By Nnamdi Ikeh-Akabogu (FRSCN)

A seatbelt helps to prevent injury in the event of a car crash by reducing the velocity of a body as it experiences a sudden decrease in speed.

Due to the body’s inertia, which is its ‘resistance to a change in speed or direction of travel’, a passenger in a vehicle will want to continue travelling forwards once the car has reached a sudden stop.

If the vehicle is travelling at 50mph and crashes into a brick wall, instantly reducing its velocity to zero, the passenger will continue moving forwards at 50mph unless there is something in front of them to create a ‘stopping force’.

This is because the velocities of the car and passenger are independent.

A seatbelt spreads the stopping force needed to decelerate the passenger across their body.

This prevents the body from hitting the windshield or steering column of a car at high speed, which could easily result in injury or death.

The belt is designed to apply most of the stopping force required to the pelvis and rib cage, both of which are relatively robust.

Since stress is inversely proportional to the area at which a force is being applied, we can deduce that if the stopping force is spread across a larger area, the less stress the body will experience in the event of a crash.

Since an abrupt stopping force could contribute to a passenger’s injury, the material from which a seatbelt is constructed is designed to allow for a small amount of movement as the body tries to move forwards.

Lengthening the time taken for the body to come to a stop helps to reduce the impact that the body experiences.

Typically, a seatbelt will include a ‘retractor mechanism’ that causes a spring inside the retractor to apply a rotational force to the spool when it is being pulled.

This means that the belt tightens once it becomes loose to reduce the amount of ‘slack’ in the material, helping to secure the passenger to their seat.

Modern seatbelt mechanisms also include a pretensioner, which pulls the belt inwards once the car comes to an abrupt stop. The pretensioner is attached to a chamber of combustible gas and a central processor, which can detect a rapid decrease in the car’s velocity. Once this is detected, an electrical current is sent across two electrodes, which causes the gas to be ignited. This creates pressure that pushes on a piston in the chamber, causing the seatbelt spool to quickly rotate, winding up any loose material.

The traffic expert is DCC Nnamdi Ikeh-Akabogu, DCC Morning and Evaluation (M&E), Special Duties and External Relations (SEDER), FRCC HQ, Abuja.

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