Think skull

[Profpointy]

[QUOTE 3025818, member: 9609"]If I was right ? I was asking a question !
And as for falling of a low step - if someone was to fall head first into a kerb stone with their whole bodyweight behind their head and their hands tied behind their backs then I would expect a fractured skull.

Anyway, I would still like to know if a 3½stone block dropped 6 foot would deliver about 500J, took me a lot of research and calculations to come up with that - so could be a million miles out![/QUOTE]

Energy to lift a weight is force x distance. thus force is weight - which on earth is 9.81 Newtons per kg (let's call it 10) x height lifted in metres. So 3.5 stone = 49lbs = 22kg - giving 220 Newtons of force / weight. 6foot is just under 2m - so that's around 400 Joules - so you're there or thereabouts.

 

[Wobblers]

[QUOTE 3023400, member: 9609"]Well actually I didn't, I think skulls and helmets are very different materials and have very different jobs and can't be compared that way - I will come to that later.

I was surprised at the high figure needed to fracture a skull (my steel toe capped boots are only rated at 250J) So I tried to understand what a 500J impact would be (the following took me a very long time to work out and I'm not confident of its accuracy or worth) What I come up with was a concrete building block (typical block in the building trade - weighs 3½ stone) dropped from 6 foot. On impact that would deliver about 500J of energy - far too much for any skull surely??

But really I don't understand any of that, the angle of the strike would also be a big factor. And how would the test be done, skull pressed against a solid surface then the block dropped, because if the person was for arguments sake swimming, the chance of damage would be different as the water under the head would take some of the energy.

So back to the helmet - a helmet is not only in addition to the skull, it creates a space for deceleration, The head coming to a stop in a 1/10 of a second is a lot better than it coming to a stop in 1/1000 of a second (which it might if the skull made a direct strike against a kerb stone). For instance hitting your unprotected head onto a grass surface could be similar to a helmeted head striking concrete. The grass and the polystyrene both offer a little give. The helmet has to be a good idea (even if I rarely chose to wear one)[/QUOTE]

You need to recalibrate your energy scale! Your steel toecaps may only be rated for 250 J, but that's because steel (as with all ductile metals) absorbs energy by plastic deformation and there's an awful lot less of it - only a few tens of grams of metal to absorb a fair quantity of energy. 500 Joules is not that much: your ankles, knees and muscles have to absorb over 1500 J when you jump off a reasonably high wall. Yet most people can manage that without injury. The forces and energy required to cause injuries have been measured using cadavers (the first experiments involved dropping bodies head first down stairwells in an American university!) using very well established protocols: materials scientists have been doing this sort of thing for a very long time. Typically, the skull would be restrained so that all the impact energy would be transferred to it rather than to other modes.

Your assumption that as a helmet may be expected to reduce peak forces, it must be beneficial is just that: an assumption. It is not supported by the evidence, which fails to demonstrate any benefit. And again, this is a simplistic view. One mechanism that a helmet uses is to transfer the load via the straps away from the head - to the neck. (That's why proper helmet fitting is so important for protection.) The cervical vertebrae and especially the spinal cord are quite vulnerable structures. It is a poor solution to transfer significant load to them. Yet that is exactly what a helmet will do in any impact where there is a significant torque. But if you're travelling at speed, that is exactly what will happen should you fall off and head contact the ground.

 

[Fab Foodie]

.... my brain hurtz ....

 

[Canrider]

Not wishing to start an argument but this bit about helmets only being effective up to 12mph, what's the fall-away form that? Does the protection become insignificant at 13mph or does it drop away gradually p to a much higher speed but still offer reasonable protection?

Getting back to this question, if we ignore everything* about helmet shape and size, angle of impact, even potential energy and just consider the helmet as an absorber of kinetic energy..

For a given mass, at a speed of 12**, the helmet will be absorbing (0.5)*(mass = 1)*12^2 = 72 units of kinetic energy.
Let's assume that at this speed, the helmet just manages to absorb all the kinetic energy from the impact, and it can't absorb any more
At a speed of 15, the helmet has to deal with (0.5)*(1)*15^2 = 112.5 units of energy.
Subtract the two and you have 40.5 units of energy left over in the second impact. Since the helmet just absorbs 72 units of energy, those 40.5 units are not absorbed and have to be dealt with by the skull.
But, since kinetic energy increases with the square of speed, the kinetic energy in the second impact that the helmet doesn't absorb is equivalent to the kinetic energy of an impact at a speed of 9: SQRT(40.5) = (0.5)*(1)*9^2.

So, given all those assumptions, a helmeted impact at a speed of 15 is like an unhelmeted impact at a speed of 9.

*That's a lot of ignoring!
**Not bothering with units here as they don't affect the energy calculation

 

[Profpointy]

Just wondering a bit more about the "absorbing a set amount of energy" thing - whether skull, toe-caps or helmets. Doesn't the time or force aspect have an impact (excuse pun) ?

500J absorbed over half and hour (a silly example, OK) is f-all, wheras over a nanosecond, it's gonna hurt.
So isn't force or impulse to the brain going to be more important ?
Isn't it ? .. actual question, not rhetoric as I'm not sure really

 

[Profpointy]

[QUOTE 3026047, member: 9609"]Of course it does, that's why you can jump 20' into deep water and enjoy it, jump 20' onto concrete and it will spoil your day,

mr McWobbles example above with the body being able to absorb a large force when jumping off a wall is only possible because the feet ankles knees hips spine absorb the energy over a period of time - jump off a reasonably high wall with legs straight and stiffened and the impact will destroy you.

If the polystyrene of a helmet increases the time the skull comes to rest from 1/100 of a second to a 1/10 of a second then I reckon the shock load will have been reduced by 10 fold.[/QUOTE]

Quite - despite being currently in the "sceptic" camp, we should still try to be solid in argument and not make stuff up like the filthy liddites (joking)

 

[ufkacbln]

Actually head first from ground level if all the force was into your head would be quite a biff (not using arms to stop yourself for whatever reason) .... so I wasn't saying the figure was wrong necessarily

Anyone who has been in the Military will have seen someone faint on a Parade Ground, and that tends to be a direct descent to the Tarmac with no hands

 

[theclaud]

I fell off my bike on some unexpected black ice, no time to put a hand out (thank goodness). The side of my head hit the tarmac pretty hard, causing a tender bruise. OK I admit I was wearing The Woolly Hat That Saved My Life. I might have mentioned this before, but it is evidence isn't it?

Hmmmm. Did it crack or squash?

 

[ufkacbln]

Hmmmm. Did it crack or squash?

... or bleat?

 

[benb]

I fell off my bike on some unexpected black ice, no time to put a hand out (thank goodness). The side of my head hit the tarmac pretty hard, causing a tender bruise. OK I admit I was wearing The Woolly Hat That Saved My Life. I might have mentioned this before, but it is evidence isn't it?

Make wooly hats mandatory immediately!!

 

[ufkacbln]

Reminds me of the Falklands... the UK Armed Services "sense of humour" noted the woolly hats and started calingthe locals "Bennys" after the Character in Crossroads

An edict was than published that as the locals found this insulting it was to cease immediately

So they became "Stills"

As in no matter what the command structure said they were STILL "Bennys"