﻿ Mature Driver

 Required amount of time to complete: 25 minutes

MAN, MACHINE AND ENVIRONMENT
MODULE 5: CRASH DYNAMICS

SUBJECT 1: Effect Of Speed On Force Of Impact

One should avoid dangerous driving situations (excessive speed, running red lights or stop signs, etc.). The most dangerous situation to avoid is the head-on collision, followed by the multiple vehicle collision. As the driver you can use many evasive actions to avoid any collision, but action must be taken to avoid a head-on collision. You can turn the steering wheel, use your brakes, let off on the accelerator prior to impact or do combinations of all three to lessen the severity of the collision1.

What is the effect of speed on the force of impact?

Simply put, the greater the speed of any vehicle, the greater the force of impact. If you are going the same speed, the greater the vehicle weight, the greater the force of impact. For example, if both vehicles are going 20 mph and one is a sub-compact and the other is a tractor-trailer, the collision with the tractor-trailer will have the greater force of impact. This is a straight-line relationship. The more weight, the more force at impact.

If the vehicles are the same weight, the vehicle with the higher speed will have the greater force of impact. If one vehicle is going 20 mph and the other is going 60 mph, the one going 60 mph has nine times the force at impact than the one going 20 mph. This is a squared relationship. Three times the speed will have nine times the force of impact (32). Four times the speed will have sixteen times the force of impact (42). Five times the speed will have twenty five times the force of impact (52), and so on.

The force at impact is what can kill. A three thousand-pound car traveling at 70 mph has 15.8 million pounds of force to release in a crash. The release of this energy is what causes the car to get damaged or destroyed in the crash. This is also what injures or kills the occupants of the vehicles. Speed kills. Great speed increases the probability of injury in a crash2.

What are the two collisions that happen in a crash?.

1. The impact of the car with the object.
2. The impact of the driver with the inside of the car.

If a car leaves the road at 35 mph and hits a large tree, how long does it take to dissipate the kinetic energy contained in the vehicle and everything in it?
It takes about 7/10 of a second.

The damage caused to a person striking his seat belt at a relative speed of 15 mph (35 mph forward momentum minus 20 mph speed of belt) and "riding down" the crash (decelerating) over the next 3/10 of a second is much less than that sustained by a person striking the front of the passenger compartment at a relative speed of 35 mph and stopping in 1/10 of a second3.

RESOURCES:
American Automobile Association, Responsible Driving, Chapters 8 and 12, number 1.
Governor's Office of Highway Safety, Speed and Speed Limits, Atlanta, Georgia, October 1998, number 2, 3.

MAN, MACHINE AND ENVIRONMENT
MODULE 5: CRASH DYNAMICS

SUBJECT 2: Concept Of Second Collision

When it looks like you cannot avoid the collision that is about to happen in front of you, you are actually in two collisions. The first is the car with whatever you hit or whatever hit you. The second collision is when you are stopped by the windshield or steering column if you do not wear your safety belt. Additionally, any object that is not secured in place will go flying to the front of the car, striking what is in front of those flying objects.

What happens in a crash?

Newton's first law of motion states: A body remains at rest unless a force makes it move. A force is required to change the speed or direction of a moving body. This law means that it will take a force to start and stop an object in motion1.

Below is the 7/10 of a second impact (including people and objects in the vehicle):

1/10 sec. - In the first tenth of a second: the car and everything inside are going at the speed of the vehicle, for the purpose of demonstration we will pick 35 mph. The front bumper strikes the tree and begins to deform. The front center of the car slows to 0 mph; the rest of the car and its occupants continue moving forward at 35 mph.

2/10 sec. - In the second tenth of a second: the bumper continues to deform as the energy of the crash is being dissipated; the radiator and fan begin to crush; the engine and frame strike the tree and begin to decelerate.

3/10 sec. - In the third tenth of a second: the frame and body of the car continue to deform; the passenger compartment, front dash and windshield have decelerated to 20 mph. The car's passengers are still traveling forward at 35 mph.

