Crash Dynamics: Newton’s Second Law

In crash survival engineering, the key is slowing you down as slowly as possible. This is because Newton’s Second Law dictates that the force, F you feel during a crash is equal to your mass, m multiplied by your acceleration, a at the moment of impact.

Acceleration here is referring to the change in speed or velocity, v (which is the deceleration during a crash) divided by the duration (time difference, t).

In a mathematical expression, t is considered an inverse proportion to a which means, whenever t is increased, a will decrease and vice versa. Taking into account the reduced acceleration, a into the equation of force, F where F is directly proportionate to a, when a is reduced, F will also be reduced.

To understand it in layman terms, if you brake from 80 kmph, it will take longer to stop compared to a sudden stop if you hit a wall at the same speed. The slowing down process (where you increase the time of stopping) is what saves you from injuries. A sudden stop means a significantly more force is exerted to you.

This is the reasons why vehicles are designed with:

✔ Crumple zones – the systematic crumpling adds time to the whole car stopping.
✔ Seat belts that can stretch – the stretching adds microseconds to the body of occupants stopping.
✔ Air bag is crucial for crash survival but only if the body is positioned correctly prior to impact – it cushions occupants.

It is also the reason old cars with ‘solid’ builds should be retired.

It is also the reason why we always say, your child restraint harness should be SNUG, not TIGHT.

Now, look at the picture. When the restrained child is still ‘slowing’ down during the crash, the unrestrained child had already ‘stopped’ when he hit the back of the driver’s seat.

The law of physics is applicable as long as you are on earth. Distance, near or far your journey is, DOES NOT have anything to do with crash dynamics.

So, you still don’t want to restrain your children in safety seat?

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