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# Auto and Falling Object Interaction

## Auto and Falling Object Interaction

(OP)
Does anyone know of a study relating auto speed/weight to being stopped by or rolling under a vertically falling object of various weights and velocities and geometries?  In other words, when a car is hit by a vertically falling object, at what point does a car get stopped dead in its tracks with the object remaining in place crushing the car or roll under it and continue traveling with lesser damage?  I can imagine a very heavy object and a fast-moving car coming to a halt.  On the other hand, a slow-moving object and a fast-moving car, may have the car still partially crushed but moving out from under the object.
Replies continue below

### RE: Auto and Falling Object Interaction

That is just about the most odd request I believe I seen on these forums.  What exactly are you after.  It certainly does not appear to be 'automotive' oriented!

Rod

### RE: Auto and Falling Object Interaction

(OP)
Rod -- I am actually working on a forensic investigation where a motorist driving by somewhere between 20 and 65 mph (accounts differ) and was hit by a several-ton weight accidentally dropped from a crane.  True story.  Never knew what hit him. Completely crushed car to halfway down doors.  We are trying get a handle on the velocities of car and object to determine exactly what happened.  We know the weights involved and the final position of car and object.  Velocity of object is important to drop height and how it was let go.

### RE: Auto and Falling Object Interaction

Back in the early 60's when I was in highschool, we studied vectors.
Most practical application was allowing for winds, side slip and currents when navigating boats or aircraft.
I can't remember any formulas now, but as I recall you simply graphed the velocities with the two knowns as the X and Y axis. In this case use the momentum
(half the mass times the square of the velocity as I remember after 40 years)

Regards
pat

### RE: Auto and Falling Object Interaction

Adjacent to my jobsite in Sylmar, CA in 1971 during the big earthquake an overpass dropped on the connector road from I-5 to the 405 freeway and landed on a southbound pickup, probably traveling at 65+ mph.  The bridge was concrete in structure, as best I remember about 200 tons+ broke loose at both abuttments at the same time and dropped 15 feet.  The truck was pulled from the rubble and I saw that there were marks on the roof about a foot long (the truck was only a foot high at this point).  My guess is that was the distance for deceleration. The wreck had been removed from the roadway so I couldn't tell if there were skid marks or such. This particular instance was well documented in the L.A. area so some forensic studies may still exist.

Best of luck in your search.  This subject is something that hits very close to home.  I have lost loads a couple of times and had workmen injured as a result.  Not good.

Rod

### RE: Auto and Falling Object Interaction

This would seem to be a two part problem.
1) The first part would be to determine the coefficient of friction of the combined objects.  For this, a drag test with the crushed car and weight (on the same type roadway) would provide a reasonable first approximation of the friction coefficient.
2) The second part is the impact phase which would be a simple linear momentum problem in the vehicle travel direction.
This would at least provide you with an estimate of the minimum speed for the vehicle.
Kevin

### RE: Auto and Falling Object Interaction

I'm not a forensic scientist but i do watch a lot of CSI so here goes...

Actually i think there would be to many variables, was the vehicle able to support the weight for a fraction of a second and continue rolling, or did and axle break, did both axle's break? These would all have an effect on the friction applied to the road do to the amount of the vehicle that was contacting the road. I think you would have to recreate it to be sure. But it seems to me there should be other evidence at the scene such as skid marks or gouges on the road. I know you can figure out vehicle speed from skid marks, but i imagine trying to figure it from gouges from vehicle parts would be totally different.

-Jon

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