Shock absorbing qualities of the human body

Scenario: Woman Jumps in Front of Speeding Car to Save Boy’s Life (Warning: Graphic Video)

Over in this thread, Machine Elf attempts to make the case that in the video the woman’s actions cut the impact on the boy’s body in half. I say nonsense. I ask this purely in the name of the straight dope on the issue, fully acknowledging I might be wrong but I am curious, did the woman’s actions (whatever the intent) lessen the impact on the boy by half?

I don’t see anything stopping the car other than the impact with her body. And as the car rolls back, you can see the impact dent where the front of the car struck her. So, yes, I think it’s pretty clear that if she had not been there, the boy would have been hit much harder. I’d say Machine Elf’s interpretation of what happened looks correct.

You mean other than the building wall which actually shakes because of the impact.

If you ‘frame by frame’ it you will see she gets hit on the passenger side front and the indention you reference is clearly from the boy.

I disagree that it’s “pretty clear”.

I would have to agree, the woman, being larger would have to be squished before it could touch the boy, so that would absorb some of the impact.

The rest i chalk up to the damage absorbing qualities inherent in little boys

I have not disagreed that the woman would absorb some of the impact first, but half?

Two ways to look at it:

  1. The car transmitted some total amount of force to the two bodies, yes? Whatever force of impact was absorbed by her was impact not absorbed by him. She is more surface and mass and likely absorbed more than half the car’s force. (The car does not look like it ever hit the wall; the force to the wall was transmitted by the bodies.)

  2. What I am seeing looks like she is next to him against the wall and again is a larger, wider object than he is. Significant force could not even begin to be transmitted to him until her body was compressed to his width, and from that point on in the collision would be divided between the the total that was her compressed contact area with the car and his. Imagine for comparison one larger and one smaller ball of modeling clay and compare the impact to the smaller ball under the conditions of an object smashing with force X into it alone vs that object with the same force X into both it and the larger/wider ball against the wall.

I would say that the statement that “the woman’s actions cut the impact on the boy’s body in half” is incorrect … her actions cut the impact by significantly more than half.

In any case, both got hit hard

I take your points, but…

Your points would only be true if the point of contact came while both of them were standing against the wall which was not the case. The initial point of impact was several feet away from the wall. Many people are hit by cars with no wall involved and still die.

My contention is that even if the car stuck her first it would not have slowed down any appreciable amount before striking the boy. A point that can be readily seen as the boy flies onto the wall after being struck.

There are two different impacts events you see in the video: the woman’s body could have and probably did cut the *first * impact to the boy in half or more. This is the initial impact of the car to the two people, which sent them flying along with the car into the wall. Consider though that the car retained most of it’s kinetic energy after that first impact. Had there been no wall the injuries would have been less to most involved.

But if you watch the video over and over you see the car did in fact subsequently hit the wall or something else. The car clearly keeps moving after the boy and woman have been hit and are flying through the air, so something else must have stopped it and there is a visible movement of the wall. Watch the cars’ tire and see that it keeps rotating after the initial impact with the people until the wall is encountered after which it rolls backwards after hitting the wall. Note the cars’ front hood doesn’t dent on the initial impact but does dent after bouncing off the wall. Also watch the driver’s bald head bounce around not after contact with the 2 people but after the second impact with the wall (or whatever stopped the car).

Then watch the impact of the boy; it’s actually his body hitting the wall and bouncing back then hitting the car a second time while the car is still moving forward that does the majority of the damage/impact to him. His pelvis area looks like it may have been pinned between the car and the wall on second impact considering how he fell straight down after the car rolled back and the blood stain on the ground was in the same place where his hips were after the fact at the end of the video when he was moved. The woman rebounds and hits the car a second time too but the impact appears less severe. Watch the timing of the wall shudder and note that it happens when the car would have made impact not when the people would have.

So I’d say yes the woman’s mass absorbed 50+ % of the initial impact which by its’ self may have only caused minor injuries had they been thrown clear (as happened to the second boy). But the majority of the impact and damage to the boy happened as he rebounded off the wall and collided a second time with the car as it continued on it’s trajectory after the initial impact. Considering both impacts, I’d say the woman had a pretty small effect.

