Seat Belt Function and Physics

Seatbelts have two basic functions in the event of a crash: to restrict movement and disperse energy. In a vehicle accident there are really two collisions. The first collision is between the vehicle and another object, for example another vehicle. The second collision is between the vehicle occupant and another object, such as another passenger or the interior of the car. Seat belts are basically designed to prevent this second collision or at least lessen the severity of the injuries caused by it.

When a car is traveling at 30 miles per hour and hits another object it will abruptly stop or quickly decelerate. Unless restrained, a passenger in the car will continue traveling at 30 miles per hour until they are met by an equal and opposite force. The tendency of an object in motion to continue in motion until an external force is applied is known as Newton’s First Law of Motion.

Seat belts and other vehicle safety equipment, such as airbags, are designed around this principal and they are intended to provide a safe method of slowing the body down in the event of a sudden change in speed or direction. The task of the seat belt is to stop the body along with the rest of the car. By restricting the vehicle occupant’s movement in a crash, the seat belt prevents, or at least significantly delays, the occupant’s collision with other objects such as the dashboard or another occupant. Such restraint should also prevent the occupant from being ejected from the vehicle in the event of a crash.

The kinetic energy involved in a vehicle accident can be significant. Seatbelts are intended to reduce the destructive power of these forces by dispersing the energy from a crash across a large surface area and to parts of the body that are more capable of withstanding such forces without serious injury. Also, the energy produced during a crash gets loaded into the seat belt and transferred to the vehicle structure at the anchor points. Statistics show that a properly wore seatbelt that is free from defect will reduce injuries and save lives.

Of course, the effectiveness of a seat belt in preventing injury and death is dramatically increased when combined with other safety features such as airbags and sturdily constructed vehicle roofs. Many modern automotive manufacturers now include multiple airbags in their vehicles which, when properly designed and constructed, can provide passengers with additional protection during front, side, and rear collisions. Unfortunately, seat belt and airbag design and manufacturing defects are very common and result in thousands of unnecessary injuries and occupant deaths every year.

When a seat belt fails due to defects in its design or manufacture and a vehicle occupant gets injured or killed as a result, the seat belt defect can be seen as a legal cause of the injuries or death. As such, the manufacturer of the seat belt, along with other parties in the chain of distribution, can and should be held liable in a court of law.