The impact depth of a projectile is the distance it penetrates into a target before coming to a stop. The physicist Sir Isaac Newton first developed this idea to get rough approximations for the impact depth for projectiles traveling at high velocities.

Newton's approximation for the impact depth

Newton's approximation for the impact depth for projectiles at high velocities is based only on momentum considerations. Nothing is said about where the impactor's kinetic energy goes, nor what happens to the momentum after the projectile is stopped. The basic idea is simple: The impactor carries a given momentum. To stop the impactor, this momentum must be transferred onto another mass. Since the impactor's velocity is so high that cohesion within the target material can be neglected, the momentum can only be transferred to the material (mass) directly in front of the impactor, which will be pushed at the impactor's speed. If the impactor has pushed a mass equal to its own mass at this speed, its whole momentum has been transferred to the mass in front of it and the impactor will be stopped. For a cylindrical impactor, by the time it stops, it will have penetrated to a depth that is equal to its own length times its relative density with respect to the target material. This approach is only valid for a narrow range of velocities less than the speed of sound within the target or impactor material. If the impact velocity is greater than the speed of sound within the target or impactor material, impact shock causes the material to fracture, and at higher velocities to behave like a gas, causing rapid ejection of target and impactor material and the formation of a crater. The depth of the crater depends on the material properties of impactor and target, as well as the velocity of impact. Typically, greater impact velocity means greater crater depth.

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