Board falling from a truck?

Discussion in 'Physics' started by spinnaker, Mar 16, 2013.

  1. spinnaker

    Thread Starter AAC Fanatic!

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    I saw a television show about automobile accidents. In the one scenario, an accident victim, using a dash cam, actually filmed what could have easily been her own death.

    She was traveling on a highway. There is a truck in front of her traveling in the same direction and about the same speed. The truck was carrying lumber and lost a board off the load. The board stuck the victims windshield at what they said was 90 miles an hour.

    How could this be possible? If both vehicles were traveling in the same direction and at the same speed, wouldn't the board appear to drop straight down as in relation to the victim? At worst it might strike the victim's vehicle at 10-20 MPH assuming she was passing.

    Unless I misunderstood the scenario and they were actually traveling in different directions then I could easily see the 90 MPH estimate.
     
  2. Wendy

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    Mar 24, 2008
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    Did it bounce off the concrete? There could have been elastic rebound.
     
  3. MrChips

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    Once the board leaves the truck, wind resistance would very rapidly reduce its speed to zero.
     
  4. hexreader

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    Exaggeration is very common (almost obligatory) on TV shows.

    Facts are far less common.

    Though this does not make the incident less scary for the victim.
     
  5. spinnaker

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    There was no "exaggeration". How would it be possible when the whole thing was caught on the woman's dash cam? While 90 mph might be an exaggeration, it could not be far off because the board went through the windshield and embedded in the seat beside her. This would take a significant amount of force.
     
  6. spinnaker

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    Possible. How could rebound build up such force?
     
  7. hexreader

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    You seem to want it both ways.

    First you say that 90MPH is unreasonable, next you say that there is no exaggeration.

    I agree with your first guess that the board hit at some lower (relative) speed.

    Something heavy going through a windscreen at 20-30MPH would cause a fair bit of damage. "Lumber" sounds like something heavy.

    "Lumber" sounds like something that may also be long, so perhaps the front end of the "Lumber" hit the road, while the car drove into the back end of the "Lumber".

    ... all just guesswork and supposition without seeing the video.
     
    Last edited: Mar 16, 2013
  8. spinnaker

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    Here is is here.

    http://www.youtube.com/watch?v=Jqpg7nI4rKs


    There is a version that is slowed down too but for whatever reason the person posting it chose to use profanity video (no idea why people seem to feel the need to do this).

    Anyway, it does look like the board strikes the pavement first. The truck was actually traveling faster than the victim's vehicle but it looks like it is possible that when the truck passes the trailer that the board could have gone under the wheels of the trailer vehicle. I suppose this could cause the board to shoot at the victim's vehicle.
     
  9. DerStrom8

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    I'm not sure about that. If both vehicles are moving forward, the wind from the front would push the board backwards. The way I would see it is this:

    Let's assume both vehicles are traveling at 65mph (an average highway speed). A board comes loose and catches the wind (blowing backwards in relation to the vehicles). The wind caries the board back at a fraction of the speed of the truck (we could say 25mph). The people on the show, obviously not physicists, would say that a car traveling 65mph in one direction hits a board flying at 25mph in the opposite direction means it's equivalent to the board hitting the car at 90mph.

    That's the only way I can see that working.

    Regards,
    Matt
     
  10. #12

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    Nov 30, 2010
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    The way I see this is that the board came from neither vehicle. It was laying on the edge of the asphalt and the left vehicle tipped it up by driving one wheel on the edge of the board. Relative to the sign beside the road, the board speed is nearly zero and car speed seems like about 60 MPH.

    You might agree with my interpretation or not.
     
    Last edited: Mar 16, 2013
  11. hexreader

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    Apr 16, 2011
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    Yes, I do agree.

    60 MPH sounds about right to me now that I have seen the video.
     
  12. spinnaker

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    I was sort of thinking the same myself. I was wondering how the board could fall off of the truck and make it to the left side of the trailer vehicle.

    Your explanation would explain how the board was on the left. It was merely road debris and if indeed the truck on the right was carrying lumber, it was merely a coincidence. There very well could have be a caravan of lumber trucks so one of their co-workers could have dropped the board.
     
  13. MrChips

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    I see the same thing. There was debris on the road and the 2x4 was flipped up off the road.
     
  14. WBahn

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    The board didn't fall off the truck. It was lying stationary on the left side of the road and was kicked up by the left tire of the trailer she was following (or possibly one of the left tires of the tow vehicle, but I think it was the trailer). That means that the lumber was probably travelling at low speed, probably in the same direction as her, when her car hit.

    Also, it was clearly not filmed with a dash cam. It looks more like a handheld cam either by the driver herself or someone in the back seat. Can't imagine why they were filming at all.
     
  15. thatoneguy

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    Freeze it at 32 seconds.

    Board is laying on edge of road, then trailer bumps over it and kicks it up. Wheelbase of trailer must be wider than wheelbase of towing vehicle.
     
  16. strantor

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    I think we all agree it was already on the road and the trailer kicked it up.

    Take a 1ft ruler and lay it perpendicular to the edge of a table, with one inch protruding over the edge. Now place your face 1ft above and 1 ft back from the ruler. Now let your hand lethargically fall onto the edge of the protruding ruler and observe how the speed with which the opposite end of the ruler contacts your face is some multiple (I'm guessing somewhere around 11X) of the speed of your hand.

    I see no reason why the board could not have been going 90+ mph when it hit the windshield.
     
  17. WBahn

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    Why is anyone trying to defend this claimed 90mph number when I haven't seen anything that even hints where this number came from?

    Imagine you are standing on the side of the road and that there is a piece of lumber lying there next to a road sign. Now a car comes from the right and hits the piece of lumber and it kicks up. Do you expect it to come to rest to the left of the road sign or to the right of it?

    I would expect it, most of the time, to end up a relatively short distance to the left of the sign.

    Now imagine hanging that piece of lumber so that it was motionless over the road and the car hits it doing 60mph. Is there anything in the video that argues that the impact was at a higher speed than that?
     
  18. Evil Lurker

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    Aug 25, 2011
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    90mph or not, that board was hauling some serious @ss when it went through the windshield.

    And as far as the cursing, I would have **** my ******* pants (use your imagination) if that came through my windshield. That could have easily killed the driver... by far and away curse worthy in my book!
     
  19. Metalmann

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    :confused:

    Very lucky woman, indeed.

    I suppose someone that studies this everyday, could come up with an impact speed. Frame analysis? Angle at moment of impact?
    Seems to me that the speed, would be whatever speed she was driving. The board may have just shot up, but had no forward or reverse movement of it's own.:confused:
     
  20. DerStrom8

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    Guess I should have watched the video before responding :p
     
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