Crash and helmet

For all discussions about this "lively" subject. All topics that are substantially about helmet usage will be moved here.
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 30 Sep 2022, 1:34pm
mattheus wrote: 30 Sep 2022, 9:50am
Jdsk wrote: 30 Sep 2022, 9:47am dmrcycles mentioned heat leaving the system, not heat being generated in the system.

Jonathan
Good spot. I'll await dmr's confirmation of that view (and whether he thinks that affects his answer). I'd like to be crystal clear on this ...
I was talking about the system being solely the three objects. They come together with a certain momentum. If there is deformation there will be heat generated and that leaves the system and the momentum of the 3 items combined therefore less.
Just leaving this here: https://en.wikipedia.org/wiki/Inelastic_collision
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 30 Sep 2022, 4:02pm
dmrcycle wrote: 30 Sep 2022, 1:34pm
mattheus wrote: 30 Sep 2022, 9:50am
Good spot. I'll await dmr's confirmation of that view (and whether he thinks that affects his answer). I'd like to be crystal clear on this ...
I was talking about the system being solely the three objects. They come together with a certain momentum. If there is deformation there will be heat generated and that leaves the system and the momentum of the 3 items combined therefore less.
Just leaving this here: https://en.wikipedia.org/wiki/Inelastic_collision
And.... I know all that, its basic 1st year physics. Kinetic energy of an elastic and inelastic collisions is not even what I am talking about. The point I am making is if we consider any collision which external forces apply if you consider losses due to heat of friction (from deformation) leaving the system, then the bodies themselves you are considering do not conserve momentum. They lose momentum. A car freewheeling down a road will slow to a stop. Its momentum has changed. The total momentum of the planet and molecules is constant but talking about the car as the system its momentum is not conserved.
Its in Newtons law it talks about only conservation of momentum unless other forces apply. Friction is a force that applies here and heat leaves the system.
The principle of conservation of momentum states that for a closed system the total momentum remains the same before and after a collision.

The key is the word closed system. If heat or sound escapes its not a closed system. We are only looking at the bodies impacting when considering a helmet collision. The head slows down, energy is lost, heat and sound generated and leave the reference field of the car and head. The sum momentum of the head and the car is less after the collision and eventually zero as all the energy dissipates out of the system and car and head and come to rest. When at rest momentum of the car and head is zero.
Jdsk
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Re: Crash and helmet

Post by Jdsk »

dmrcycle wrote: 30 Sep 2022, 4:38pmKinetic energy of an elastic and inelastic collisions is not even what I am talking about. The point I am making is if we consider any collision which external forces apply if you consider losses due to heat of friction (from deformation) leaving the system, then the bodies themselves you are considering do not conserve momentum. They lose momentum. A car freewheeling down a road will slow to a stop. Its momentum has changed. The total momentum of the planet and molecules is constant but talking about the car as the system its momentum is not conserved.
Its in Newtons law it talks about only conservation of momentum unless other forces apply. Friction is a force that applies here and heat leaves the system.
The principle of conservation of momentum states that for a closed system the total momentum remains the same before and after a collision.
Yes.

Momentum is conserved in closed systems whether the analysis is classical, relativistic or quantum mechanical.

If external forces act on something then that something is not a closed system. If heat leaves something then that something is not a closed system.

Jonathan
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 30 Sep 2022, 4:38pm
mattheus wrote: 30 Sep 2022, 4:02pm
dmrcycle wrote: 30 Sep 2022, 1:34pm

I was talking about the system being solely the three objects. They come together with a certain momentum. If there is deformation there will be heat generated and that leaves the system and the momentum of the 3 items combined therefore less.
Just leaving this here: https://en.wikipedia.org/wiki/Inelastic_collision
And.... I know all that, its basic 1st year physics. Kinetic energy of an elastic and inelastic collisions is not even what I am talking about. The point I am making is if we consider any collision which external forces apply if you consider losses due to heat of friction (from deformation) leaving the system, then the bodies themselves you are considering do not conserve momentum.
Well that's rather strange. Because an inelastic collision IS one where deformation occurs.

It's right there in the Wiki article (and my A-level physics lessons, but I don't have a transcript handy, sorry ...) Which clearly states that momentum of the system is conserved.

Are you saying it's wrong? Or your 1st year physics lessons were wrong?
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 30 Sep 2022, 5:03pm
dmrcycle wrote: 30 Sep 2022, 4:38pm
mattheus wrote: 30 Sep 2022, 4:02pm
Just leaving this here: https://en.wikipedia.org/wiki/Inelastic_collision
And.... I know all that, its basic 1st year physics. Kinetic energy of an elastic and inelastic collisions is not even what I am talking about. The point I am making is if we consider any collision which external forces apply if you consider losses due to heat of friction (from deformation) leaving the system, then the bodies themselves you are considering do not conserve momentum.
Well that's rather strange. Because an inelastic collision IS one where deformation occurs.

