# 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:** 3
**Author:** ![Willcross](https://avatars.discourse-cdn.com/v4/letter/w/b2d939/32.png) [@Willcross](https://boards.straightdope.com/u/Willcross)\ **Post date:** [April 16, 2013, 11:53am UTC](https://boards.straightdope.com/t/head-on-collision/655742/41 "2013-04-16T11:53:29Z")
> [@Princhester](#): > > 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. That’s true, but a rocket has a stream of reaction mass. A jump is one impulse. Wouldn’t any upwards acceleration be counter acted by your downward acceleration as you fall back down to your death? It seems to me it would only slightly delay the impact, but I admit I may be wrong on this. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 16, 2013, 12:23pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/42 "2013-04-16T12:23:50Z")
> [@Willcross](#): > > That’s true, but a rocket has a stream of reaction mass. A jump is one impulse. Wouldn’t any upwards acceleration be counter acted by your downward acceleration as you fall back down to your death? It seems to me it would only slightly delay the impact, but I admit I may be wrong on this. It also slightly reduces the speed at impact. Without the jump you accelerate at something close to 9.8 m/s^2 the whole time, with the jump you reduce or reverse the acceleration for a short while. Unless you jump back to the height at which you started free falling the downward acceleration post-jump won’t balance out the speed reduction. ---
**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 16, 2013, 12:25pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/43 "2013-04-16T12:25:41Z")
> [@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 have not calculated the accelerations experienced in the collision. This is the relevant factor. To calculate peak acceleration, we need to know how much the cars deformed. To determine that, we need to know how much kinetic energy was absorbed in the collision. Let us assume the car is moving to the right, and the SUV is moving to the left. To determine how much kinetic energy is absorbed in the collision, we need to measure vehicle velocities in a frame of reference that moves with the combined center of mass of both vehicles. In both cases, the combined center of mass of the vehicles is moving to the left. In the first case, there’s only a mass differential; the combined COM is moving to the left at 16.67 MPH. In the second case, the mass and speed differentials result in the combined COM moving to the left at 30 MPH. Let us define vehicle speeds in a frame of reference that moves with this combined COM. In the first case, the car is approaching the combined COM at 66.67 MPH, and the SUV is approaching at 33.3 MPH. In the second case, the car is approaching the combined COM at 80 MPH, and the SUV is approaching at 40 MPH. In the second case, 1.44 times as much kinetic energy is absorbed in the collision. Assuming linear elastic properties of the vehicle bodies, this results in 1.2 times as much deformation, and therefore 1.2 times as much peak deceleration (at the moment when both vehicles reach zero speed relative to the combined COM). Passengers will experience 1.2 times as much force. That’s definitely a worse collision. > [@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. It absolutely will make a difference. See [post #12](http://boards.straightdope.com/sdmb/showpost.php?p=16195328&postcount=12) and [post #20.](http://boards.straightdope.com/sdmb/showpost.php?p=16195783&postcount=20) If you can adjust your velocity relative to the elevator, then you are also adjusting your velocity relative to the earth. ---
**Author:** ![Mama\_Zappa](https://avatars.discourse-cdn.com/v4/letter/m/71e660/32.png) [@Mama\_Zappa](https://boards.straightdope.com/u/Mama_Zappa)\ **Post date:** [April 16, 2013, 3:47pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/44 "2013-04-16T15:47:39Z")
I’ve always wondered, in the elevator scenario, whether it wouldn’t be a good idea to **lie down** on the floor of the elevator. Yes, you’ll still have that sudden deceleration - which will mostly likely result in blenderized innards and so on, but at least you’d skip the impact of, say, your head slamming into the floor. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [April 16, 2013, 5:48pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/45 "2013-04-16T17:48:31Z")
