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?.
The impact of the car with the object.
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: Your motor vehicle's owner manual.
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)
Unbelted vehicle occupants reach 0 mph by striking the windshield,
steering column and dashboard. True False
Modern vehicles are designed to crush when they crash to absorb kinetic
energy. True False
The second collision is when the windshield or steering column stops
you if you do not wear your safety belt. True False
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
The increase of speed is a squared relationship. Three times the speed
will have nine times the force of impact. True False
As the vehicle continues to deform at the beginning, the passengers are
still traveling forward at the speed of the vehicle. True False
The ______ at impact is what can kill. size length force
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
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. 
