Effects of Collisions on People
In order to explain exactly how people are thrown around in an auto accident, you need to remember Newton's 1st law; a body in motion will remain in motion until an external force acts on it. Since the forces that are being applied to the car during a crash are not being directly applied to the passenger, the passenger will keep moving when the car has stopped moving. This means they either go out the window or hit the dash or steering wheel. The speed that the passenger hits the interior of the vehicle is roughly equal to the speed of the vehicle before it comes to an abrupt stop.
The solution is a restraining device, like a seat belt. Seat belts, however, are not always the perfect solution.
Photo courtesy of www.intech-eng.com
While a seat belt can restrain a torso effectively, it can not restrain the head. An air bag, however, can prevent the head from swinging through the whole range of motion and damaging the neck. If your vehicle is not equipped with an air bag, there is no need to worry excessively; certainly no need to be so concerned that you stop wearing a seat belt. The statistics show that you are about 45% less likely to die in an auto accident if you are wearing a seat belt (information from www.nhtsa.com). The benefits of a seat belt far outweigh the danger of a possible neck injury.
Impulse
A seat belt or air bag not only prevents a person from bouncing around inside the vehicle, it spreads out the time that the decelerating force is applied to a person. The decelerating itself does not harm a person; it is sudden decelerating over a very short period of time that causes damage. The term that is used to represent force over time is impulse. Impulse is defined as the integral of force over time. If force is plotted on a graph, then the impulse is the area under the curve.
The first graph would represent a situation where a person collides with the steering wheel or dash, and all the force is transmitted in only a few fractions of a second. The second graph represents the use of a restraining device which spreads the impulse out over a longer period of time. Even though the same amount of force is transmitted in the process of decelerating the person, the second graph represents a much safer situation.
Severity index
There are several methods of quantifying the severity of an injury sustained in an automobile accident. The Gadd Severity Index was developed with data from tests on human cadavers, and supported with real accident data.
The simplified formula is GSI = a^5/2 * t
Where:
a = acceleration (or decelerating) in g (1 g = acceleration of earth's gravity,
or 9.80 m/sec^2)
t = time in seconds
A human head can sustain values as high as 1,000 without serious injury, as long as the peak value does not last for more than 10 or 15 milliseconds. For comparison, a hammer hitting a nail into wood gives a value of about 3,000, a baseball hitting a concrete wall is about 10,000, and a hammer hitting a concrete wall is about 3,600,000.