What Happens In a car accident

This is a reprint of an old post because it is still important information:
When a motor vehicle accident occurs, the outcome is highly variable depending on multiple factors involved in the accident like:

  • Speed of the vehicles involved
  • Size of the vehicles involved
  • Road conditions for the vehicles involved
  • Direction of the vehicles involved
  • Position of the seats of the vehicle passengers
  • Positions of the passengers in the vehicle
  • Type of restraints on the passengers in the vehicle
  • Type of deployment for the passenger restraints

One key thing to remember when evaluating an accident is to remember one of Newton's laws that is that an object in motion will remain in motion until a sufficient equal and opposite force brings that object to a stop. Another thing to remember is that momentum can be transferred from one object to another. Often it is assumed that because the vehicle did not show much damage on the exterior, then the passengers should sustain little damage. The other assumption is that because the impact speed was low, the damage to the occupants is low. Let's handle the speed issue first. How slow is slow? NFL lineman usually clocks in at 4.9 to 5.2 seconds for a 40 yard sprint. That works out to about 16 miles per hour. How much damage can a NFL lineman cause to a person standing still, when he hits them in the middle of the back going full speed? What happens when we double the weight of the lineman and cut the speed in half? The force is still the same. What happens if we triple the mass of the lineman and cut the speed to one third as fast? Still this is the same force. According to the New York Times the average car sold in 2003 weighed 4021 pounds. An average NFL lineman weighs 300 pounds. The average car weighs 13 times more than an average NFL lineman. That means that the average car traveling about 1 mile per hour would have as much force as an average lineman does when he hits someone running at top speed. So if the average car hits an object at 5 miles per hour, that would equal an NFL lineman hitting the object at 80 miles per hour. What do you think would happen to that object? The bigger the vehicle doing the hitting, the greater the force is. 
The vehicle damage does not determine the amount of damage to the passengers inside the car. This point is a little complicated to explain but imagine two cars of equal size. The car in front has the brakes firmly in place applied to great tires with a firm grip on the road. The striking car is of equal size and hits the front car at 10 miles per hour. In this scenario the front car driver has strong and immovable pressure on the brakes and is firm in the seat with head firmly against the head rest. The cars are of equal size and the brakes and friction against the road is acting as the equal and opposite force pressing against the striking car. The results will be skid marks on the surface of the street, rear end damage on the front car, and front end damage on the back car equaling 10 miles per hour times 4000 pounds. Because the passenger was firm in the seat and firm on the brakes, the passenger did not travel independent of the car and the protective seat along with both vehicles absorbing the force of the impact through the crumpling metal protected the driver of the front car. The cars sustained extensive damage and the passenger in the front car was pressed into the cushioned seat and sustained little damage. The elasticity of the seat shot the drivers head forward at a reduced speed because the momentum was absorbed by the seat and the car metal. So the outcome for the driver of the front car is better than what it could be. The driver of the back car had head speed forward, because an object in motion will remain in motion until forces equal and opposite can act on that object to bring it to a stop. The air bag deployed and absorbed the force of the head traveling forward, so the driver of the back car might also fair good despite the gross look of the cars. Let's use this same scenario but change the front car driver's pressure on the brake and position in the seat so that the drivers head is six inches forward from the head rest. The striking car hits the front car and the front car leaps forward at 10 miles per hour in 0.053 seconds. The brake is not well deployed because the front car leaping forward has the driver pressed into the seat and the foot has left the brake. The front car has sustained little damage because it acted like a billiard ball bouncing away from the impact, but the front car driver's head travel back at 10 miles per hour in 0.053 seconds for 6 inches. The driver's head hit the head rest and recoiled forward about the same speed less the cushion absorbing the momentum. The driver's soft tissue in the neck sustained a great amount of damage although the car appears lightly scathed. Now just imagine the vehicle in the back was twice the size of the front car, but the speed was half and much. Just like before, the force is the same, and the outcome will be similar. 
Next post will be about the soft tissue damage that the driver's body undergoes when involved in this type of accident.