Cracked Windshield Physics

Videos of high speed steel projectiles breaking through plexiglass targets.
Top: A projectile traveling at about 50 MPH impacts a 1 mm-thick target.
Bottom: A 126 MPH projectile hits a plexiglass target.
Video Credit: N. Vandenberghe/Aix-Marseille Univ.

May 6, 2013

Researchers have found a surprisingly simple relationship between the amount of radial cracks in plexiglass targets and the velocity of impacting projectiles.

According to the researchers from Aix-Marseille University in Marseille, France, the number of resulting cracks would double if the impact velocity were quadrupled. For example, a projectile traveling at 50 MPH would result in half as many cracks as one traveling at 200 MPH.

The researchers hope this relationship may be used in applications including ballistics, forensics, and even planetary science. So next time you see cracks radiate from a point on your windshield, count the number of lines; it might just reveal how fast that rock hit your car.