Crumple Zones in Automobiles
by Christopher Erickson
In recent years, the automobile industry has attempted
to improve safety through a number of technological developments. One technique
which has been proven to be successful, involves the use of crumple zones
positioned in specific areas of an automobile. Crumple zones are created
by the integration of variable grades of steel and fiberglass into the
front and rear-end assemblies of the automobile. Occasionally, crumple
zones are used in the actual frame of the automobile, creating a point
for the frame to buckle when subjected to extreme stress. These crumple
zones yield during impact, redirecting the energy of the collision---often
reducing the chance of injury to the driver.
Example of front and rear crumple zones
in a recent Volvo model
The following diagrams illustrate the effect of crumple
zones in automobiles. In figure 1, a steel block travels at a constant
velocity towards a cement wall, representing an automobile without crumple
zones. Initially, the block has kinetic energy, represented by the expression,
mv2/r. As the block
collides with the wall, it exerts a force on the wall, after which the
wall exerts an equal and opposite force on the block. The magnitude of
this force is illustrated by the amount of kinetic energy regained by the
block. At the moment of impact, the steel block immediately rebounds in
an elastic manner, regaining nearly all of its kinetic energy, and consequently
experiencing a large force.
In figure 2, an aluminum can travels at a constant
velocity towards a cement wall, representing an automobile with crumple
zones. As the can collides with the wall, it does not regain all of its
initial kinetic energy. Instead, some of the kinetic energy is transferred
into heat and sound energy, resulting in a smaller force experienced by
(replace any MV^2/R expressions
The energy equation governing the previous example is
Where m is mass, vo is initial velocity, and vf
the final velocity of objects 1 and 2. NC refers to non-conservative kinetic
energy, or the sound and heat energy lost in the collision.
Crumple zones in action at the Toyota crash testing site
The amount of force experienced by the car has been
shown in the previous example to have been lessened by the action of crumple
zones. Crumple zones also decrease the severity of an accident by creating
a phenomenon known in the automobile industry as "controlled deceleration."
Generally, this means that if the time it takes for an automobile to come
to rest or change direction is increased, the force experienced by the
automobile is decreased.
This phenomenon is expressed more formally through
the application of Newton's second law of motion, or:
where a can be represented by the change in velocity
over the change in time, or:
Substituting the previous equation for a in Newton's
second law yields:
From this equation, it is clear that as the time of the
collision decreases, the force experienced by the automobile increases
dramatically. For example, if a 1000 kg car collides with a wall at 14
m/s (32 mph), the force experienced by the car is expressed as:
(The resulting negative sign indicates direction of the force)
The graph of this function is shown below, illustrating
how force changes as time changes:
The key point is that at small intervals of time,
a slight increase in time results in a large decrease in force. The better
the crumple zone, the more effective it is in increasing the time of a
Be sure to also check out these sites:
experiment I performed involving acceleration and deceleration in elevators.
Volvo Home Page
The Volvo home page, including great information on safety development.
B.U. Page A collection of links relating to B.U. and the Boston
Apple Computer Need
I say more?
Cutnell and Johnson. Physics. 4th
ed, New York: John Wiley and Sons, 1997.
Bueche, Frederick. College Physics.
ed, New York: McGraw-Hill, 1989.
Mashaw, Jerry. The struggle for auto safety.
Cambridge, Mass: Harvard Press, 1990.
Toyota Automobiles Inc. http://www.toyota.com
Volvo Motors Inc. http://www.volvo.com
Please E-Mail me with any comments or suggestions