Euan said:>>>>> "Claes" == Claes <[email protected]> writes:
Claes> It seems that statistics can not solve this one. How about a
Claes> simple test. You wear nothing on your head, I smack a
Claes> baseball bat on your head, just hard enough to crack you
Claes> scull, then we do a test with your head again, healed up and
Claes> all, and smack at the same force, you think you head would
Claes> not crack this time?
You would have to have a very fine gradient in the velocity of the
baseball bat.
Bicycle helmets absorb kinetic energy (KE). The formula for KE is:
KE = 1/2 * M * V^2
It's tempting to think that a bicycle helmet that's rated for a 19 km/h
impact will take 19km/h off of any impact speed and make a difference.
This isn't the case.
Let's say the mass is 10kg and the velocity is 19km/h. The kinetic
energy is 1805.
Now let's take an impact at 40km/h. The kinetic energy is 8,000.
So we take away the 1805 from the 8,000 which leaves 6,195.
Re-arranging the equation a bit we can find out how much speed the
helmet's taken off the impact. The effective speed of the impact is
35.2km/h.
The higher the impact speed, the more ineffective the helmet is and it's
an exponential curve. At 60km/h the effective speed of impact is
56.9km/h. At 80km/h the effective speed of impact is 77.7km/h
I ride consistently at speeds over 35km/h. A collision at that speed
whilst wearing a helmet would make the collision speed 29.39km/h. I
don't think that's going to make a huge difference to the extent of a
head injury incurred, but that's a personal judgement.
Add in the fact that I weigh considerably more than 10kg and that makes
a helmet almost irrelevant.
--
Cheers | ~~ __@
Euan | ~~ _-\<,
Melbourne, Australia | ~ (*)/ (*)
This doesn't smell right - surely it is the distribution of the energy of an impact through time and across an area that determine the likelihood of damage. You have shown that the distribution of energy through time is little changed, especially for higher speed impacts, but not shown that energy is dissipated across a wider area of the head. For example, a 20kg plate can be supported by balancing it on your head, but put a nail in the centre of the plate and you'll pierce a nice hole in your head if you try to balance it in the same way. This example says nothing about velocity, but something about the distribution of force...
Ritch