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Q: Crashing cars - any difference? ( Answered ,   7 Comments )
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 Subject: Crashing cars - any difference? Category: Science > Physics Asked by: ap12-ga List Price: \$5.00 Posted: 30 Jan 2003 06:22 PST Expires: 01 Mar 2003 06:22 PST Question ID: 155209
 ```A car is driving 50mph and hits a wall. Two exactly the same cars are driving 50mph and hit each other heads on. Is there any difference for passangers, if wall can't absorb any energy?``` Request for Question Clarification by omnivorous-ga on 30 Jan 2003 06:32 PST ```Two cars driving 50 mph have a relative impact speed of 100 mph -- or twice as much energy to dissipate as the car hitting the wall. Is this the comparison that you wish to make? Best regards, Omnivorous-GA``` Clarification of Question by ap12-ga on 30 Jan 2003 07:06 PST ```I want to establish if there is a difference in impact on driver, if he hits the wall compare to head-on collision with another car, which is exactly the same, the same weight, same type and driving the same speed.```
 Subject: Re: Crashing cars - any difference? Answered By: krobert-ga on 30 Jan 2003 08:41 PST Rated:
 ```ap12-ga, I looked at the question clarification by you and at the comment by michael2-ga and have a critical component to add. You can't look at closing speed and put 100 mph into the energy equation: Kinetic Energy = 1/2 mass times (velocity squared) You have two cars, each with their own mass and reference frame. Don't think about "inertial reference frames" because we're talking about a crash, not simply relative velocities. The total collision has twice the energy that the independent collision into a wall has. Why? Kinetic Energy = (2 Cars) times 1/2 mass times (velocity squared) However, you have two cars to dissipate the energy. So divide the total energy by two. So yes, the total energy of hitting a wall head on is exactly the total energy of hitting a oncoming car traveling at the same speed. Except now you have _two_ people and cars in need of repair. Having said all that, your still dealing with a total of twice the energy compared to a simple wall-crash. If the energy is not distributed equally between the two cars (very likely), the poor driver in the car that gets the bigger chunk of the energy will have a rougher time than the other guy. NOTE: This answer _definitely_ uses your assumption that the wall can't absorb any energy. Let me know if you need a clarification of this answer. Best to you, krobert-ga``` Request for Answer Clarification by ap12-ga on 30 Jan 2003 11:51 PST ```Really good, thanks a lot. Can we categorically say, that impact will not be twice as bad, or alternatively, will not be the same as one car hitting the wall with the speed 100mph?``` Clarification of Answer by krobert-ga on 30 Jan 2003 12:47 PST ```For the impact to be twice as bad, all the energy from both cars would have to be transferred to one of them. Just thinking about this, it's pretty impossible for that to happen. One car would have to come out just as good as it came in. So, in the real world... yes, we could categorically state that the impact will not be twice as bad. Of course this doesn't take into account a tumble down a hill or impact with any other vehicles after careening from the first impact. As the old saying goes, your mileage may vary. - krobert-ga```

 Subject: Re: Crashing cars - any difference? From: michael2-ga on 30 Jan 2003 06:42 PST
 ```The energy dissipation is four times as much at a relative closing speed of 100mph as it is at 50mph (as energy is proportional to the square of the velocity). So the head-on collision will be far, far worse.```
 Subject: Re: Crashing cars - any difference? From: pocoloco-ga on 30 Jan 2003 10:38 PST
 ```I agree with krobert-ga that the head-on collision of the two cars has twice the energy. In your original question, you asked "Is there any difference for passengers?" To further explore this aspect of the question, I suggest that you look at some articles on the kinematics of trauma. The following excerpt from the book "The Basic EMT" will serve to introduce you to this field: "In motor vehicle collisions three separate impacts (collisions) occur: (1) the vehicle crashes into an object, (2) the unrestrained occupant collides with the inside of the vehicle [edited: or unrestrained parts of the occupant collide with the vehicle], and (3) the occupant's internal organs collide with one another or with the wall of the cavity . . . In frontal collisions, the sum of both vehicles' speeds become the velocity at which damage is produced." This gets to the physics of the impact. However, the actual injuries suffered by a passenger in such a crash depend critically on whether or not they are restrained, how they are restrained (there is a big difference between seatbelts and a seatbelt/airbag combination), and (if they are unrestrained) whether they end up going "up and over" or "down and under." You can order The Basic EMT from Amazon for \$51.95 http://www.amazon.com/exec/obidos/tg/detail/-/0323022561/qid=1043951691/sr=1-7/ref=sr_1_7/002-1180689-0840034?v=glance&s=books or The slide presentation "Things That Go Bump in the Night - Kinematics of Trauma" does a nice job of summarizing factors that affect the different degrees of trauma that passengers will experience in a crash such as the one you describe. file:///C:/WINDOWS/Temporary%20Internet%20Files/Content.IE5/CHOX2BKX/256,1,Things That Go Bump in the Night Kinematics of Trauma For a discussion of "up and over" vs. "down and under" injuries, see the article "Trauma Mechanisms." http://www.nordictraumarad.com/Syllabus/Traumamechanisms.pdf search term="kinematics of trauma"```
 Subject: Re: Crashing cars - any difference? From: racecar-ga on 30 Jan 2003 11:49 PST
 ```The answer to the question is that both cases are exactly the same. It's true that in the real world, any wall will budge at least a little bit, and one of the cars might have hit a slight bump before the collision and so be a little higher than the other, or the left/right asymmetry of the cars might mean that they crumple differently than they would if hitting a wall, but I don't think these issues are in the spirit of the question. If two identical and left/right symmetrical cars collide head on, with no offset, the plane of impact remains motionless as both cars squash toward it. This must be the case because the situation is exactly the same from both sides. Since the impact point is motionless, there could just as well be a solid wall there from the point of view of either driver.```
