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G force calculation of hitting the head

  1. Jul 10, 2015 #1
    Hello,
    Could you guys please do me a favor and calc the g force of a 8.8lbs head that accelerates with 1m/s or 1.5m/s and than hits a solid metal objebt with the back of the head which moves 0.5 mm.

    This is no homework. It happened to me.
     
  2. jcsd
  3. Jul 10, 2015 #2
    http://arxiv.org/pdf/0711.3804v1.pdf

    Here's a paper we wrote on the subject of head impacts.

    Hitting stuff hard that moves very little is a bad plan.

    There is some give in the scalp and some bending in the skull, but not much.

    Accelerations can be high.
     
  4. Jul 11, 2015 #3
    This looks very interessting but way to complicated for me.

    I don't know how to do it
     
  5. Jul 11, 2015 #4

    Drakkith

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    Without knowing the velocity of your head at the moment of impact or how long you were accelerating, there's little we can do. Can you put this in context for us? Were you in an automobile accident? The details may help us figure out some approximation.
     
  6. Jul 11, 2015 #5

    No I wasnt in a car accident. I wanted to look through a window and pulled the curtains back(not to the side). I ve overseen this big metal bar attached on the wall and leand my body and head back until it collided. The impact was moderate hard.

    The velocity while hitting the object has to be 1 or 1,5 m/s. I think I did not decelerate before the impact. The distance when I started to move backwords was 30 cm. I cannot tell how long I was accelerating,
     
  7. Jul 11, 2015 #6

    Drakkith

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    I'm not sure it's possible to get an accurate number then.
     
  8. Jul 11, 2015 #7
    So the only thing that would be needed is how long I accelearated?

    I calculated 0.6s
     
  9. Jul 11, 2015 #8

    Drakkith

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    Even with that I'm not sure what we could really do. It's a complicated problem. But honestly I don't expect that the g-forces exerted on your head were very high. There's no way you were moving that fast.
     
  10. Jul 11, 2015 #9
    After rethinking this I would say youre right.

    I needed approximately 0.5 s for a 30cm distance. So my velovity was 0.6m/s.

    Now it should be doable? It os no problem when the calc is not 100% correct.
     
  11. Jul 11, 2015 #10

    Drakkith

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    Now it depends on how fast your head decelerated. And that I don't really know how to find out. If we assume that it takes 0.27 seconds (the time of the collision in the paper linked in the 2nd post) then the deceleration is only 2.22 m/s2, which is less than 1/4 g. I expect that the time taken to come to a stop is different than 0.27 seconds, but I don't know by how much. Still, that should give you a decent estimate.
     
  12. Jul 11, 2015 #11
    0.27s is far too long. It was a rapid stop as metal give not much room. I would drop this number down to 0.1s

    About 50-60g would be requird for a concussion,
    10-20g is a subconcuasive impact.

    I am sure it is around 20g
     
  13. Jul 11, 2015 #12

    Drakkith

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    But your skull does. :wink:

    That puts the acceleration at 6 m/s2, or just under two-thirds of a g.
     
  14. Jul 11, 2015 #13

    Drakkith

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    Note that you can sustain an injury at much lower g-forces than commonly stated if the force is concentrated in a small area, such as when you hit your head on an metal bar.
     
  15. Jul 11, 2015 #14
    Yes I only hit a small part of my head(more the edge of this thing)

    So in summary we end up that it cannot be calculated.

    At least an assumtion?
     
  16. Jul 11, 2015 #15
  17. Jul 11, 2015 #16

    Drakkith

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    If you're referring to post #33, then it appears they've made a mistake with the units. Converting everything to meters (which is what you have to do) gives me 9.16 g's, not 91.6 g's. The calculated values in post #35 are also wrong. For the 0.5 mm deflection, the g-force is 0.19 g's, not 1.9.
     
  18. Jul 11, 2015 #17
    Ah ok. You know certainly more like this users about physics.

    So what is a reasonable value for my incident yesterday? I know physicians don't like estimations
     
  19. Jul 11, 2015 #18

    Drakkith

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    Assuming your head is a rigid object, a velocity of 0.6 m/s, and that the bar deflects 0.5 mm, I get 3.7 g's.
     
  20. Jul 11, 2015 #19
    That looks so little. The bump was mediocre intense. Now think how hard you have to hit your head to get 20-60g. Why did you take 0.5mm? How high are the gs when we reduce this to 0.1mm??

    The nfl does reasearch on head to head collisions. Most of thestart at twenty and go up to 50 and more.
     
  21. Jul 11, 2015 #20
    Where do you have this info from? Imagine the bar to be 4m long, 10cm wide and high. It hit me almost with the edge in the area where the ear ends a bit above.
     
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