4/10 sec. - In the fourth tenth of a second: the frame of the car decelerates to 20 mph and continues to dissipate the energy of the crash. The safety belts and passengers continue forward at 35 mph.

5/10 sec. - In the fifth tenth of a second: the safety belts begin to deform by stretching to decelerate the passenger in a comparatively gentle manner. Occupants decelerate to 25 mph, the car frame has decelerated to 15 mph. Unbelted occupants continue forward at 35 mph. Loose objects from the back seat and deck continue forward at 35 mph.

6/10 sec. - The sixth tenth of a second: the safety belts have reached their deformation limits. Belted occupants decelerate to 10 mph, the dashboard and windshield decelerate to 0 mph. The car body, frame and engine continue to absorb the energy from the crash. Unbelted occupants continue forward to strike the dashboard, steering column and windshield at 35 mph. Loose objects from the rear seat and deck come flying forward to strike the front seat passengers at 35 mph. The unsecured objects in the back of the vehicle could strike with enough energy to cause injury or death.

7/10 sec. - In the seventh tenth of a second: the frame and body have finished deforming and rebound in the opposite direction (reaction to the crash action); the belted occupants rebound from the safety belts, their heads continue back to come into contact with the head restraints. Unbelted occupants reach 0 mph by striking the windshield, steering column and dashboard, they deform and crush. Their internal organs, still going 35 mph, strike their rapidly decelerating body frames2.

The crash is essentially over!!! The belted-in occupants count their lucky stars and continue on with their lives. The unbelted occupants are carted off to the hospital or morgue for an extended stay; some longer than others.

RESOURCES:
Telford, Laurie, Laws of Motion, University of West Florida, October 1998, number 1
Governor's Office of Highway Safety, Speed and Speed Limits, Atlanta,Georgia, October 1998, number 2.

MAN, MACHINE AND ENVIRONMENT
MODULE 5: CRASH DYNAMICS

SUBJECT 3: Energy Absorption

As you drive around, your car is "carrying" around kinetic energy. This energy is the force that needs to be dissipated in a collision.

How do modern vehicles absorb the energy from a crash?

Modern vehicles absorb the energy from a crash by the way they deform or "crush" when they are in a crash. The energy of the crash is used up crushing the metal of the vehicle. The passenger compartment is the strongest portion of the vehicle. The car doors have internal steel beams to protect the passenger compartment.

With the body of the motor vehicle using up the energy of the crash, does it make a difference where the crash occurs on the vehicle?

The direction of impact in the collision makes a very large difference. As you recall from earlier in the course, the greater the speed of the vehicle, the greater the force of impact. As an example, you are going 70 mph and the car you strike head on is going 70 mph, what do you have? Simple, a 140 mph collision. Even with the modern cars that are designed to absorb the energy of impact, the 140-mph crash has approximately 31.6 million pounds of force to dissipate.

RESOURCES:

These questions are for practice only. You must only pass the final exam.
(REMEMBER: You must answer ALL questions listed below correctly before you can move to the next module)

1. Unbelted vehicle occupants reach 0 mph by striking the windshield, steering column and dashboard.
True
False
2. Modern vehicles are designed to crush when they crash to absorb kinetic energy.
True
False
3. The second collision is when the windshield or steering column stops you if you do not wear your safety belt.
True
False
4. One of the two collisions that happen in a crash is:
impact of the car with the object
impact of the car with the air
both
neither
5. The increase of speed is a squared relationship. Three times the speed will have nine times the force of impact.
True
False
6. As the vehicle continues to deform at the beginning, the passengers are still traveling forward at the speed of the vehicle.
True
False
7. The ______ at impact is what can kill.
size
length
force
8. If one vehicle is going 20 mph and another is going 60 mph, the one going 60 mph has ______ times the force at impact as the one going 20 mph.
three
six
nine
twelve