I figured three impacts, and saw them this way (referencing the times in this video, not the OP’s):

#1: the initial contact with the car, at t=11 seconds. Yesterday I had thought that the woman and boy were struck independently, in which case the woman’s presence would not have significantly changed things for the boy; the math of momentum transfer events would have the 4000-pound car traveling just about the same speed after the collision, regardless of whether it hits 50 pounds of meat or 200 pounds of meat. In viewing that same footage today, I see that the woman’s body was between the boy and the car - so instead of being hit by hard metal, the boy was hit by much softer meat - not an airbag, but a meatbag. He still got launched at pretty much the same speed as if she had not been there, but those extra few milliseconds of time to accelerate his body to that speed makes a huge difference in the force exerted on the boy. I’m quite certain the woman’s presence helped the boy during this collision.

#2: the woman and boy contact the wall, at t=14 seconds. After being launched from the car’s grille, the woman and boy travel on separate paths and each victim has his/her own completely independent impact event with the wall. The wall does not move appreciably during this event, so the woman’s presence does not help/hurt the boy during this collision.

#3: the car crushes the woman and boy against the wall, t=15 seconds. This is the collision I was focusing on in the other thread. After the car rolls back and the bodies fall to the sidewalk, there does not appear to be any damage to the wall, which suggests that the car itself never made direct contact with the wall. This means that the entire force of impact was delivered through the bodies of the boy and woman. This was where my marshmallow analogy came in:

[ul][li]Drop a book on one marshmallow, it crushes by X millimeters.[/li][*] put two marshmallows side by side and drop the book from the same height; the marshmallows crush by Y millimeters, where Y is less than X, indicating a smaller impact force/energy delivered to each marshmallow.[/ul] Likewise, if the car crushes only the boy against the wall, the boy feels the entire impact force; if there are two boys side by side, each boy feels roughly half the impact force; if there’s a thick woman next to the boy, the woman will be compressed by a greater percentage than the boy (they will both be compressed to the same final thickness), so the woman will surely feel more impact force than the boy. This means that the woman’s presence helped the boy during this collision.

Had there been no wall the car would have run over both people, resulting in injuries.

Machine Elf disagree with your part two because of part one. Because the boy was propelled forward with some lesser amount of force than he otherwise would have been if she had not been there (still plenty) his contact with the wall was with that lesser amount of force.

Part 3 is clearly true.

And assuming that **[mmmiiikkkeeeB]'s view of the tape is correct and there was another portion that was the boy bouncing off wall into oncoming car before they both got compressed then that same decreased force from initial impact still applies in how much his velocity back at the car contributing.

My interpretation though remains that the greatest injury impact was the being compressed between the car still moving at nearly the same speed and the wall - a very short period of time that enough force was transmitted through the two bodies in combination to both offset the car’s forward velocity and bounce it back some. The car in a short period of time changed from its apparent substantial forward velocity to zero and slightly into reverse. The mass of the car times that total change in velocity over that brief time period was the force of that particular part of the collision F=mass*|V2-V1|/t. Given that we all accept the car’s velocity did not appreciably change in any other portion it seems likely that the period with the massive change in velocity of that mass was when the most force was transmitted. And yes whether next to him or over him her larger body absorbed some of that force first and then split it with him from there taking the lion’s share of it being both more mass and more surface area. Even more so by her body already being compressed and thus likely taking more force to compress further.

BTW I think your mashmallows analogy work better than the modeling clay one I came up with. Well done.

This we don’t know; they might have been run over or thrown enough to the side (or the car slightly change directions over the next 50m) such that they could have been partly or entirely missed after the first impact which threw them forward. We do see the second boy get hit and thrown at an angle such that the car wouldn’t have run him over had it been able to continue. There’s also the possibility of the driver reacting and swerving/slowing down after the first impact over the subsequent few seconds had he had an open path; as it was the second/third impacts happened almost instantly and he had no time to do anything to lessen the severity.

And even if one or both people did get run over their bodies would have taken less force than they did after getting pinned against the wall where they took essentially all the cars force. I’d rather have a 4000lb car roll over a hopefully non-vital part of my body on rubber tires and keep going with 95% percent of it’s momentum than to use that same part of my body to stop it dead in it’s tracks.

That’s why I say no wall would have resulted to less damage to all, including the car.

Yes that is possible, possibly less damage to all if that happens. But in your own words, this we don’t know. They also might’ve been run over by the car. Let’s be glad there weren’t more injuries, or fatalities.