study of the available evidence, eyewitness correlations, analysis, and conclusions,” Neurosurgery, 54, No. 6, 1298-1312 (June 2004)
·· Wohl, C. G., “Scientist as detective: Luis Alvarez and the pyramid burial chambers, the JFK assassination, and the end of the dinosaurs,” American Journal of Physics, 75, No. 11, 968-977 (November 2007)

Related reference
· Muller, M. K., and J. N. Majerus, “Energy absorption capability of recycled plastic bottles,” Polymer Engineering and Science, 42, No. 7, 1580-1589 (July 2002)
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