Crash and helmet

[pjclinch]

<snip>

Plus there's the statistical view:
the number of "Cracked helmet saved my life!" stories VASTLY outweighs the number of serious head injuries in the days before helmets.

Do you happen to have a reference for that statistic, please?

I don't have a formal citation, but a bit of pondering suggests it's not an unreasonable extrapolation.

We know the spec of a cycle helmet, most often EN1078, and we know it's to take a fall from stationary to hard ground with no other parties involved (that's what the testing regime simulates).
We also know, as dmrcycle pointed out, that in absorbing a full-spec load of energy it's entirely reasonable to expect a helmet to crack.

If we look around at where falls from low speed to hard ground are common then I'd suggest school playgrounds are a good candidate. Falls with head injuries are so common that the primary school my kids went to had a special protocol. If a child fell and didn't hit their head they'd get some TLC and an I've Been Brave! sticker, but if it was a head injury, those being more serious, additional measures were deployed. That meant another sticker (I Banged My Head!) and a form letter home to parents. Note that's a form letter, not a specially prepared one, and the I've Banged My Head! stickers were ordered in sheets from a catalogue, just like gold stars for workbooks. Both of those points indicate a common injury.
Now imagine that children have to wear helmets for free play (just as they often have to do for Bikeability lessons in the same playground, because the RA says they might fall or collide, just like they do in free play on a daily basis), and how now we'll see quite a number of cracked helmets as a result.

While trips and falls can kill from head injuries, they rarely do in terms of rates (you still get quite a number as so many people fall over). While they are the second highest cause of hospitalisation with head trauma (after car crashes), the fact is that the vast majority are dealt with by the body with a bit of R&R and feeling sorry for oneself. So if the number of children that start appearing with cracked play helmets start having assumptions made that they'd surely have cracked their skull and/or been killed without the helmet at the same rate helmeted cyclists seem to, everyone would be terrified to send kids out to play without head protection, but it would be confirmation bias and culture rather than an objective risk assessment.
Just like the post that started this thread, while one can safely assume that a cracked helmet has probably saved one a very nasty headache, it's a very big assumptive leap to take it as read that one would have surely ended up listed as a serious/fatal injury after a hospital visit. Yet any trip to the comments/followups of a social media post where it's suggested that helmets are a bit of a distraction can be guaranteed to cause plenty of "but a helmet saved me from terrible injury!, it was cracked!" anecdotes to come out of the woodwork.

Pete.

[mattheus]

<snip>

Plus there's the statistical view:
the number of "Cracked helmet saved my life!" stories VASTLY outweighs the number of serious head injuries in the days before helmets.

Do you happen to have a reference for that statistic, please?

I said "statistical". Big difference!

[Steady rider]

https://www.cyclehelmets.org/1209.html
not precisely answering the question but could be helpful.

[Stevek76]

If I tell you my helmet cracked then you can't tell whether that's failed pathetically to do its job properly or whether it's absorbed every last joule it possibly could have, only fracturing when it could compact no more. The latter is what is meant to happen (assuming I'm at or above spec limit) and is a Good Thing, the former very much isn't. They can both result in a cracked helmet.

As long as the parts don't fly apart leaving your head to continue to incept the ground I doubt this matters too much, and it's unlikely that they will fly apart given the friction of being stuck between a head and the ground. The parts can and will continue to crush after any crack if the collision is genuinely serious.

Sorry. Typo. Yes it does crush it’s all relative. Helmets do however seem to be slightly denser than the packing type. What I was trying to say is that the polystyrene will compress say 5mm but then there will be a fracture. The fracture is not like a pane of glass and is a slow propagation and non linear and slows down the head further (more like a tear than a shatter so to speak. The resultant distance they head can move is say 10’s of mm slowed down by the slow fracture. My point is a cracked helmet is part of the design and a good thing not a failure of poor design or an old helmet.

I did say *only* crack. My entirely anecdotal perception is that the vast majority of MHSML posts on the social medias tend to show a helmet with no or minimal (<1mm) crushing but cracked, probably from a manufacturing fault or a mix of age and poor treatment leaving partial fractures already present in the helmet.

[Chris Jeggo]

<snip>

Plus there's the statistical view:
the number of "Cracked helmet saved my life!" stories VASTLY outweighs the number of serious head injuries in the days before helmets.

Do you happen to have a reference for that statistic, please?

I don't have a formal citation, but a bit of pondering suggests it's not an unreasonable extrapolation.

<snip>

Pete.

Thanks,
Chris

[Chris Jeggo]

https://www.cyclehelmets.org/1209.html
not precisely answering the question but could be helpful.

Thanks for that, too.
Chris

[Mick F]

Can I suggest people do what I did?
Estimate the weight of your head by dipping it in a full bucket of water, and measuring how much water overflowed.
Guesstimate the weight of your head at maybe 70% of the water weight.

Hold your (old) helmet a metre off a solid concrete floor with a bag of wet sand equal to the weight, and drop the helmet repeatedly until it cracks. I used more than weight than 100% water.

Three times dropped.

viewtopic.php?f=41&t=79382&start=75

[drossall]

This will test faithfully what would happen if you chopped off your own head, held it in your hands, and dropped it on the floor.

