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The Physics Behind Car Crashes
Guinnessy writes "Physics Today has an article on Vehicle Design and the Physics of Traffic Safety. The article analyzes in detail typical crashes experienced between cars, and cars with SUVs'. According to Marc Ross, Deena Patel, and Tom Wenzel, "The evidence is compelling that body-on-frame light trucks cannot safely coexist with passenger cars under existing conditions. That problem is critical because so many light trucks are used nowadays as car substitutes." They suggest some ways in which both cars and SUVs' can be redesigned to improve safety. Meanwhile Detriot News reports on a Pediatrics journal study says that claims that children are no safer in SUVs than cars because of the rollover risks."
KE = (m/2)*v^2
or the more common form: KE = 1/2mv^2
This incidentally does make it in the same class as E=m*c^2 (due to magnitude/exponentiation) but that's more or less irrelevant.
As others have pointed out, this has nothing to do with E=mc^2.
In addition, in a large vehicle the conservation of momentum is on your side. It's not the crash that kills you it's the sudden acceleration (your body going from 30 to 0 mph in a second). But if your vehicle doesn't change velocity much (because of its large mass, mass==resistance to acceleration) then you're fine. The other guy dies. That's why there don't have to be seat belts on buses (though it would still be a good idea). If a bus crashes into something, it's probably not going to stop very fast.
Large mass is not why TFA says SUV's are dangerous, it's because they tip over.
Whoever is in the car that undergoes the largest acceleration is worse off. Since mass is by definition the resistance to acceleration, the ones in the larger car win. If two equal mass cars collide they both undergo the same acceleration. Ignoring car vs SUV strength, if two cars have a head-on collision at 30mph, the people inside are just as well off (or just as injured) as if it were two SUVs.
I find that hard to believe, but even if it's true, it's hardly representative of most SUV vs. passenger car crashes. It's just not. Perhaps the H2 is just poorly constructed in terms of side-impact protection or something, I can't say since I'm not familiar with it, but imagine that same collision occurring head-on. It barely matters what equipment each car has, because the H2, having much more mass, is going to decelerate much less violently. It's just not going to experience as serious a collision as the smaller car will.
There are many studies which support this (besides TFA), and I've personally seen (as an EMT) some really hideous SUV/Car collisions, and can tell you: the SUV usually wins. Sometimes there are mitigating factors, like the SUV will roll over and throw out and crush anyone that's not seat-belted, while the car will basically stay in one place, but if you have the choice between being in a Honda Civic and a GMC Suburban with similar safety equipment when they run into each other, you'd be insane not to pick the Suburban. Plus, the increased size of the car makes it likely that you'll get extricated from the vehicle a lot faster (again, assuming no rollover).
The biggest problem with SUVs, traditionally and still, is that they tend to roll over during hard panic maneuvers, or when hit from the side. Where I used to live, the biggest risk was people driving them too fast for conditions (no, your SUV does not mean you can drive on ice) and laying them over -- generally nonfatal though. But in a straightforward front-end or rear-end collision, there's something to be said for surrounding yourself with several thousand extra pounds of steel. Granted, you get that advantage at the expense of the person in the smaller car, and you have to be more careful taking it around a corner, but it's there nonetheless.
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Actually, looking at the videos and pictures of the Landwind crash-test, it becomes quite obvious that the whole car is one big crumple-zone. The car seems to simply disintegrate.
See for yourself
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While it says that the most common accidents - rear enders - were decreased, it goes on to say "Rollovers and side impacts with fixed objects - crashes that typically follow a complete loss of directional control - had the highest increases with ABS. Nonfatal crashes increased by 28 percent, and fatal crashes by 40 percent. "
Meganne safety rating is off the scale. The occupant safety level is so good that they are having to redesign the tests because of it. At 35 mph collision of any type (front, side flat, back and side pillar) there is no damage to the occupants. At all. The test result is all green with a possible minor spot of yellow (which corresponds to a bruise). Just look at the EUROncap safety tests http://www.euroncap.com/. Same for every Renault made in the last 3 years.
t ails.php?id1=1&id2=201) is safer then being in a Hummer.
At the same time the Hummer is not even on the list. In fact if it was, it would have pulled a 2-2.5 star rating at best. Same as a suicidal box like Fiat Ceicento. I have seen a crashed Hummer so this unscientific opinion is based on seeing what happens when it smashes. If you are driving this POS you have a death wish.
Essentially even being in a something microscopic by American standards (like Modus http://www.euroncap.com/content/safety_ratings/de
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From the
National Highway Traffic Safety Administration.
"Do cars with ABS stop more quickly than cars without?
ABS is designed to help the driver maintain control of the vehicle during emergency braking situations, not make the car stop more quickly. ABS may shorten stopping distances on wet or slippery roads and many systems will shorten stopping distances on dry roads. On very soft surfaces, such as loose gravel or unpacked snow, an ABS system may actually lengthen stopping distances. In wet or slippery conditions, you should still make sure you drive carefully, always keep a safe distance behind the vehicle in front of you, and maintain a speed consistent with the road conditions."
ABS is about control, not stopping distances. It allows you to retain control of your vehicle by preventing the wheels from locking up. As such, ABS will not reduce your velocity as much as tradtitional brakes and you will therefore impact with more energy.
However, the benefits of ABS and being able to avoid the collision outweighs the costs of reduced stopping force. So for the untrained driver who simply hits the brakes and waits for impact, ABS can result in higher impact velocities. but used correctly, ABS can help you avoid a collision altogether.
So in some cases (when you cannot maneuver) it is more dangerous, in some cases it is safer.
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That's not necessarily what's being done.
Many of the largest SUVs (Suburban for example) are on a longer redesign schedule than your standard passenger vehicle. Going 8 years between a full redesign means you're still getting crash technology that's nearly a decade old.
Making matters even better, is that the largest of SUVs, by virtue of their GVWR (a number that's made up by the manufacturer) are not required to meet several bare-minimum government safety standards.
The current Suburban, with a GVWR of 7000-8600 lbs, is exempt from government roof crush tests. (PDF)(As is any vehicle with a GVWR over 6000 lbs.) Unfortunately, passing them wouldn't mean much either - as the minimum only requires a vehicle with windshield intact to withstand 1.5 times its weight pressed down on the roof.
In a rollover, the windshield is usually gone by the end of the crash, and forces can easily exceed 1.5 times vehicle weight.
The current standard has been in place - and unchanged - since 1971, and many SUVs aren't required to pass even that lax standard. Many are also exempt from other minimum standards, like Steering Wheel Rear Displacement (only applies to cars with GVWR 4,000 lbs and less), and basic braking standards (applies to vehicles with a GVWR of 7,716 lbs and less).
Detroit, of course, insists the existing rules are adequate - the rules that often don't apply to their vehicles.
Even better, it's been shown that they misrepresented data from their own tests to the NHTSA.
Obviously, we all have to be more proactive in researching the safety of the vehicles we purchase - and not just go out and buy something "big" that "feels safe". Often, we may be safer in that type of vehicle, but only in multi-vehicle-non-rollover crashes - and to what expense? Raising the weight of your vehicle may reduce the risk to your family by 1/3 in some wrecks, but you've increased it threefold to the other car.
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