When Isaac Newton formulated his laws of motion in 1687 the automobile had not yet been invented. Therefore, he had no way of knowing how his conclusions would affect the investigation of automobile crashes centuries later. The first law of motion is well known: an object either remains at rest or stays in motion at a constant velocity unless acted on by an external force. In this law the term velocity means both speed and direction. In a real world example, if a man is driving his car at fifty-five miles per hour and an opposing vehicle crosses the center line and hits him head on his vehicle will come to an abrupt stop. The driver’s body, however, continues to move at the pre-crash speed and direction until he comes into contact with some part of the vehicle. This contact is sometimes referred to as the second crash.
In investigating serious automobile crashes involving permanent injury or death it is critical to determine how the injuries occurred. This requires a determination of what the occupant struck during the crash sequence. All motor vehicles now contain sophisticated safety features that are designed to allow the occupant of a vehicle to gradually ride down the forces in a crash so as to avoid serious injury or death. These safety features include seat belts, air bags and the structure of the car itself. We all know what seat belts and air bags are. The structure of the car is also critical. Cars are now designed in such a way that the frame and the skin of the car absorb the energy of a crash and direct it away from the occupants. A well designed and manufactured vehicle can prevent serious injury and death in many instances. There are some crashes, however, where the forces are too great to protect the occupants from permanent injury or death.
Occupant kinematics is an analysis of the motion of the occupants inside the vehicle during a crash. An accident reconstructionist will look for occupant contact marks on the inside of the vehicle. These are sometimes called witness marks. When the occupant strikes some part of the interior of the vehicle his velocity is brought in line with that of the vehicle. The pre-crash speed of all the vehicles will determine whether this second impact collision of the occupant with the vehicle creates forces that result in serious injury and death.