[Jdsk]

From another thread:

I don't recall any errors in dmrcycle's posts on momentum:
viewtopic.php?p=1726915#p1726915
and subsequent.

He stated that momentum was not conserved in inelastic collisions. That is incorrect. (Despite his quoted physics qualifications.)
I suggest you review the discussion, it was quite clear.

Which post was that, please?

Thanks

Jonathan

[dmrcycle]

From another thread:

I don't recall any errors in dmrcycle's posts on momentum:
viewtopic.php?p=1726915#p1726915
and subsequent.

He stated that momentum was not conserved in inelastic collisions. That is incorrect. (Despite his quoted physics qualifications.)
I suggest you review the discussion, it was quite clear.

Which post was that, please?

Thanks

Jonathan

I never stated that. I do understand momentum. I said that in a closed system momentum is conserved. If energy leaves the system the bodies that are left do not conserve momentum. Two objects collide if heat and friction losses are considered the two bodies have less momentum after the collision. "Their" momentum is not conserved. Mattheus just kept repeating "conservation of momentum" is fundamental. Not understanding it all depends on what objects you are considering.

[mattheus]

From another thread:

He stated that momentum was not conserved in inelastic collisions. That is incorrect. (Despite his quoted physics qualifications.)
I suggest you review the discussion, it was quite clear.

Which post was that, please?

Thanks

Jonathan

Well, to be honest, I couldn't be bothered to trawl through this thread for a single simple answer. Fortunately our resident collision expert (with published papers, doncha know!) has kindly provided this statement:

I said that in a closed system momentum is conserved. If energy leaves the system the bodies that are left do not conserve momentum. Two objects collide if heat and friction losses are considered the two bodies have less momentum after the collision. "Their" momentum is not conserved.

I can't tell you what to think, jdsk, but the case is pretty clear to me.

[Chris Jeggo]

From another thread:

He stated that momentum was not conserved in inelastic collisions. That is incorrect. (Despite his quoted physics qualifications.)
I suggest you review the discussion, it was quite clear.

Which post was that, please?

Thanks

Jonathan

Well, to be honest, I couldn't be bothered to trawl through this thread for a single simple answer. Fortunately our resident collision expert (with published papers, doncha know!) has kindly provided this statement:

I said that in a closed system momentum is conserved. If energy leaves the system the bodies that are left do not conserve momentum. Two objects collide if heat and friction losses are considered the two bodies have less momentum after the collision. "Their" momentum is not conserved.

I can't tell you what to think, jdsk, but the case is pretty clear to me.

If energy leaves the system it is not a closed system.

[Jdsk]

From another thread:


Which post was that, please?

Thanks

Jonathan

Well, to be honest, I couldn't be bothered to trawl through this thread for a single simple answer. Fortunately our resident collision expert (with published papers, doncha know!) has kindly provided this statement:

I said that in a closed system momentum is conserved. If energy leaves the system the bodies that are left do not conserve momentum. Two objects collide if heat and friction losses are considered the two bodies have less momentum after the collision. "Their" momentum is not conserved.

I can't tell you what to think, jdsk, but the case is pretty clear to me.

If energy leaves the system it is not a closed system.

Yes

Jonathan

[Jdsk]

From another thread:

He stated that momentum was not conserved in inelastic collisions. That is incorrect. (Despite his quoted physics qualifications.)
I suggest you review the discussion, it was quite clear.

Which post was that, please?

Thanks

Jonathan

I never stated that. I do understand momentum. I said that in a closed system momentum is conserved. If energy leaves the system the bodies that are left do not conserve momentum. Two objects collide if heat and friction losses are considered the two bodies have less momentum after the collision. "Their" momentum is not conserved. Mattheus just kept repeating "conservation of momentum" is fundamental. Not understanding it all depends on what objects you are considering.

Yes.

Momentum is conserved in closed systems.

If the system consists of more than the two billiard balls there is no principle that the momentum of the two balls is conserved.

Jonathan

[Chris Jeggo]

The law of conservation of momentum follows from Newton's second and third laws.
2nd law: force = mass x acceleration, F = m a.
2 particles A and B on a collision course along the same straight line with velocities u-A and u-B.
Bang! Collision! Maybe they bounce elastically, maybe they cohere, maybe something in between.
Call the final velocities v-A and v-B.
Integrate Newton's 2nd law with respect to (wrt) time . The integral of force wrt time is called impulse I.
So I-A = m-A (v-A - u-A) and I-B = m-B ( v-B - u-B).
(masses are constant; acceleration is rate of change of velocity, or velocity is integral of acceleration)
3rd law: action and reaction are equal and opposite, so F-A = - F-B at any instant, so I-A = - I-B.
So m-A (v-A - u-A) = - m-B ( v-B - u-B).
So (m-A v-A) + (m-B v-B) = (m-A u-A) + (m-B u-B).
i.e. final total momentum = initial total momentum. - - QED.

I shall curb my enthusiasm for going through the equivalent derivation in relativistic physics. Likewise for sub-atomic physics, where photons have momentum but no mass. The latter is a different sort of QED (look it up!).