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EXSTRACT C.

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Part B: The influence of the impact on a body inside the struck car.

Chapter B.1 Acting forces towards the body when driving and when velocities change.

In the former chapter I have shown the mechanisms affecting the cars in a collision.

To evaluate whether or not persons inside the cars have been exposed to high loads, I have to study how the body might be affected during such an incident.

Fig B.1a shows a person driving a car at a constant velocity on a straight and horizontal road. In this condition it's only the forces caused by the mass of person acting between the person and the chair. See Fig. B.1b

When the car is increasing it's velocity the acceleration a causes a pressure acting from the back of seat accompanied by a horizontal force between the seat and the lower part of the body. The total sum of accelerating forces are:

Sum F = m * a

The horizontal force acting between the seat and the lower part of body Ff can not exceed the FRICTIONAL FORCE See Fig. B1.c. In the figures above only the real acting forces are shown, not the inertial "forces" in the opposite direction.

When the car is decreasing it's velocity during a retardation or a negative acceleration -a , all forces acting to reduce the velocity of the person is the horizontal force Ff which can not exceed the frictional force between the frictional force f and, if the retardation is high enough to strain the safety belt, forces acting through that belt or, in a strong collision, additional forces cting between structures in front of the body like the steering wheel etc.

An airbag, which is rapidly inflated in front of the person if the retardation of the car exceeds a specified limit, is emptied as the head is slowed down to the velocity of the car, reducing the retardation of the head and relieving the neck region from the high retarding stretching forces combined with an extreme forward inclination.

The forces acting from the back of the seat towards the person during the increase of velocity is an acting force increasing the velocity of the person. The person feel this acceleration like being pressed into the back of seat.

This pressure is a result of the resistance of the body against changes of it's velocity. This resistance, named as the "Inertial Force" is a reaction to the real acting force F.

1. The behaviour of a body when affected by high forces acting in a very short time.

A question to be discussed when studying the "whiplash-mechanism" including the effects on a body is how the body will respond to such loads.

The acting forces towards a body in a car crash will range the scale from the lowest to the most extreme. The deformation of the cars in a high velocity frontal collision will last about 0,070 seconds.

A rear end collision will last from about 0,100 to about 0,200 seconds. During this period of time, the struck car may be accelerated from 0 to about 60 km/h and even more. The persons inside the hit car may achieve a significant higher forward velocity. Accelerations of the head during tests of frontal collisions are measured even at approx. 100 G. It means an accelerating force of 6,9 kN towards a head with a mass like 7,0 kg.

Affected by high impulses acting like very high forces in a very short period of time I find it reasonable to assume the behaviour of the body to be similar to the behaviour of a body of an unconscious person, responding to the acting impulses almost like it is lifeless.

Chapter B.2 Movements and loads acting on a person inside a vehicle exposed to an impact.

1. Acceleration of objects inside a vehicle exposed to an impact.

Due to an objects resistance against changes, any object within another object during an acceleration will remain in it's velocity until it is exposed to an accelerating force from the surrounding body. According to the "Law of inertia".

The behaviour of such an object will depend on how the object is connected to the vehicle.

An object fixed to the vehicle by bolting or welding will follow the accelerations of the vehicle unless this is not overloading the strength of fastenings. If so the structure might be deformed and broken giving a movement of the object related to the vehicle.

An object connected to the vehicle in a flexible way will, according to the "Law of inertia", be accelerated as the fastenings are deflected opposite to the direction of changes.

A free rolling object with almost 0 friction will remain in it's velocity unless it's picked up by a vertical part of the vehicle.

If the object inside a vehicle during acceleration, either increasing or decreasing it's velocity or changing it's direction of motion, is not fixed to the vehicle, the acceleration of the object will be like an impact between this object and the vehicle unless the acceleration a/g (g is the acceleration due to gravitation.) does not exceed the coefficient of friction between the object and the vehicle.

A person seated inside a car during any acceleration has to behave like an object not fixed to the car. In every change in the velocity of car it will be a delay between the acceleration of car and the seated body causing a difference between their velocities.

For the Next page. Part B continues.

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The contents of, or any part of this compendium, are not allowed to be made public or copied without my permission.