quattro Digest, Vol 6, Issue 46

[Larry C Leung]

Brett, et al,

To answer your question, which is a common misconception post Physics
classes
if not clarified too well:




> Date: Tue, 13 Apr 2004 22:30:18 -0400
> From: Brett Dikeman <brett at cloud9.net>
> Subject: Re: Question on baby safety
> To: mailinglist at endosquid.com, quattro at audifans.com
> Message-ID: <a05200f00bca252068608@[192.168.1.2]>
> Content-Type: text/plain; charset="us-ascii" ; format="flowed"
> 
> 
> What I've never quite understood was the emphasis on getting the 
> seat 
> as tightly belted in as possible...everything I learned in physics 
> contradicts that advice.
> 
> Brett
> -- 


Because Seat Belts are not non-giving steel straps. They are designed to
stretch
when the forces on them exceed a preset value (the chevron pattern on
street
car seat belts allows this type of stretch by elongation and the
narrowing of the
belt, it's less obvious in racing harnesses). The stretch means that the
belt will
extend the period of time required to bring the belted individual (or
baby seat), 
to a stop, i.e. the car's body will likely stop before the belted
persons, whom
will continue until the belt is done stretching. From the Impulse
formula, 
J = F x t, where J is impulse, which is equal to the change of momentum 
(m x change in velocity, in this case, m is the mass of the person, the
change in velocity
is that of the original speed of the car before impact coming to a rest),
if you note,
the Force on the person is reduced in direct proportion to the extension
of the time
it takes to bring the person to rest. As noted, the Audi's belts (mAC)
will extend 
that time. What's bad is if the person is loosely belted. Then they may
impact a 
less giving surface, such as the dashboard. It won't likely stretch, thus
stopping the
body nearly instantly, or time nearly zero. Thus the impact force of
hitting the dash
goes up, usually quite dramatically. Airbags are designed to do the same
thing,
extend the time of impact by hopefully being inflated BEFORE the (in the
US
unbelted) person goes flying into the bag. The bag's deflation then
serves to 
extend the time of impact. The problem with bags is if the person hits
the bag
while it is still inflating (i.e. sitting too close or being in a car
seat, ESPECIALLY
rear facing seats, which are quite near the dash!), the bag acts
essentially like 
the hard dashboard or steering wheel hub, thus slowing the occupant MUCH
to rapidly. 

Moral of the story, wear your belts, keep them tight and wear the lap
belt
low (so that the belt supports your pelvic bone, not support your
intestines. 
They don't have too much strength!). 

LL - NY, a.k.a. the Physics Teacher