 Subject: Re: Crashing cars - any difference? From: pocoloco-ga on 30 Jan 2003 17:11 PST
 ```Dear ap12-ga, As our Comments are coming to different conclusions on the physics of the accident (total energy dissipation), I thought it might be useful to you if I identified experts in the field. Searching on the term "physics of motor vehicle accidents," I found the following useful sites: 1. THE PHYSICS CLASSROOM This site is the headquarters for a high school physics tutorial. It works through the physics of several very specific accident scenarios. These are animated scenarios with physics explanations (and calculations!) including a car and truck in a head-on collision (an "elastic" collision) at http://www.physicsclassroom.com/mmedia/momentum/cthoi.html and a moving diesel engine crashing into a stationary flatcar (an "inelastic" collision) at http://www.physicsclassroom.com/mmedia/momentum/dft.html What are elastic and inelastic collisions? The instructor says "Certain collisions are referred to as elastic collisions. Elastic collisions are collisions in which both momentum and kinetic energy are conserved. The total system kinetic energy before the collision equals the total system kinetic energy after the collision. If total kinetic energy is not conserved, then the collision is referred to as an inelastic collision." (NB: In an inelastic collision, total energy is not lost. But, kinetic energy is changed into other types of energy such as sound energy and thermal energy, so the kinetic energy goes down. This gets to your assumption that the wall can't absorb energy.) I am not a physicist, and I have not attempted to calculate the forces in your accident. It appears to me that this site's examples give you the tools to do so, and that - no matter what - your head-on collision is going to have more impact on the driver than the crash into the wall. However, I was too quick to say (in my earlier comment) that it would have twice as much energy. 2. ACCIDENT RECONSTRUCTION TECHNIQUES AND ACCURACY http://www.mchenrysoftware.com/genintro.html Brian McHenry, who has developed software to analyze accidents, discusses Newton's Laws of Motion as they affect motor vehicle accidents. You may contact him at McHenry@mchenrysoftware.com. 3. ADDITIONAL RESOURCES The Glenbrook South (High School?) Project Information Sheet on Auto Collisions and Auto Safety includes additional links that you may find useful. http://www.glenbrook.k12.il.us/gbssci/phys/projects/yep/autos/austupa.html Lots of people seem to be getting interested in your question!```
 Subject: Re: Crashing cars - any difference? From: hedgie-ga on 30 Jan 2003 22:17 PST
 ```One cannot vote on truth, of course. But perhaps one more comment will help ap12 to make his mind. The reasoning of the answer allowed for the incorect conclusion, actually made by michael2. One does not divide total energy by number of particles to get the peak acceleration. Deceleration of the car is what casuses the injury, as it propels the passenger against the windshield, as corectly explained by pocopolo. So question can be modelled by two billiard balls colliding, or one such ball hitting heavy stationary wall. Is the the deceleration profile different in such two cases? In his second comment pocopolo confused things: The wall may be elastic or plastic and that has nothing to do with it being stationary or not. So we assume that wall and balls (and cars) have same elastic properties. Let's consider elastic collisions only. In that case ball rebounds the same way: It just changes the direction of velocity (in the same time interval) and threfore the effect (measured e.g. by peak deceleration) is same in both cases. So, racecars is essentially right (I hope it is not based on experience :-) but there are some implicit assumptions . If you would consider balls made of different materials and mass, a wall which has less then infinite mass (a net), it would be slightly more complicated. I hope this clarifies things hedgie```
 Subject: Re: Crashing cars - any difference? From: pocoloco-ga on 31 Jan 2003 06:47 PST
 ```Hello, all! This morning, I sent an email to Brian McHenry, an engineer who specializes in accident analysis (see his resume here: http://www.mchenrysoftware.com/bgm.htm). I referred to his company, McHenry Software (http://www.mchenrysoftware.com/genintro.htm) in my second comment above. His company produces software for accident reconstruction and analysis. I copied Mr. McHenry on our entire discussion and asked him if he could comment. He emailed back (within an hour!) as follows: QUESTION: A car is driving 50mph and hits a wall. Two exactly the same cars are driving 50mph and hit each other heads on. Is there any difference for passengers, if wall can't absorb any energy? ANSWER: The main question is 'Is there any difference for passengers'?The Answer is NO. DISCUSSION: The passengers in each vehicle experience a 50 MPH change in speed, impact speed change or DeltaV. Of course in scenario 1 (car-to-barrier), the occupants of veh 2 are stationary, watching those in Car 1 crash, and therefore they do not see any speed change, so, if you want to get technical, the speed change WOULD be different for the veh2 occupants in test 1. One other point: A minor difference between car-to-car and car-to-wall is that in the car-to-car scenario, since the other car is not a wall/barrier, there may be some minor differences due to the fact that to line up the impact exactly in the theoretical sense is impossible so the offset, etc would make the car-to-car slightly different than the car-to-barrier.```
 Subject: Re: Crashing cars - any difference? From: michael2-ga on 31 Jan 2003 15:00 PST
 ```Yes, this is clearly correct, and if I had thought about it for more that 5 seconds I should have realised my earlier comment was quite wrong. Sorry. Michael2```