|MadSci Network: Physics|
It is amazing how much damage a high-speed impact of one material into another can do. In your example of a lead projectile impacting into a steel plate at approximately 4000 feet per second (~1.2 km/s) a hemispherical cavity or crater is formed. The ejecta from this impact have traveled up to 100 meters in distance, and, even though the ejected particles are fairly small in size, they were moving fast enough to etch glass and punch through paper. A number of experiments involving impact crater formation have observed similarly fast-moving ejecta. In fact, the earliest ejected particles from impact craters can be moving as fast as the initial projectile that impacted the surface. In your example, that would mean that the fastest moving ejected particles, at the very earliest times after impact, could be moving at 4000 feet per second (~1.2 km/s). Even particles ejected halfway through the crater formation could be moving fast enough to etch glass 100 meters away. These ejected particles are moving fast enough that they create small craters when they impact a surface – these craters are called "secondary craters" because they were formed by ejecta from the first, or primary, crater. Secondary craters are observed in laboratory experiments as well as on planetary bodies, such as the Moon. For more information on impact craters, check out the Canadian National Geophysics Terrestrial Impact Crater Database: http://gdcinfo.agg.emr.ca/crater/ index_e.html A general text that includes a good section on Impact Cratering is "The New Solar System" by J K Beatty, CC Petersen and A Chaikin. (1999, 4th Edition, but any edition will do). Cambridge Univ. Press. If you’d like to read some scientific papers that deal with ejection velocities, let me recommend: M J Cintala, L Berthoud & F. Horz (1999) Ejection-velocity distributions from impacts into course-grained sand. Meteoritics and Planetary Science, Vol 34, p.605-623. This paper discusses experiments where the ejecta particle velocities were measured. Graphs within this paper show how the ejecta velocity starts off very high and decreases with distance from the crater center. K R Housen, R M Schmidt & K A Holsapple (1983) Crater ejecta scaling laws: Fundamental forms based on dimensional analysis. Journal of Geophysical Research, Vol 88, p. 2485-2499. This paper is more of a theoretical treatment of ejecta velocities, but compares a number of experimental results with theory. J L B Anderson, P H Schultz & J T Heineck (2001) Oblique impact ejecta flow fields: An application of Maxwell’s Z Model. Lunar and Planetary Science Conference 32, #1352. (Available online at: http://www.lpi.usra.edu/ meetings/lpsc2001/pdf/1352.pdf ) The experiments that I’ve been doing directly measure impact crater ejecta velocities. Check out my most recent abstract that includes a graph showing the velocities we’ve measured in the laboratory – very fast indeed!
Try the links in the MadSci Library for more information on Physics.