Collisions Damage


Sorry to bother you, but I have an on going argument at work concerning damage at impact with two cars. Both cars are same weight or mass and traveling same velocity (say 60mph) I say that each car will suffer the same amount of damage as if they hit an immovable object like a solid wall going 60 mph. My co-workers claims the damaged would be the same as one car hitting a solid wall at 120mph. I realize the total amount of force or energy would be equal to one car hitting that wall at 120 but since all that energy is divided by two vehicles then the impact force for each vehicle would have to be halved so each car would look like it just hit a solid wall at 60 mph? Right or Wrong?

p.s. Do you think it unhealthy for air traffic controllers to discuss such things?.


Here is the way I would think about the problem. Imagine a car to be a moving crushable object which hits a much more massive object and dissipates the entire energy of its motion by the heat generated in crushing so that it comes to a stop in contact with the "immovabal" object. The one vehicle at 60 mph case. It will be crushed enough to account for all its kinetic energy, an amount equal to one half its mass times its velocity squared. If two identical cars each moving at 60 miles per hour have a perfect head on collision, each coming to rest in contact with the other at the point of impact, the total kinetic energy of the two vehicles will be twice that of the previous case. That energy will be shared between the two vehicles equally so each will crush the same amount as in the first case as you suggest. Only if one vehicle came away undamaged would the whole kinetic energy go into damage on the other vehicle.

You might make the point to your fellow worker that the infinite mass of the wall in the first case has been replaced by the 60 mph velocity of the second car in the two vehicle case so that the collision from the perspective of one of the cars is the same. It is sort of like the second car, coming along at 60 mph renders the point of impact an "immovable" object.

Regarding the p.s., I hold a ComSEL license so I have always suspected that ATC guys had their minds on things other than airplanes:). Seriously though, a healthy curiosity about how the universe works is a prerequisite to sound decision making in your line of work. I hope this answer to your question is adequate. If you need more, let me know.

J. D. Jones
M. Casco Associates