Michael Gueterman writes:

> "Engineers"...It figures :)
>
>  Regards,
>
>  Michael "It's not the stresses or the strains, but the jerks
>   that get you every time" Gueterman
>  (saying lifted from an 'engineer' friend of mine long ago)

As an official card-carrying engineer, I thought that perhaps I should
explain Michael's "engineering" joke. The explanation is that it was the
jerks that killed Princess Diana (although you probably already thought that
you knew that).

The first time-derivative of distance is speed, the second is acceleration,
and the third is technically called jerk. Because we all sit in the gravity
well of the earth, we are under a constant acceleration of 1G. But when that
acceleration is changed dramatically -- and in a short time -- a significant
jerk is placed on the body. Unfortunately, due to varying inertias and
mechanical constraints, not all parts of the body respond to the imposed jerk
equally. The result is that the body's internal organs are differentially
ruptured and torn from their moorings. Vascularization tears are among the
most common forms of trauma under high-intensity jerk, resulting in massive
internal bleeding.

Anything that can be done to the design of a car that will minimize the
intensity of the jerk imposed on the occupants will save lives. That's
exactly the tact that Volvo began taking twenty-five years ago. In a Volvo,
the chassis is built with shear pins in the structure so that significant
front or rear collisions cause the chassis forks to telescope inward,
shearing each pin as they do. Simultaneously, the engine and drive train are
designed to break free their mooring bolts, fold upwards, and ride underneath
the cabin, which is a unitized, protective framework. The skin of the car is
also made to fold, accordion-like, further increasing the amount of energy
absorbed and lengthening the time of the collision.

Indeed, that's all that it takes to reduce jerk. Simply increase the amount
of time over which the collision occurs. It is the reason that race car
drivers can get up and walk away with only bruises after hitting a wall
(albeit at a glancing blow) at 250 mph.

Race cars also employ a second primary mechanism for reducing the amount of
energy transferred to an occupant. They shed as much mass as they possibly
can as quickly as they can, most especially the engine and tires. Once these
heavy masses are gone, the driver is now enclosed in a lightweight,
low-inertia, very tough chromalloy welded cage that is very well padded. It
can bounce dozens of times, from surprising heights, and still not seriously
injure the passenger.

Race car drivers use a five-point harness to keep themselves tied to the
protective cage. It is extremely important to let the chassis take the brunt
of the impacts and lenghten out the time of the collision. If Diana had been
tightly strapped into the back seat, she probably would have survived. The
car, judging from the amount of damage done, was probably not going over 50
mph. The rear half of the car was barely deformed. Unfortunately, she (and
every one else in the car) would have almost certainly survived if the tunnel
in which she crashed was built in the manner that tunnels are built in the
US, which continuous concrete barriers so that a head-on collision with a
structural element would have been nearly impossible. Glancing richocets down
a tunnel, even if those impacts caused the car to flip several times, would
have been eminently survivable, especially in a Mercedes, if the passengers
were well strapped in.

Wirt Atmar