# Head-on Collision **URL:** \ **Category:** Factual Questions\ **Created:** [April 15, 2013, 2:34am UTC](https://boards.straightdope.com/t/head-on-collision/655742 "2013-04-15T02:34:24Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![steabo](https://avatars.discourse-cdn.com/v4/letter/s/c68b51/32.png) [@steabo](https://boards.straightdope.com/u/steabo)\ **Post date:** [April 15, 2013, 3:03pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/21 "2013-04-15T15:03:41Z")
Prompting so much head-scratching, these questions are clearly not coming from a physics guy. [quote=“sitchensis, post:8, topic:655742”] I believe the “logic” is that you will then impart the majority of the force onto the other vehicle, and thus save yourself at the cost of the other car. Indeed, this is what I meant, thinking that the situation might be analogous to the relationship of an SUV to a Smart, not that you would save yourself, but that the effects on you and your passengers might be less. Another analogy might be two billiard balls; the one moving faster does not come to an abrupt a stop as the other, and its direction is less likely to be reversed. And, no, it’s not for homework; I was just curious, but not in a Christopher Walken (Annie Hall) kind of way! The elevator question is one I have wondered about since I was a kid (I missed the “Mythbusters” episode). I did originally imagine that you would continue to stand normally, rather than start to float like in a simulated zero-g environment, but I also assume that holding yourself down and launching off the floor would offer some benefit, too. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [April 15, 2013, 3:05pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/22 "2013-04-15T15:05:57Z")
> [@Chronos](#): > > Warning: If you follow **Machine Elf** ’s link, your display mode for threads will be changed to “threaded mode”, which may be disorienting if you’re used to the default linear mode. To change it back, click on “Display Modes” at the top right corner of the thread. :smack: Sorry, I did that badly. [here is a direct link](http://boards.straightdope.com/sdmb/showpost.php?p=15528865&postcount=8) to just my reply in that thread. Any chance a moderator could edit my first post in this thread to include this link? ---
**Author:** ![hogarth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hogarth/32/1773_2.png) [@hogarth](https://boards.straightdope.com/u/hogarth)\ **Post date:** [April 15, 2013, 3:13pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/23 "2013-04-15T15:13:27Z")
> [@sitchensis](#): > > I believe the “logic” is that you will then impart the majority of the force onto the other vehicle, and thus save yourself at the cost of the other car. > > This is how it was explained to me 20 years ago. I agree it doesn’t make sense but it could be the basis for the question. It almost sounds like it would make sense to me if the situation were phrased as a car with no brakes about to hit a fence. Is it better to hit the fence and come smashing to an abrupt halt or to speed up and try to break through? Presumably the logic is still the same: adding more energy to the crash = bad. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [April 15, 2013, 3:30pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/24 "2013-04-15T15:30:30Z")
> [@hogarth](#): > > It almost sounds like it would make sense to me if the situation were phrased as a car with no brakes about to hit a fence. Is it better to hit the fence and come smashing to an abrupt halt or to speed up and try to break through? Presumably the logic is still the same: adding more energy to the crash = bad. Exactly, and this was my initial thought even before reading any responses. Why add more energy to the collision? Think of it this way. Imagine the other car is standing still. Would you rather hit it at 20 mph or at 60 mph? ---
**Author:** ![Cheesesteak](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cheesesteak/32/3694_2.png) [@Cheesesteak](https://boards.straightdope.com/u/Cheesesteak)\ **Post date:** [April 15, 2013, 3:32pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/25 "2013-04-15T15:32:26Z")
Theoretically, let’s say you have a car with an invulnerable safety cage, and a rocket strapped to the back. At the instant you connect with the other car, the rocket goes off with a force exactly equal to the force imparted by the other car. Your car will plow through the other car, with no change in velocity, at which point the rocket turns off and you coast to a stop. I think the “hit the gas” concept evokes this idea. It’s not so much your velocity at impact that causes a problem, it’s your change in velocity\*, and hitting the gas will counteract the forces that are changing your velocity. In actuality, any benefit is comparable to the amount of pain you experience when hitting the gas, pretty much zero. This “pretty much zero” also only has an effect between the time the collision begins and the time the drivetrain stops functioning. You’re going to get much more of a benefit by reducing your speed in advance of the collision. \*The fall was just fine, the trouble was with the sudden stop at the end. ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [April 15, 2013, 4:01pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/26 "2013-04-15T16:01:15Z")
