The Physics of Car Crashes
I have the theory that: if you drive a vehicle into a static, unbreakable wall, you will feel the same G-force and get the same injuries as if you would drive into your exact copy but mirrored (same car, weight, velocity, angle) head to head.Everybody that I talk to say there would be more energy transferred to the drivers and more injuries, like there would be some extra energy created, but I don't agree. I think the effect should be exactly the same when the energy is divided between the bodies no matter if it's a wall or your mirror image/clone.
A couple of people bring up the particle accelerator and tell me that there's a reason why they accelerate two particles against each other. "It will create more energy, thus damaging the particles more, as with the cars and their drivers."
But I think that only shatters the atoms more, like throwing two glass bottles really hard and they shatter all over more than just throwing a glass at a wall. Cars don't shatter like that so I don't think it applies when the bodies come to a stop.
There are several concepts at work here and I thought about it a bit and have responded in the article "What Is the Physics of Car Collisions?"
Let me know what you think about my response. Did I miss any key points?
Comments
I don’t think the analogy of the two cars colliding to particles in an accelerator is valid. In the particles case, they will interact via nuclear forces and their kinetic energy can result in the creation of larger mass particle(s). But two twin BMW’s colliding won’t ever make a Mack truck (or at least the probability is very very low!)
The two cases (A and B) are totally equivalent, in that the energy density in the crash volumes are identical. In one case energy K is restricted to the half-plane volume (HPV), and in the other case energy 2K is distributed throughout the full-plane volume (2HPV). The energy density in both cases is K/HPV.
>From Cars to Particles:
It is unnecessary to bring up relativity. The real key to this question is that it is impossible to slam a nuclear particle into a brick wall. The only thing you have to slam it against is other nuclear particles which have roughly the same mass. This is analogous to saying that there is no brick wall that a car can slam into. The only thing available is another car or perhaps a truck.
If only one car (particle) is moving conservation of momentum dictates that the fragments must careen down the road (beam tube) at a high rate of speed meaning that a large fraction of the energy is taken up moving the pieces.
However if two cars (particles) going in opposite directions collide conservation of momentum dictates that the total aggregate must stop leaving far more energy available to tear things to pieces. It is much more that just twice.
HTH
Jim
Okay, i’ve had a long think about this, and i believe it is impossible to apply the laws of physics to make such a comparison. Surely, you may say that the collisions are similar in which the car collides with a wall and and all the kinetic energy is transformed into strain and heat energy- and when the two cars collide the sum is simply divided equally amongst the cars which is the same effect on the car in the first case.
But beyond the realm of energy, such a collision can never exist and the laws of motion and momentum can not be applied. Allow me to demonstrate
F= (mv-mu)/t (Newtons 2nd Law)
Fwall = (0-0)/t = 0 N(seeing as the final velocity is also zero)
Therefore if there is no collision force being exerted on the wall there is simply no collision which is why to compare other collisions to this case is nearly impossible when discussing Newton’s Laws, but is acceptable when discussing energy.
hope this helps