Counting cracks in glass gives speed of projectile

Simple relationship between velocity and number of spokes in star-shaped fracture

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A shattered windshield has a story to tell. The key to hearing it is counting the cracks.

BB-sized pellets burrow through millimeter-thick Plexiglas plates in these high-speed camera photos. A 80-kilometer-per-hour pellet creates four cracks (left), while a 204-kph one creates eight.