> [@Cheesesteak](#): > > Theoretically, let’s say you have a car with an invulnerable safety cage, and a rocket strapped to the back. At the instant you connect with the other car, the rocket goes off with a force exactly equal to the force imparted by the other car. Your car will plow through the other car, with no change in velocity, at which point the rocket turns off and you coast to a stop. Yeah, only - You don’t have an invulnerable safety cage. - You don’t have a propulsion system that can counteract the force of hitting the other car, or even come remotely close. - Even if you did, you’d have to engage it at the exact moment of collision, not beforehand. Engaging it beforehand only increases the power requirements. So, the answer (as Cheesesteak points out) is clearly “Don’t hit the gas.” This is a case where the obvious answer is correct. ---
**Author:** ![tim314](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tim314/32/3468_2.png) [@tim314](https://boards.straightdope.com/u/tim314)\ **Post date:** [April 15, 2013, 4:10pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/27 "2013-04-15T16:10:29Z")
> [@hogarth](#): > > It almost sounds like it would make sense to me if the situation were phrased as a car with no brakes about to hit a fence. Is it better to hit the fence and come smashing to an abrupt halt or to speed up and try to break through? Presumably the logic is still the same: adding more energy to the crash = bad. Well, wait a second, I’d like to discuss the fence scenario more. With the car crash, when you’re going faster the force of the other car might not be enough to stop you, but it’s still going to affect you about as badly. Rapidly going from 60 mph to 10 mph isn’t really better than going from 50 mph to 0. But if you actually _break though_ the fence, it seems that would reduce the total impulse you receive from the fence, because it stops exerting its force as soon as it gives way. Anecdotally, when I was taking Tae Kwon Do, it hurt less to break a board successfully than to fail in the attempt. Although maybe that’s more a function of _how_ I hit it than how _hard_. ---
**Author:** ![Daylate](https://avatars.discourse-cdn.com/v4/letter/d/9dc877/32.png) [@Daylate](https://boards.straightdope.com/u/Daylate)\ **Post date:** [April 15, 2013, 4:44pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/28 "2013-04-15T16:44:37Z")
OK, what if the car or the elevator were on treadmills? What then? Huh?? ---
**Author:** ![JBDivmstr](https://avatars.discourse-cdn.com/v4/letter/j/b4bc9f/32.png) [@JBDivmstr](https://boards.straightdope.com/u/JBDivmstr)\ **Post date:** [April 15, 2013, 7:23pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/29 "2013-04-15T19:23:00Z")
> [@Daylate](#): > > OK, what if the car or the elevator were on treadmills? What then? Huh?? Well, _ **that** _, just changes everything! :rolleyes: ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [April 15, 2013, 8:22pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/30 "2013-04-15T20:22:48Z")
I know this is not completely accurate since auto collisions are not elactic but just to gauge what happens when you speed up I looked at the conservation of momentum/energy equations. A car and an SUV (twice as heavy as the car) collide head-on at 50mph. The car goes 83 1/3 mph backwards and the SUV goes 16 2/3 mph backwards i.e. the car has a delta-v of 123 1/3 mph and the SUV has a delta-v of 66 2/3 mph. Now the SUV speeds up to 70 mph to “drive through” the car. Car delta-v is 160 mph viz sponge them off the seats and the SUV delta-v is 60 mph. Holy fuck it works! I know a better physics student than I will come along and point out why my methodology is wrong. ---
**Author:** ![Gagundathar](https://avatars.discourse-cdn.com/v4/letter/g/a183cd/32.png) [@Gagundathar](https://boards.straightdope.com/u/Gagundathar)\ **Post date:** [April 15, 2013, 8:28pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/31 "2013-04-15T20:28:18Z")
OK, y’all. This just went into the “Thor Tumbling In The Hulk Cage From 30,000 Feet” category. On another note, how much DOES Wiley Coyote weigh and what is his terminal velocity? ---
**Author:** ![WhyNot](https://avatars.discourse-cdn.com/v4/letter/w/c77e96/32.png) [@WhyNot](https://boards.straightdope.com/u/WhyNot)\ **Post date:** [April 15, 2013, 8:38pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/32 "2013-04-15T20:38:27Z")
I was thinking something completely different with the car question, and that was the \*angle \*of the impact. Is it better, worse or about the same for a car to make a hard head on collision on the front of the bumper or the top of the bumper/hood of the car? Press the brake, and the front of the car tips down. Press the accelerator, and you’ll hit more straight on - but not more straighter than doing nothing at all, I don’t think. I have absolutely no idea how the physics work out, but I’m betting they have to crash test cars under different conditions to account for such things. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [April 15, 2013, 8:45pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/33 "2013-04-15T20:45:00Z")
Don’t forget that a car accident is not just a one time only hit but may consist of several small collisions. Adding extra energy into the system might result in a stronger more damaging secondary collision, a roll over, or some other bad result. ---
