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Old 02-17-2010, 11:52 AM   #1
Jonah Baby
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Projectile Crash Force

Projectiles in a crash.
Is there a mathematical formula to figure how many pounds of force a projectile may/can/will impact another object/passenger?
I'd like a formula which I could plug in variables, like speed, vehicle hitting stationery object OR non-stationery (like another vehicle), and the estimated weight for said projectile, etc.

I'd like to make the terrifying consequences of heavy or light, loose objects acting as projectiles a grim reality in a very quick and precise way - scare tactic, if you will.

Lots of rep to a good answer!
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Old 02-17-2010, 11:55 AM   #2
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Re: Projectile Crash Force

I was taught it was the weight of the object x the speed you're traveling
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Old 02-17-2010, 12:35 PM   #3
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Re: Projectile Crash Force



In physics, it's actually mass multiplied by velocity. For everyday usage, it's the weight of the object X the speed traveled.

So, that 20 lb. bar bell kept in the back of the car is going to hit someone with ~600 lbs. of pressure if you're traveling at 30 mph.
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Old 02-18-2010, 12:08 AM   #4
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Re: Projectile Crash Force

I'm going to throw the metric one out there. force (Newtons) = mass (kilograms) x acceleration (meters per second squared). Acceleration is defined as change in speed over time for that change to take place. For quick conversions and comparisons. Force of gravity is 9.80N, 1kg = 2.2lb, 1 meter = 39", 1 mile = 1.6km

F= m(vi-vf)
........∆t

Factors such as area of contact relative to weight will also play a role in how much force is exerted on the impact.
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Old 02-18-2010, 08:13 AM   #5
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Re: Projectile Crash Force

Hey, mythbusters actually did a show on killer tissue box. Then they used heavier and heavier objects. IT was pretty cool.
It is in this episode.
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Old 02-18-2010, 09:27 AM   #6
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Re: Projectile Crash Force

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Originally Posted by gecikuh View Post
I was taught it was the weight of the object x the speed you're traveling
This is a decent enough approximation for most parents. Some (who have studied physics) will know that it's not accurate. There are many other factors, including the type of crash, the crush of the vehicle, the time between the launch of the object and its impact, and so on. But for those who understand that it's not accurate, they will get the point. A heavier object will impact with more force at the same speed. The same object will impact with more force at higher speed. And crash forces are crazy higher than what one would like to believe they are.
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Old 02-20-2010, 12:03 AM   #7
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Re: Projectile Crash Force

Basically, a collision between a stationary and non-stationary object would more or less necessitate the same formula. Assume Vehicle 1 is the vehicle in motion, and initiating contact. Also assume that both vehicles, if moving, are doing so on the exact same vector.

Change in velocity = Velocity of vehicle 2 - Velocity of vehicle 1.
∆V=(V2-V1)

Assume ∆V is transferred to the loose object in your vehicle as its new velocity, vectorized accordingly to the direction of impact. (as we have assumed both vehicles are moving along the same vector, this is negligible, but in reality, some velocity is lost here, as well in the force lost in the crumpling of metal between vehicles) At this point, assuming the vectorized force is enough to break the inertia of the loose object (in reality, some velocity is lost in this transfer), and assuming there is a clear path for the loose object to follow along the force of influence (which in an object loose on the floorboard is unlikely):

F=((M*V)/∆T)

Force = (Mass x Velocity)/change in time

Therefore, the force of the impact of the loose object is equal to its mass multiplied by its velocity, over the change in time.

So, if you have a 1 pound object, which is traveling at 1 foot/sec, the force of said object would be equal to 1 foot/lb. (the same units used in measuring torque)

However, impact is not measured in such terms. In order to calculate the force/surface area of the impact you would need the variable of surface area impact. I.E. A knife's edge, with the same amount of force behind it, would generate a much higher P.S.I (pounds per square inch) than would a bowling ball.

In conclusion, physics can't really help scare a knowledgeable person into believing the umbrella on the floorboard would turn into a lethal projectile in any/most/all collisions, as too many variables are unknown. (How much is not transferred between vehicles due to not hitting squarely, how much force is used in the crumpling of vehicles, how much is lost in inertia, how much is lost in ricochets, and finally angle of impact of loose object and surface area of impact) While the possibility exists of a poorly mounted child seat breaking loose and lethally striking the passenger in front, there are too many variables to plug into a simple java application and get a regurgitated answer.

