[Phys-L] Re: collision question

["Spagna Jr., George" <gspagna@RMC.EDU>]

Herb asks:

-----Original Message-----
From: Herbert H Gottlieb [mailto:herbgottlieb@JUNO.COM]
Sent: Thursday, February 17, 2005 8:18 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: collision question

> Which would be more damaging, driving into a very massive concrete wall
> or having a  head-on collision with a similar car moving toward you
with
> the same speed?

Assume identical cars at the same speed.  As seen from the center of
mass frame, the two cars come to rest at the COM.  The work on each car
is done over the "crush length" designed into the vehicle frame and
body, dissipating (1/2)mv^2 wrecking each vehicle.

If the concrete barrier in the second case is taken to be infinitely
massive, the same work is done wrecking the vehicle.  If the barrier is
not infinitely massive (or completely rigid) then some of the work is
done on the barrier, so less work is done on the car.  For a low-speed
collision the damage to the vehicle might be less hitting a wall than
hitting another car head-on.

As a practical matter, I suggest avoiding both types of collisions!

********************************************
 "Eternity is very long,
                 especially toward the end."
                                -  Woody Allen
********************************************

Dr. George Spagna
Chair, Physics Department
Randolph-Macon College
P.O. Box 5005
Ashland, VA 23005-5505

phone: (804) 752-7344
fax: (804) 752-4724
e-mail: gspagna@rmc.edu
http://faculty.rmc.edu/gspagna