How long does it take to stop when crashing?

SigmasonicX · Post by **SigmasonicX** » Fri Feb 09, 2007 7:46 pm

I suppose it might be a bit odd for one of the supposed experts to be asking a question here, but whatever.

When an object, say, falls and crashes into the ground, the acceleration from the falling speed to zero seems instantaneous. However, there is a very slight time interval in which the object loses speed. Is there any way find this time interval for some special occasions, such as a man falling onto concrete?

In case you're wondering, for my Physics Honors project my group has to find a movie scene that defies the laws of physics and explain why, and we chose a scene where a (non-superpowered) character falls an insane height onto concrete on his back but manages to survive. We already have a source stating that bodies travel a quarter of an inch when crashing into concrete (and we can easily find the time, and more importantly the force applied to the character, using this information), but we would like to find other sources for confirmation.

deleted-71447 · Post by **deleted-71447** » Fri Feb 09, 2007 11:04 pm

Hi Sigmasonic,
Wow, that's a doozy of a topic. Here is a paper that discusses the history and some of the main issues involved in injury biomechanics research. http://www.verdstein.com/scibuds/JBiome ... l_1989.pdf
The answer to your question about deformation of concrete will depend on the force of impact, the thickness and composition of the concrete, as well as the characteristics of the underlying soil. Maybe you can avoid these complications by invoking the conditions in the film as a starting point - for example, if there is no visible deformation of the concrete in the movie, you could use that as a constraint for your calculations.

SigmasonicX · Post by **SigmasonicX** » Fri Feb 09, 2007 11:51 pm

Hmm, I see. This is one of those extremely advanced things. Well, for now I'll assume that the distortion caused is 1/4 of an inch multiplied by the height of the fall in stories (the 1/4 of an inch thing comes from an article breifly mentioning that a fall from one story onto concrete yielded a "stopping distance" of that amount). Either way, it should be able to undeniably prove that no human could have survived the fall.

For those who are curious, the scene my group is using is the scene from the end of the James Bond movie GoldenEye when James and Alec fight atop a giant satelite dish. James knocks Alec off one of the cradles and a he falls for over 7 seconds straight before hitting the ground. Despite the fact that no human could live through this, Alec survives the fall (though he is later killed by the dish falling apart).

deleted-71576 · Post by **deleted-71576** » Sat Feb 10, 2007 7:33 am

I wish I could find my freshman biophysics course book from MIT. It went into great detail on this very topic.

The acceleration distance (which dictates the acceleration time, depending on the material) is key. Humans have fallen out of airplanes, had their parachutes fail, reached terminal velocity before hitting the ground, and walked away. The most common condition was a snow bank greater than 3.5 feet in distance, increasing their deceleration time as much as is necessary to prevent vital organs from being destroyed.

Do some google searchs on collision severity, LD95 (or LD50) of falls, closed head injuries from falls, and thoracic aortic injuries from falls or thoracic aorta deceleration injuries etc.

Also, keep in mind that while the concrete doesn't deform much (I have a hard time believing it deforms even a 1/4"/story), the human body does. Also the greater the surface area that absorbs the hit, the less the collision severity (hence the advice about lying down on the floor of a plummeting elevator, not trying to jump up and down.)

Also, if memory serves, the LD50 from a fall is 3 stories. I think the LD95 is about 6 stories, so people can fall greater distances than you think.

BTW, how are you going to find out the height of the middle of the dish at Arecibo? In other words, how far did Alec Trevelyan fall? Of course, you can calculate the distance it takes a human to reach terminal velocity and if it is greater than that it's irrelevant, but I think that is a greater distance than on the dish.

Anyway, random ramblings for now. I'll see if I can find a few good links for you in the medical literature.

deleted-71576 · Post by **deleted-71576** » Sat Feb 10, 2007 7:35 am

BTW, make that Deceleration distance in my second paragraph. Aargh, it would be easier to edit these thing.

deleted-2131 · Post by **deleted-2131** » Sat Feb 10, 2007 11:41 am

The key here is really Newton's second Law. If you manipulate the variables in Newton's second law (net force = mass times acceleration) and then redefine them, you get the definition of momentum (p): p = mass times velocity and delta p = force times time. That is, momentum is changed by exerting a force on an object for a given time. As times increases, the force applied decreases. (This is why an egg dropped from a height onto concrete breaks, while an egg dropped from the same height into a column of water doesn't), So, you can calculate a person's momentum when the person is terminal velocity and then divide that by the time it takes for the person to hit the ground. This will give you the force exerted on the body. So, if we assume from you previous posts that the person falls for seven seconds, he may or may not have reached terminal velocity. If we assume the time it takes the body to stop is half a second (which is probably being generous) then the force exerted on the body is twice the change in p. If this force is greater than that a human body can withstand (someone with more medical knowledge help) then the person is injured or dead.

deleted-71576 · Post by **deleted-71576** » Sat Feb 10, 2007 12:10 pm

Thought I'd interupt the physics with a little human interest story. Here's a link to a person who survived a 17 story fall. He survived a 160 foot fall onto as asphalt overhang. And as I said earlier, people have survived free fall without parachutes.

http://www.twincities.com/mld/twincitie ... 521707.htm

I'm working on finding some of the physics on deceleration injuries and injury severity.

The most susceptible structures in the body are the brain, and the thoracic aorta. Severe deceleration injuries to them are what cause most of these deaths.

deleted-71576 · Post by **deleted-71576** » Sat Feb 10, 2007 12:12 pm

The newspaper article states that, "Richard G. Snyder wrote one of the groundbreaking studies on such falls in 1963. The paper, "Human Survivability of Extreme Impacts in Free-Fall," examined the physical and biological factors involved."

Sounds like a great paper for you to find. Could be a little hard, because it is 44 years old.

deleted-71576 · Post by **deleted-71576** » Sat Feb 10, 2007 12:16 pm

OK, I'm getting a number of interesting (albeit gory concept) hits from a google search using "human surviving falls"

This link has references to a number of papers/books on the topic. It's know as the Free Fall Research Page:
http://www.greenharbor.com/fffolder/ffreading.html