In 1941 the Germans invented the cumulative anti-tank shell. On the cone of the shell is a fuse. On impact it causes detonation and ignites the entire charge. The projectile penetrates the entire armor. In 1944 such German shells came into our hands and into the hands of the Allies. A wide experiment was started. Many additional effects and paradoxes were discovered. They began to find out what was flying, what was penetrating? At first they thought it was an armor-busting projectile, that the armor was pierced by a jet of hot gas. No, it turned out that it is metal, and in the most inexplicable way: in front of the plate at a speed of 8 km/s, inside the plate at 4 km/s, behind the plate again at 8 km/s" (from the opening speech of Academician M.A. Lavrentiev, Presidium Chairman of SB AS USSR, to the students of the Summer Physics and Mathematics School in 1971). Explain this phenomenon. Determine with what speed the wall of a metal conical cavity overlapping the charge moved, if the angle at the apex of the cavity is 30°.