Seat Belt

How Does It Work?

Recently, I was on a trip to Hazaribagh to attend the engagement function of my niece. After getting down at the Barkakana railway station, which is about 50 kilometers away from Hazaribagh, I was greeted by my nephew who had come to receive me and take me to his hometown by a car. Once, all of us sat in the car, I fastened the seat belt as per my habit that has developed by virtue of living in the capital of the country.

Looking at me with bewildered faces, my nephew and the driver shouted in unison, ‘Mamaji, there is no need to fasten the seat belt. You are now at Hazaribagh. No need of fear from the police. This is not your Delhi where the cops are always on prowl.’


I just smiled and said, ‘Seat belts are there for your safety. You guys should also wear the seat belt whenever you are traveling by a car.’

Apparently, they did not listen to me and continued the journey without wearing the seat belt. This is a common problem in most parts of the country. When it comes to following the seat belt discipline for those on the rear seat, there is a minuscule portion of people who wear the seat belt at all. Recently, a topnotch business leader met his tragic end in a car accident near Mumbai. The revealing truth that came from the new reports indicates that people on the front seats survived as they were wearing seat belts. But unfortunately, passengers on the rear seat died because they were not wearing seat belts.

Seat belt is a simple device which helps in saving many lives. Let us try to understand how does a seat belt save the life and how does it work.

Science Behind Seat Belt

You may have read about inertia while reading the First Law of Motion by Newton. The law says that every object tries to maintain its inertia and refuses to change its state of motion until an external force compels it to change its state of motion. This may sound complicated to many of you, especially to those whose minds were wandering during science lessons. To understand this, let us take an example of a book which is kept on a table. The book is in the state of rest, and will remain in this state until and unless an external force compels it to move. This means until someone pushes the book, the book is not going to budge from its original position. This happens because the book (the object) tries to maintain its inertia.

Let us take another example, that of a moving object. Let us imagine that you are traveling by a bus, and you are standing in aisle because you did not get a seat. While the bus is in motion, your body is also in motion along with the bus. What happens when the driver suddenly applies the brake? The upper portion of your body suddenly leans forward. Let us see why does this happen. While the bus is moving, your whole body is moving along with the bus. When the driver applies the brake, your legs come into state of rest, along with the bus. But your torso tries to remain in the state of motion. Due to this, your torso is thrown forward, i.e. in the direction of movement of the bus.

Let us take the example of a person traveling by a car. His body is in state of motion when the car is in motion. If the car meets an unfortunate accident, the car comes to a sudden halt. As a result, the upper portion of the passenger’s body is thrown forward with a great force. If the jolt is too much, the person’s head and chest will end up hitting the windowpane and steering wheel, resulting in severe injuries to his body. If the person is wearing the seat belt, then the seat belt will absorb most of the impact and distribute the impact on a larger area of the body of the passenger, and will help in minimizing the impact.


Mechanism of Seat Belt

Three Point Seat Belt

Now-a-days, all the cars are equipped with three-point seat belt which means that a seat belt is harnessed at three points. Two points are attaches to the car’s body and the third point gets attached to the buckle which is fitted on the seat. This combination helps in distributing the force of the impact across the pelvis, chest and shoulder of the passenger.

The top end of the seat belt is attached to a spool and spring mechanism, through which the seat belt can be coiled and uncoiled. When you pull the seat belt, it uncoils from the spool. When you release the seat belt, it gets coiled on the spool. But your question can be the seat belt appears to be quite loose. So how is it going to save a person?

Locking System in Spool

Pendulum and Lock Under Normal Condition in Seat Belt Retractor Pendulum and Lock in Seat Belt Retractor When Car Suddenly Stops

In fact, the spool and spring system is equipped with a locking mechanism which works in case of sudden and fast tension on the seat belt. There are two systems in place to ensure this. One of the systems is composed of a pendulum and a lock. When the pendulum suddenly moves forward, under sudden deceleration, it pushes the lock up which ends up locking the cogs in the spool. It helps in preventing further release of the seat belt. Another system is for backup, if the pendulum mechanism fails. The second system also works up to lock the cogs in the spool, and thus helps in preventing further release of the seat belt.

So, next time whenever you are traveling by a car, wear the seat belt irrespective of whether you are traveling on a road in Delhi or Darbhanga, irrespective of the fact that cops are watching or not.


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