It's right there in the Wiki article (and my A-level physics lessons, but I don't have a transcript handy, sorry ...) Which clearly states that momentum of the system is conserved.

Are you saying it's wrong? Or your 1st year physics lessons were wrong?
What it says is correct and what I say is correct. An in elastic collision not all the energy is converted into kinetic energy. Deformation doesn't have to occur though. A vehicle collision is inelastic as energy is lost as heat and sound. Momentum is conserved of all the items in the system if no external forces apply. Momentum is mass X velocity. If we were to consider a closed system where you took the total momentum of all the molecules including the vibration of heated molecules then momentum is conserved. However, it's really hard to keep that system as a closed system as the heat and sound energy will dissipate away. Therefore considering just the head and the car as the items you are measuring for the momentum, their combined momentum is reduced because of the external force of friction and conversion to heat.
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 30 Sep 2022, 8:55pm
mattheus wrote: 30 Sep 2022, 5:03pm
dmrcycle wrote: 30 Sep 2022, 4:38pm

And.... I know all that, its basic 1st year physics. Kinetic energy of an elastic and inelastic collisions is not even what I am talking about. The point I am making is if we consider any collision which external forces apply if you consider losses due to heat of friction (from deformation) leaving the system, then the bodies themselves you are considering do not conserve momentum.
Well that's rather strange. Because an inelastic collision IS one where deformation occurs.

It's right there in the Wiki article (and my A-level physics lessons, but I don't have a transcript handy, sorry ...) Which clearly states that momentum of the system is conserved.

Are you saying it's wrong? Or your 1st year physics lessons were wrong?
What it says is correct and what I say is correct. An in elastic collision not all the energy is converted into kinetic energy. Deformation doesn't have to occur though. A vehicle collision is inelastic as energy is lost as heat and sound. Momentum is conserved of all the items in the system if no external forces apply. Momentum is mass X velocity. If we were to consider a closed system where you took the total momentum of all the molecules including the vibration of heated molecules then momentum is conserved. However, it's really hard to keep that system as a closed system as the heat and sound energy will dissipate away. Therefore considering just the head and the car as the items you are measuring for the momentum, their combined momentum is reduced because of the external force of friction and conversion to heat.
It's still a completely normal Inelastic Collision, as described by the WIki page! There is nothing special happening in the polystyrene of the helmet - that is already covered in the standard description of Inelastic Collisions. It's right there in the first two paras:
An inelastic collision, in contrast to an elastic collision, is a collision in which kinetic energy is not conserved due to the action of internal friction.

In collisions of macroscopic bodies, some kinetic energy is turned into vibrational energy of the atoms, causing a heating effect, and the bodies are deformed.
I don't know why you think helmets create some new special physics that isn't covered by this :-/ ???
mattheus
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Re: Crash and helmet

Post by mattheus »

ASIDE: upthread someone posted a (fairly?) recent study showing increased head injuries for helmet wearers. Can anyone help me find it?!?

(I think it was either on the cuk site, or they had some link to it. The reason for my request is an interaction with an organisation that is making us wear helmets. TIA!)
Steady rider
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Re: Crash and helmet

Post by Steady rider »

https://www.researchgate.net/publicatio ... jury_rates
https://www.researchgate.net/publicatio ... _warranted
Cycling UK https://www.cyclinguk.org/campaign/cycl ... s-evidence
https://www.cyclinguk.org/briefing/cycle-helmets
In any case, there are serious doubts about the effectiveness of helmets. They are, and can only be, designed to withstand minor knocks and falls, not serious traffic collisions. Some evidence suggests they may in fact increase the risk of cyclists having falls or collisions in the first place, or suffering neck injuries.
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 3 Oct 2022, 11:34am
dmrcycle wrote: 30 Sep 2022, 8:55pm
mattheus wrote: 30 Sep 2022, 5:03pm
Well that's rather strange. Because an inelastic collision IS one where deformation occurs.

It's right there in the Wiki article (and my A-level physics lessons, but I don't have a transcript handy, sorry ...) Which clearly states that momentum of the system is conserved.

Are you saying it's wrong? Or your 1st year physics lessons were wrong?
What it says is correct and what I say is correct. An in elastic collision not all the energy is converted into kinetic energy. Deformation doesn't have to occur though. A vehicle collision is inelastic as energy is lost as heat and sound. Momentum is conserved of all the items in the system if no external forces apply. Momentum is mass X velocity. If we were to consider a closed system where you took the total momentum of all the molecules including the vibration of heated molecules then momentum is conserved. However, it's really hard to keep that system as a closed system as the heat and sound energy will dissipate away. Therefore considering just the head and the car as the items you are measuring for the momentum, their combined momentum is reduced because of the external force of friction and conversion to heat.
It's still a completely normal Inelastic Collision, as described by the WIki page! There is nothing special happening in the polystyrene of the helmet - that is already covered in the standard description of Inelastic Collisions. It's right there in the first two paras:
An inelastic collision, in contrast to an elastic collision, is a collision in which kinetic energy is not conserved due to the action of internal friction.

