Torque:

Technically, it's DYNAMICS, not PHYSICS.

Nanna-nanna-(hijack)-boo-boo.

 

actually i find the dicussing rather interesting. But theres a pretty easy way to look at it. Most of you have seen crash tests that run from 40 into a solid wall. I imagine running into a solid wall at 120 has to be tremendously more powerful. Looking at the picture of the lightning, thats not 120 mph worth of damage.

 

I do not have closed minded attitude towards learning as is obvious by my obtaining an advanced degree. I do have a problem with people that think they know it all. As for "piled higher and deeper", what degree do you have? There is no reason to put me or my education down personally like you have. I did no such thing to you. All I did was enter input into the thread and did not bash you personally. Your above statement reflects upon your maturity level I guess....

 

Everyone by now knows that we should not be dwelling on this. It's ridiculous to discuss the dymanics of the problem when in the real world, someone died and someone else's life will never be the same.

However, I'm going to hash it one more time because some of the information here is absolutely misleading (including some things I wrote incorrectly as well--I blame it on a long day since I bought a new car, RX-8 to go with the L). None of it is absolutely wrong, but people reading this may get the wrong idea on it. Plus, I said some things poorly and want to set the record straight. Hopefully this clarifies things.

Flame suit is off, because I probably deserve it for not letting this rest.

In essence, we've compared 3 different things:
1. The impact of two vehicles hitting each other head on. To make it simple, we HAVE to assume that these two vehicles are identical, traveling at a constant velocity (i.e., no acceleration), etc, etc. Which kinda makes it much more academic than real, but whatever...

2. The impact of a vehicle hitting an IMMOVABLE, non-deformable object head on. Again, most objects are not completely immovable or deformable, but what the heck--let's run with it.

3. The impact of a vehicle moving at a constant velocity colliding with an identical vehicle entirely at rest. Same assumptions come into play.

In short, no one has been completely wrong in what they have said (although some wrong things have been said). However, the way it has been said is very misleading.

In all 3 of the above cases, we have to assume each vehicle is identical in mass (and in determining deformation, materials, construction, geometry, defects, etc.). If a vehicle is moving, let's assume they're all moving at the same speed.

When a vehicle takes damage, it is because it has to absorb energy, in this case, the kinetic energy (energy is neither created nor destroyed, it only changes form).

CASE 1
Now, in case number 1, each vehicle has the same kinetic energy. When the two vehicles collide, there is 2x the kinetic energy that each had (because there are 2 vehicles colliding with the same energy). Let's call this amount of energy that each has before the impact K. So in this case, the total energy at impact is 2K. Thus, after all of the sound waves, light, whatever, the leftover energy is absorbed equally by each vehicle (each vehicle will absorb half of the total energy not given off as light, heat, etc.)

So let's say that, in this case, each vehicle absorbs X amount of energy (which is something less than K).

CASE 2
Now, in case number 2, a vehicle moving of the same type and moving at the same speed as those in case 1 strikes an immovable object (like a brick wall, only so tough that it is indestructable). Now the vehicle hits a wall with an amount of energy equal to K from above. The wall itself has no energy. After the impact, the wall takes no damage (because it's a theoretical wall that doesn't deform or move, so it doesn't absorb anything). So after the explosions and noise and such, the vehicle has to take an amount of damage equal to X (because it's the only thing that can absorb damage).

So, it seems our theoretical physicists have it right, eh?

Not exactly. This all started because of Sal's comment about two vehicles hitting head on having twice the impact force of one hitting something like another vehicle at rest. That's at least how I would interpret it. To put it another way, 2 vehicles should have twice as much energy as one, right.

Sort of.

See the confusion comes from comparing apples to oranges to pears, which is what is happening with this thread.

Any typical person I imagine doing a comparison like this imagines two vehicles hitting head on vs. 1 vehicle either hitting another one at rest or hitting a movable object. That changes everything, and IMHO, is similar to case 3...

CASE 3
Now a vehicle identical to those above and moving at the same speed hits a nonmoving vehicle, identical to those above.

The energy of the first vehicle is K (just like before). The energy of the second vehicle is 0--just like the wall scenario.

But now, there's one difference. We don't have an invincible wall.

Since the energy remains the same, after all of the explosions and noise, each vehicle absorbs half of the total, leftover energy, or X/2.

So, in short, 98Cobra:

You're right with your intuition. BOTH vehicles absorb energy (which produces damage) equal to the total energy at impact. If they're identical, hit head on, etc, each absorbs half. This is different than if one vehicle at 60 mph hits an immovable wall. In that case, the vehicle takes the same damage as 2 vehicles hitting one another at the same speed.

Cobrakid:
You're talking about stationary walls. That's scenario 2. In that case, our engineers are right on. Counterintuitive, eh?

Sal:
A better way to look at the collision problem is that 2 vehicles colliding head have twice the impact force of one vehicle moving at the same speed, colliding with a vehicle at rest. In that way, Ford was right on in what they were saying. It all depends on how you say it.

Doug:
...

You're absolutely right.

In closing, I'll say that if I run a deformation simulation using finite elements on 2 vehicles hitting one another vs. 1 vehicle hitting another at rest (which is what I meant to say--I kept referencing walls after a while, but I was focused on vehicles at rest from my initial post), then one will have half the deformation of the other.

I'm done.