**Author:** ![Bad\_Astronaut](https://avatars.discourse-cdn.com/v4/letter/b/e19adc/32.png) [@Bad\_Astronaut](https://boards.straightdope.com/u/Bad_Astronaut)\ **Post date:** [April 15, 2013, 9:04pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/34 "2013-04-15T21:04:59Z")
> [@Saint\_Cad](#): > > With the collision question the obvious answer is better it is better to break as the amount of energy in the crash varies with the square of the speed. The better question is would you rather die in a head-on collision or **almost** die in a head-on collision? Many people might think they would rather die than be horribly crippled but they may well be mistaken. [This study](http://www.ncbi.nlm.nih.gov/pubmed/690806) is often cited as evidence that happiness is completely relative. The study found that people who had been paralyzed in accidents were just as happy as people who had won the lottery, in both cases within a year and a half of the accident or the windfall. ---
**Author:** ![Willcross](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Willcross](https://boards.straightdope.com/u/Willcross)\ **Post date:** [April 15, 2013, 9:35pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/35 "2013-04-15T21:35:52Z")
Jumping in an elevator isn’t going to make any difference even if it was perfectly timed. It’s not about stopping momentum relative to the elevator, it’s the motion relative to the earth that is the concern. If I drop you out of a clear plastic box from an airplane, do you think jumping at the last second would help? ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [April 15, 2013, 9:51pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/36 "2013-04-15T21:51:39Z")
Head-on Collision! Apply directly to the forehead! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [April 15, 2013, 10:13pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/37 "2013-04-15T22:13:39Z")
Jumping in the elevator _will_ make a difference; it’s just that there’s a very small window of circumstances where that difference will be significant. For a very short drop, it won’t matter, because you’ll be fine either way, and for a very long drop, you’ll be dead either way. But there’s some marginal zone in between where jumping could, theoretically, mean the difference between death and major injury, or between major injury and minor injury. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [April 15, 2013, 10:23pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/38 "2013-04-15T22:23:12Z")
> [@Saint\_Cad](#): > > I know this is not completely accurate since auto collisions are not elactic but just to gauge what happens when you speed up I looked at the conservation of momentum/energy equations. > > A car and an SUV (twice as heavy as the car) collide head-on at 50mph. The car goes 83 1/3 mph backwards and the SUV goes 16 2/3 mph backwards i.e. the car has a delta-v of 123 1/3 mph and the SUV has a delta-v of 66 2/3 mph. > > Now the SUV speeds up to 70 mph to “drive through” the car. Car delta-v is 160 mph viz sponge them off the seats and the SUV delta-v is > > 60 mph. Holy fuck it works! > > I know a better physics student than I will come along and point out why my methodology is wrong. You touched on it when you noted that auto collisions are not elastic. In inelastic collisions, energy is lost and “work” is done in the form of rearranging the autos in the collision. In the 70mph case, the SUV has doubled its kinetic energy allowing for more of this “work” to be done. More energy = more carnage. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 16, 2013, 11:10am UTC](https://boards.straightdope.com/t/head-on-collision/655742/39 "2013-04-16T11:10:40Z")
> [@Learjeff](#): > > If your body could withstand the force of such a jump, then it wouldn’t be damaged by the force of the landing (if your muscles could time it right.) You’re absorbing the same amount of energy in either case. Agreed. I thought about making that point. > [@](#): > > Of course, the timing issue applies in either case (jumping just before the landing, or bracing for the landing as it happens.) In the latter case there’s the possibility of detecting and reacting to the landing, but not given human anatomy and nerve impluse speed, etc. Which is why, if we could actually jump hard enough for this trick to be useful, open topped elevators would be so much safer 🙂 Close topped elevators make the timing critical since an early jump would cause you to hit your head. With an open topped elevator you could just jump relatively early. At least that way, you would decelerate to zero a few metres above the ground, then fall the last bit, which would (with your super-legs) be entirely survivable. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 16, 2013, 11:16am UTC](https://boards.straightdope.com/t/head-on-collision/655742/40 "2013-04-16T11:16:12Z")
> [@Willcross](#): > > Jumping in an elevator isn’t going to make any difference even if it was perfectly timed. It’s not about stopping momentum relative to the elevator, it’s the motion relative to the earth that is the concern. If you were right, rockets wouldn’t work. You are however dead wrong. Rockets push reaction mass away so the rocket goes the other way. Jumping in a falling elevator is exactly the same, you are pushing the elevator down so that you accelerate up. The elevator is the reaction mass, and you are the rocket. [Previous page](https://boards.straightdope.com/t/head-on-collision/655742.md?page=1) [Next page](https://boards.straightdope.com/t/head-on-collision/655742.md?page=3)