In collisions of macroscopic bodies, some kinetic energy is turned into vibrational energy of the atoms, causing a heating effect, and the bodies are deformed.
I don't know why you think helmets create some new special physics that isn't covered by this :-/ ???
I dont think there are special physics at play its all in your link. ! I really think you need to go back to learn some basics. I think yourr arguments about helmets having no effect may have some gounding in confused understanding or momentum and energy. You are disagreeing and then pointing to articles that agree with what I say. I can recommend some basic bitesize physics videos. The BBC do some good ones.

An inelastic impact is when kinetic energy ( formula 1/2mV^2) is lost. The formula for kinetic energy contains mass and velocity. The formular for momentum does too (mV). If kinetic energy is lost then the momentum of the body is reduced. The momentum of the two bodies is less after the collision. They have the same mass but velocity is reduced. The conservation of momentum applies though as the vibrating molecules now have extra momentum thats lost by the helmet velocity. However that does not contribute to injury as its heat.
mattheus
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Re: Crash and helmet

Post by mattheus »

Steady rider wrote: 3 Oct 2022, 4:25pm https://www.researchgate.net/publicatio ... jury_rates
https://www.researchgate.net/publicatio ... _warranted
Cycling UK https://www.cyclinguk.org/campaign/cycl ... s-evidence
https://www.cyclinguk.org/briefing/cycle-helmets
In any case, there are serious doubts about the effectiveness of helmets. They are, and can only be, designed to withstand minor knocks and falls, not serious traffic collisions. Some evidence suggests they may in fact increase the risk of cyclists having falls or collisions in the first place, or suffering neck injuries.
Many thanks!

[email sent, with reference. Likely to fall on stoney ground initially, but this is a war I am happy to fight as long as I can ride ... ]
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 4 Oct 2022, 12:11am I really think you need to go back to learn some basics. I think yourr arguments about helmets having no effect may have some gounding in confused understanding or momentum and energy.
Isn't it queer! The more you write on this subject, the more I believe the exact opposite to be the case.
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pjclinch
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Re: Crash and helmet

Post by pjclinch »

dmrcycle wrote: 4 Oct 2022, 12:11am
I dont think there are special physics at play its all in your link. ! I really think you need to go back to learn some basics. I think yourr arguments about helmets having no effect may have some gounding in confused understanding or momentum and energy.
My physics background is geophysics rather than pure physics, but we all use the same laws. What one does find in applied work like geophysics is that the reality of the Real World is so complex that the limited information we can glean or use is insufficient to reliably predict outcomes, and that's just with rocks where we don't have to factor in psychology.

If we say "helmets have no effect" we do need to qualify that. It's obvious that a helmet will affect interactions where one is impacted, but that's not necessarily the same thing as, say, a tangible public health benefit across a population from wearing helmets. While going off at abstruse tangents about how much people understand mechanics may be fun in itself, it doesn't actually tell anyone whether a notional rider at the start of a notional journey is at greater, lower or about the same risk if they wear a helmet compared to not wearing a helmet.

Mechanics-led approaches can't realistically model all the possible physical interactions in all possible bike crashes, and even if they could they wouldn't address psychological effects. So while they are of some interest they are also seriously limited, and that's why epidemiology looks at population statistics and not the predictions of engineers. Epidemiology has its own issues of course, with e.g. statistical approaches not necessarily applying to individuals within a population, and we need to consider those when blithely proclaiming something like "helmets have no effect" when there are clearly individual instances where they do (and they can be negative as well as positive).

Pete.
Often seen riding a bike around Dundee...
mattheus
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Re: Crash and helmet

Post by mattheus »

pjclinch wrote: 4 Oct 2022, 11:57am If we say "helmets have no effect" we do need to qualify that.
Who is this "we" ??

[Otherwise an excellent post!]
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pjclinch
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Re: Crash and helmet

Post by pjclinch »

mattheus wrote: 4 Oct 2022, 12:16pm
pjclinch wrote: 4 Oct 2022, 11:57am If we say "helmets have no effect" we do need to qualify that.
Who is this "we" ??

[Otherwise an excellent post!]
As it's a notional example, anyone you want it to be.
Often seen riding a bike around Dundee...
mattheus
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Re: Crash and helmet

Post by mattheus »

pjclinch wrote: 4 Oct 2022, 12:44pm
mattheus wrote: 4 Oct 2022, 12:16pm
pjclinch wrote: 4 Oct 2022, 11:57am If we say "helmets have no effect" we do need to qualify that.
Who is this "we" ??

[Otherwise an excellent post!]
As it's a notional example, anyone you want it to be.
So The Straw Man then? :)
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