--Rip

 

I hope nobody minds, but I posted a link to this forum on the NLOC site. Thought I would share the story of this unfortunate accident with those that don't come to this site.

 

I Hate that kind of driving!!! It was not me thank God a wife an
3 kids makes me drive slow.

Forrest
Thanks for the phone call Bud!!!!

God Bless the ones that died.

Jeff

 

I think what needs to be understood is people die every day on the road. You can be safe and live longer, but you never know when you will die. I slid off the road yesterday in my ranger and was lucky to live. I was at or below the speed 70 mph limit going up a hill. I was lucky after crashing through some trees and rolling on my side that I didn't die.


The camper shell is about 20 yards away from the truck on the hill.

 

Ripnrun:

The original question was SIMPLY:

Two (identical) vehicles hitting (precisely) head-on, each moving at 60 MPH is the equivalent of (read - "causes damage to vehicle/occupants equal to"):

A) Hitting a mythical brick wall at 120 MPH

B) Hitting a mythical brick wall at 60 MPH

The answer is "B" - hitting a brick wall at 60 MPH.

Very simple, no computer simulation needed. BASIC mechanics.

As hard as you twist it, there is NO WAY to frame the question such that the answer is "A" - hitting a brick wall at 120 MPH.

In your tortured effort to play peacemaker, you attempt to explain how Sal's original statement could be construed as correct because the TOTAL impact force (read - "energy") would be doubled due to the involvement of two moving vehicles in the crash event.

However, a vehicle hitting a mythical brick wall at 120 MPH has FOUR times the (kinetic) energy to be dissipated (by deformation of the vehicle structure) than one striking the same wall at 60 MPH. And it has twice the (kinetic) energy of TWO vehicles hitting walls or each other at 60 MPH.

The 120 MPH situation is JUST SIMPLY WRONG. The answer is "60 MPH into the brick wall". There's no way around it, no matter how badly you want it to be otherwise.

Once you get away from this simple situation, which is what this discussion was about, things get wiggy.

Hitting a parked car is indeed much preferred to the mythical wall, as the parked vehicle will take some of the energy in the process of being deformed and accelerated.

There are a million other scenarios we could discuss, but the answer would be MUCH more difficult to calculate/prove.

The original 60/120 question is a simple one.

The multitude of other scenarios get tough, and that's where the cipherin' machines (computers) come in.

Fractal - I did whack you pretty hard, maybe a little TOO hard. Pilsener Urquell will put you in a fightin' mood. Oh well. What's that university, again? I don't recall you identifying it. Maybe you inflated your educational pedigree to add force to your (tragically incorrect) point. I have a MSChE from Syracuse and a BSChE from Clarkson. Wife is MSEE from RPI and BSEE from Clarkson.

 

Ford was totally wrong and perpetuating a silly old myth when they taught mechanics that two vehicles going head-on at similar speed is the equivalent of a single vehicle at double speed hitting ANYTHING in ANY MANNER.

 

look, all i have to say is, the heavier the ball, the faster its gonna fall, end of story.

Damn easter, gald your ok.

 

:Phew:

 

Grinomyte:

You rule, you pound-of-feathers bastid!

Remember to fill your ice cube tray with boiling water - it will definitely freeze faster, or so Mom told me.

Humid air is "heavier" than dry air, too. You can feel it.

High octane gas has a higher energy content than low-octane , too.

Damn, that was funny, Chief.

 

Silver-Y2K-SVT,

The problem I have with the whole thing is that you intentionally misled people in order to promote yourself. That seems to happen a lot around here.

Sal simply stated that the collision between 2 vehicles was twice what it would be if there were only 1 car moving. As a rule of thumb, that's not necessarily far off from reality--assuming you're talking about cars hitting cars, not cars hitting immovable walls.

You introduced the brick wall concept to try and show how smart you are.

Goody goody. You're superduper smart.

(In bars, they call this type of thing a sucker bet).

Other people (like Doug, among others) stated the scenario differently--directly showing car vs. car. I didn't see any attempts to clarify.

And if it weren't for the "engineering degrees rule" bit, I wouldn't have jumped in. But all you did was take the topic on a tangent by creating confusion in what was being discussed. That's weak.

Now a bunch of people are running around thinking that if 2 vehicles collide head on, it's the same as hitting any non-moving object.

The rest of the people (that obviously know better) are thinking engineers must be stupid.

In any event, it was just a clarification. Reread the interpretations of your first message and see what conclusions people jumped to by looking at the thread again. It's just misleading.

--Rip

 

So my understanding of this is that when you have two identical vehicles hit head on at 60mph, there may be a total force of 120mph in the equation, but all things being equal, both vehicles absorb 60mph of that force, right?

I can respect you on a technical level, but from what I have seen as far as how you treat people on this forum(and I am not talking about this thread only) you still have a lot to learn. I don't want to take this thread further away from the topic than it already is, but I wonder if it is possible for you to post an answer to a question without trying to make everyone else feel smaller...

 

So much for this discussion, I suppose.

'Cobra: As far as your last question - "Prolly Not".

Additionally, re-read your posts throughout this thread. It seems you're not above the occasional cyber-bash on the less/mis-informed.

As for the "crash thang", believe what you want, I suppose. Should you ever suffer a head-on in the wrong lane, thank your lucky stars when you realize I had it correct all along.

This isn't going anywhere constructive, I suppose.