Theoretically, assuming all impacts convey a perfect transfer of energy, you could employ such tactics. However, we don't live in the 1950s where vehicles were designed to survive collisions regardless of how many humans were converted to so much ground meat, and safety features designed into the very sheetmetal and subframes protect us. To imply all modern collisions result in perfect transfer of energy to the objects and people within the vehicles is misleading and irresponsible.

Crash forces between vehicles are indeed on a massive scale. Tons of metal crashing into tons of metal at high rates of speed. However, the question posed here is not about the force of impact between vehicles, but rather the force of objects in said vehicles on other surfaces in the same environment. These by their very nature would tend to be much less dramatic, and the only constant would be the velocity of the collisions, albeit diluted.
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Old 02-20-2010, 01:02 PM   #8
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Re: Projectile Crash Force

Well, my dh did do a call once, where they really couldn't figure out what why the guy was unresponsive on the seen as it was a low speed collision, where I think the guy was hit from behind, and belted. He apparently was transporting some construction materials (sheetrock, 2x4's etc). Well, when they got to the hospital, dh happened to see the xray, and said to the doc, is that what I think it is? The driver must have had the 2x4s up on an angle in the back of the truck. When he was hit, a 2x4 impacted the back of his head, and you could see that impact on the xray.
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Old 02-20-2010, 03:18 PM   #9
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Re: Projectile Crash Force

Well sure, if you're carrying heavy unsecured loads which more or less line up directly with the back of your head, phyiscs isn't on your side any longer. That's more or less a perfect storm, where the entire force of the object (in that case, a long heavy object with a proportionally small surface area of impact) is primed for smacking you upside the head.
Yeah, that'd do it.
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Old 02-20-2010, 04:50 PM   #10
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Re: Projectile Crash Force

I guess my point was, first, we aren't talking about carseats that are not installed properly (I know all your statement wasn't about that, but you did mention it), and second, I am not sure any of us can be completely sure of where or how we are going to be hit, or where our "possible projectiles/unsecured objects" will be in the vehicle when we are hit.

Main point, we want to let people know that anything in the car that is not secured "can" be a possible projectile, and trying to spread the word about it because most people just don't think anything of it
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Old 02-20-2010, 07:01 PM   #11
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Re: Projectile Crash Force

Quote:
Originally Posted by Jennifer mom to my 7 View Post
Hey, mythbusters actually did a show on killer tissue box. Then they used heavier and heavier objects. IT was pretty cool.
I had to watch that because I actually heard a tech tell some parents once that their kleenex box was dangerous and I was a little skeptical. Turns out I was right, they busted that myth - in case you were wondering and don't have time to watch. Not that I would put a box of kleenex right in front of my kids head or anything, but I'm not going to stress about it being on the floor of the back seat.

Some interesting info they did share in that first clip though (again, in case you don't have time to watch):
- in 2001 there was an estimated 13,000 injuries in the US due to unrestrained objects in the back of the car
- in Australia there has been 2 cases of drivers being impaled through their seat by their golf umbrellas
- there has been a case in the US of a young woman being killed by her groceries in a minor fender bender.

There were no other details or sources given for those tidbits. There is also a crash test video shown of a bunch of stuff crashing into the car from the back. All that is in the first clip linked above.

They also tested a sub-woofer, a fire extinguisher, a bowling ball (which became a cannon ball), and an axe. You probably don't want any of those things hitting you in a collision - particularly the axe, which in their test landed with the blade right in the ballistic gel they were using as a stand-in for a head. I think all that was in clip 5/5. I love Mythbusters.
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Old 02-20-2010, 08:47 PM   #12
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Re: Projectile Crash Force

Quote:
Originally Posted by Jennifer mom to my 7 View Post
Main point, we want to let people know that anything in the car that is not secured "can" be a possible projectile, and trying to spread the word about it because most people just don't think anything of it
Fair enough.

However, I'd like to know how an unmounted subwoofer sounds in the back of the car. *Rattles on down the road* But then again, anything is possible in a world where celebrities drive with their children on their laps.
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