[ILUG-SOCIAL] Speed kills - or does it ?

[Colm MacCárthaigh]

On Wed, Apr 09, 2003 at 12:40:06PM +0100, Breathnach, Proinnsias (Dublin) wrote:
> > Momentum is partly due to mass.  The speed of the car is irrelevant (as
> > long as the lorry is going faster than it, obviously), the lorry is
> > still doing to crush it.
> > 
> s/weight/mass/ on my comment ... and yes momentum of truck > momentum of car
> ...
> BUT:
> 	If truck and car have similar angular momentum (travelling the same
> direction) the principle of conservation of momentum applies, leading to the
> car speeding up by a factor of about 18mph and the truck slowing slightly
> due to increased friction ... the actions of the driver on the other hand
> (applying the breaks, trying to stear away - thereby breaking the angular
> momentum) are what can cause it to become a fatal accident as opposed to a
> survivable collision with effectively minimal damage to both the car and
> truck ... the thing is that the trucks brakes *must* be applied first and
> smoothly for it to be survivable !

I don't know where you all learned your physics, but don't give
up the day job. First of all momentum has everything to do with
velocity, and everything to with mass, in fact it's the product
of both. 

Angular momentum has very little impact (groan!) on the situation,
unless they colide whilst spinning. pron: angular momentum has
nothing to do with the direction you're travelling, it's to do 
with moments of spin about a central axis. You're thinking of
plain old momentum, which is a vector.

You are right that the car will speed up, but not neccessarily
to the speed the truck was at. It's to do with conservation
of momentum, any momentum lost by the truck will be inducted
to the car (ignoring any tertiary transfer to crumple zones,
the road ... etc), but because the car has a lower mass it's
likely to go faster than the truck was moving, depending on
the specifics.

If you hurl a brick at a ping pong ball, I can guarantee you
the ping pong ball will move a hell of a lot faster than
the brick was going.

The damage in a collision in all cases is caused by force 
excerted due to deceleration, the more rapid the deceleration
the more force exerted, the more energy your car and body
has to absorb.

A car will decelerate a lot more quickly than a truck, 
and there is a lot less material to absorbe energy,
no matter what hits it. At the same time, no matter
what a truck hits, there's a hell of a lot more energy
to go somewhere.

-- 
Colm MacCárthaigh                        Public Key: colm+pgp at stdlib.net
colm at stdlib.net					  http://www.stdlib.net/