Re: Impulse

[Stefan Jeglinski <jeglin@4PI.COM>]

> Here's an interesting (IMO) statement from the "Real-life Safety"
> engineering group at Saab:  "When other vehicles are involved, the
> intensity of the crash forces is usually about half the level of the
> forces generated at the same speed against a barrier.  Accidents
> involving actual impacts against rigid objects at speeds above 40 mph
> (equivalent to about 80 mph car to car)..."

It is interesting, but pretty vague as stated. What are they trying
to say exactly? In a "zeroth" approximation, other things equal, a
car hitting a rigid barrier at 40 mph is equivalent to 2 cars, each
travelling 40 mph, colliding head on. Equivalent here meaning damage
to each car due to dissipation of its KE. It is true that the cars
are "closing" at 80 mph, but "80" doesn't enter into any calculation
of KE, to my knowledge.

Assuming delta-p = Favg*delta-t, and noting that delta-p is the same
for each car (barrier or against the other car), each case has the
same Favg*delta-t. So unless they are claiming that the car-car and
car-barrier collisions have a delta-t different by a factor of 2, I
don't see how Favg can be a factor of 2 less. But maybe larger
delta-t (smaller Favg) in the car-car case is true?

Do I view this incorrectly? My assumption that the cars are equal is
not in conflict with the spirit of the Saab statement. But I may be
missing something because this is "real-life" engineering...


Stefan Jeglinski