[File:Concussion Anatomy.png|thumb|right|Head impact can cause concussion. Sports
helmets help protect against brain injury. ]] In
mechanics, an
impact is a high
force or
shock applied over a short time period when two or more bodies collide. Such a force or
acceleration usually has a greater effect than a lower force applied over a proportionally longer period. The effect depends critically on the relative velocity of the bodies to one another.