> [@Mama\_Zappa](#): > > I’ve always wondered, in the elevator scenario, whether it wouldn’t be a good idea to **lie down** on the floor of the elevator. Yes, you’ll still have that sudden deceleration - which will mostly likely result in blenderized innards and so on, but at least you’d skip the impact of, say, your head slamming into the floor. That depends on a number of factors. But if we assume the elevator and whatever it hits at the bottom of the shaft is pretty much solid, then if you lie down the result is the same as your head hitting a solid obstacle at whatever the final speed of the elevator is. If you stand up, your body will presumably offer some resistance and your head will hit the floor of the elevator at a slower speed. So you save your head at the cost of your legs. If the bottom of the shaft has some nice springs to change the g-forces of the elevator’s impact, things change, possibly enough to make lying down better. ---
**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 17, 2013, 1:19am UTC](https://boards.straightdope.com/t/head-on-collision/655742/46 "2013-04-17T01:19:28Z")
Lying down for a fall impact is never better than standing up for it. With the appropriate springs or the like, you could make the fall survivable lying down, but you’d also make the standing impact survivable. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [April 17, 2013, 2:18am UTC](https://boards.straightdope.com/t/head-on-collision/655742/47 "2013-04-17T02:18:25Z")
> [@Chronos](#): > > Lying down for a fall impact is never better than standing up for it. With the appropriate springs or the like, you could make the fall survivable lying down, but you’d also make the standing impact survivable. I would disagree with this. Spreading the impact force over a greater area lessens concentration of energy at any one point. For example if I use my open palm and push you with 20lbs of force not much happens. If however I push you with the pint of a knife using the same 20lbs of force you get hurt. I recall reading a book about a coroner a number years ago, a section of the book felt with people trying to kill themselves. The point was made (with several examples) that if you spread the impact over time or area the chance of survival increased. One case I recall was a guy that jumped out a hospital window about 12 stories up. He landed flat on a roof over a 1 story part of the building. The roof partially collapsed. He survived due to the impact being spread over area (landing flat) and time (roof partially collapsing slowing the rate of deceleration) ---
**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 17, 2013, 2:28am UTC](https://boards.straightdope.com/t/head-on-collision/655742/48 "2013-04-17T02:28:59Z")
> [@](#): > > I would disagree with this. Spreading the impact force over a greater area lessens concentration of energy at any one point. Yes, and that’s why the standing impact is better. Standing spreads the impact over the entire length of your body. Lying down spreads it only over the thickness. ---
**Author:** ![standingwave](https://avatars.discourse-cdn.com/v4/letter/s/9de0a6/32.png) [@standingwave](https://boards.straightdope.com/u/standingwave)\ **Post date:** [April 17, 2013, 2:54am UTC](https://boards.straightdope.com/t/head-on-collision/655742/49 "2013-04-17T02:54:27Z")
> [@Chronos](#): > > Yes, and that’s why the standing impact is better. Standing spreads the impact over the entire length of your body. Lying down spreads it only over the thickness. This. Think of your legs as the crumple zones. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [April 17, 2013, 3:06am UTC](https://boards.straightdope.com/t/head-on-collision/655742/50 "2013-04-17T03:06:32Z")
> [@Chronos](#): > > Yes, and that’s why the standing impact is better. Standing spreads the impact over the entire length of your body. Lying down spreads it only over the thickness. Umm no. Look at the amount of square footage that your feet take up vs the entire back of your body. Also think of the circus act of laying on a bed of nails. Think you could stand barefoot on a bed of nails? ---
**Author:** ![clairobscur](https://avatars.discourse-cdn.com/v4/letter/c/839c29/32.png) [@clairobscur](https://boards.straightdope.com/u/clairobscur)\ **Post date:** [April 17, 2013, 3:14am UTC](https://boards.straightdope.com/t/head-on-collision/655742/51 "2013-04-17T03:14:13Z")
> [@tim314](#): > > 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. I would guess it wouldn’t change much because the difference would be that instead of going from 60 to 0, you would go from 75 to 15, roughly. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [April 17, 2013, 3:57am UTC](https://boards.straightdope.com/t/head-on-collision/655742/52 "2013-04-17T03:57:04Z")
> [@Chronos](#): > > Lying down for a fall impact is never better than standing up for it. With the appropriate springs or the like, you could make the fall survivable lying down, but you’d also make the standing impact survivable. I do believe for record breaking jumps into shallow pools of water, the technique is to land on your back, not your feet. Speaking of elevators, why are people so worried about them falling? They have brakes that stop them even if all the cables break. And each cable alone is enough to support the entire weight of the elevator. And I think building codes require 8 cables. ---
**Author:** ![pool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pool/32/4152_2.png) [@pool](https://boards.straightdope.com/u/pool)\ **Post date:** [April 17, 2013, 4:48am UTC](https://boards.straightdope.com/t/head-on-collision/655742/53 "2013-04-17T04:48:50Z")
I used to have that survival handbook and it said if you had to jump off say a fire escape and land in a large dumpster it was better to land flat on your back rather than straight down on your feet because as mentioned earlier landing horizontal on your back distributes the force of impact though it sucks if your head lands right on a sharp object. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [April 18, 2013, 2:41am UTC](https://boards.straightdope.com/t/head-on-collision/655742/54 "2013-04-18T02:41:09Z")
Yup look at how stunt men do high falls. They land on their back. ---
**Author:** ![Princhester](https://avatars.discourse-cdn.com/v4/letter/p/3e96dc/32.png) [@Princhester](https://boards.straightdope.com/u/Princhester)\ **Post date:** [April 18, 2013, 4:05am UTC](https://boards.straightdope.com/t/head-on-collision/655742/55 "2013-04-18T04:05:40Z")
I think your analysis is flawed, **Rick**. I also think that \*\*Chronos’ \*\*position is overstated since landing upright may not be the best apprach in some circumstances, but is the right approach in a falling elevator. Falling onto garbage in a dumpster, or shallow water, or stuntman’s airbag, is very different to falling onto a hard surface. If you land upright, you are creating a high point pressure at your feet, which may penetrate through the soft garbage/airbag/shallow water, resulting in you hitting the hard surface beneath - hard. This would not be good. Contrastingly, if you land on your back the impact will be over a wider area so that you are supported by a broader area of soft garbage/bag/water. This is fine as long as the surface is soft. If the surface is hard and you land on your back your skull is going to hit the hard surface directly and smash like a melon. If the surface is hard, you are going to have to sacrifice your legs and use them as crumple zones to save your head. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [April 18, 2013, 8:02am UTC](https://boards.straightdope.com/t/head-on-collision/655742/56 "2013-04-18T08:02:37Z")
But that assumes your legs are softer than your head, to be able to use them as crumple zones. ---
**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 18, 2013, 9:23am UTC](https://boards.straightdope.com/t/head-on-collision/655742/57 "2013-04-18T09:23:25Z")
They don’t have to be softer than your head, just less critical for survival. ---
**Author:** ![snailboy](https://avatars.discourse-cdn.com/v4/letter/s/97f17d/32.png) [@snailboy](https://boards.straightdope.com/u/snailboy)\ **Post date:** [April 18, 2013, 10:11am UTC](https://boards.straightdope.com/t/head-on-collision/655742/58 "2013-04-18T10:11:40Z")
I was going to mention laying down, and I was thinking more about injuring your spinal cord than your head. I can’t imagine your spine handling much acceleration before it breaks. What if you laid down and put your hands behind your head? ---
**Author:** ![Learjeff](https://avatars.discourse-cdn.com/v4/letter/l/94ad74/32.png) [@Learjeff](https://boards.straightdope.com/u/Learjeff)\ **Post date:** [April 18, 2013, 11:35am UTC](https://boards.straightdope.com/t/head-on-collision/655742/59 "2013-04-18T11:35:18Z")
> [@AaronX](#): > > But that assumes your legs are softer than your head, to be able to use them as crumple zones. And they are. They’re also below your head, which helps. Consider the difference between landing feet first and landing head first. Which does more damage to the head? I think the important comparison is the difference in rigidity bewteen your legs and body versus the ground you land on. ---
**Author:** ![AaronX](https://avatars.discourse-cdn.com/v4/letter/a/7bcc69/32.png) [@AaronX](https://boards.straightdope.com/u/AaronX)\ **Post date:** [April 18, 2013, 3:22pm UTC](https://boards.straightdope.com/t/head-on-collision/655742/60 "2013-04-18T15:22:57Z")
> [@Cheesesteak](#): > > They don’t have to be softer than your head, just less critical for survival. Then why do cars need to be designed with crumple zones? [Previous page](https://boards.straightdope.com/t/head-on-collision/655742.md?page=2) [Next page](https://boards.straightdope.com/t/head-on-collision/655742.md?page=4)