Did someone say fractures were harmless, and actually quite invigorating? Hunt them down, now!
Crash and helmet
Re: Crash and helmet
Last edited by mattheus on 27 Sep 2022, 12:07pm, edited 1 time in total.
Re: Crash and helmet
I've had that discussion before. The maths of general relativity definetly say that photons have moment. I'm less sure that this has ever been measured and could therefore be an artifact of the maths rather than a fact. It is rather about making testable predictions than just taking it on faithStevek76 wrote: ↑26 Sep 2022, 12:01pmWhat about chemical energy? You seem to be conflating energy and momentum considerably here they are different properties, for a start, energy is a scalar, not a vector. Both are, however, always conserved, regardless of what the energy does regarding getting converted between its various forms, including mass.
Well... not entirely. Classically yes it is, once relativity gets involved then not so much. Hence photons, which are massless, still having some momentum.Momentum is simply mass of an object multiplied by its velocity.
A skull fracture is hardly insignificant I'd suggest! And the resultant swelling can have its own complications. In reality both the energy and momentum transfers are going to be relevant in any collision, a reduction in energy (even if it's a small one from just cracking the plastic & EPS) is surely not to be sniffed at and a helmet will also help spread the forces across a wider area of the skull, will further reduce the chances of any fracture.
For the brain slopping about issue then no, a cracked helmet with no signs of crushing will have done nothing to help that, but I think people can be OTT when suggesting a cracked helmet has done 'nothing' even if their protective capabilities can be extremely overstated at times.
The issue comes when you delve in to quantum mechanics and conclude that mass is illusionary. As such photons are no more or less massless than a protons or a neutron or indeed anything made of them
Re: Crash and helmet
You dont need a skull fracture to have an extremely serious brain injury. Diffuse axonal injury can be devastating - saw that most frequently in "survivors" car crashes - little obvious injury but permanent damage often resultedSteady rider wrote: ↑26 Sep 2022, 8:23pm how much force to crack a helmet?
how much force to crack a skull?
Re: Crash and helmet
<googles ... >Jules59 wrote: ↑27 Sep 2022, 3:12pmYou dont need a skull fracture to have an extremely serious brain injury. Diffuse axonal injury can be devastating - saw that most frequently in "survivors" car crashes - little obvious injury but permanent damage often resultedSteady rider wrote: ↑26 Sep 2022, 8:23pm how much force to crack a helmet?
how much force to crack a skull?
What is diffuse axonal injury (DAI)? Diffuse axonal injury is the shearing (tearing) of the brain's long connecting nerve fibers (axons) that happens when the brain is injured as it shifts and rotates inside the bony skull. DAI usually causes coma and injury to many different parts of the brain.
Yes! Exactly! Skull fractures are a tiny* part of the risks in a head-impact.
[my bold]
*but I didn't say they were trivial injuries. Or fun. Or not worth worrying about. Or "insignificant". Or ... or ...
-
- Posts: 2749
- Joined: 4 Jan 2009, 4:31pm
Re: Crash and helmet
Trying to answer the questions, it will depend on several factors for a cracked skull, such as age and impact location.Steady rider wrote: ↑26 Sep 2022, 8:23pm
how much force to crack a helmet?
how much force to crack a skull?
https://pubmed.ncbi.nlm.nih.gov/1238919/
For helmets, one opinion,The results of our experiments are as follows: 1) The forces necessary to produce the first fracture (seperations of the parietotemporal suture) are between 135 kg and 385 kg. (average; 258 kg) 2) The forces necessary to produce the second fractures (true fractures) are between 195 kg and 550 kg. (average; 356.3 kg) 3)
https://www.google.com/search?q=force+t ... e&ie=UTF-8
It looks like helmets may crack at about 50-100kg and skulls at about 200-500kg.Evaluation of the helmet-only compression data showed initial cracking that occurred in the range of 100– 200 lbf. The average cracking force was found to be 140 lbf
All this probably shows is that helmets will crack at lower impact forces than that required to crack an adult skull. Other people may find more accurate results.
Re: Crash and helmet
The idea that a helmet shattering with no deformation has no effect on impact/damage to the head/brain is absurd. The idea that conservation of momentum means the head retains all the impact unless there is some deceleration force being the physics behind this is equally ridiculous. Energy transfer is the key here. Conservation of momentum (as Newton himself stated) is only true in a closed system with no external forces. Friction losses in deforming material being an external force, in this case that dissipates kinetic energy and therefore momentum. Yes considering the universe as a whole momentum is conserved but we are only considering three things, the helmet, the impact surface and the head. External forces are at work here. A bike helmet is an open structure, it's designed to break and crack to dissipate energy and actually deform. Fracture is the next step beyond deformation. If you don't believe me I only a BSc in Applied Physics perhaps listen to a professor of biomedical engineering in a video from the Royal Institution who is more qualified than me.
Watch this. https://www.youtube.com/watch?v=SgH5ZwZ1OdI
No matter what you argue about the psychology of helmet wearing, human behaviour and risk arguments you cannot argue with the physics that wearing a helmet reduces injury. It's like arguing black is white. Have a debate about society and behaviour but not the laws of physics. Thats not up for debate.
Watch this. https://www.youtube.com/watch?v=SgH5ZwZ1OdI
No matter what you argue about the psychology of helmet wearing, human behaviour and risk arguments you cannot argue with the physics that wearing a helmet reduces injury. It's like arguing black is white. Have a debate about society and behaviour but not the laws of physics. Thats not up for debate.
Last edited by dmrcycle on 27 Sep 2022, 11:07pm, edited 1 time in total.
Re: Crash and helmet
1 I think that you mean special relativity rather than general relativity.jois wrote: ↑27 Sep 2022, 12:23pm ...
The maths of general relativity definetly say that photons have moment. I'm less sure that this has ever been measured and could therefore be an artifact of the maths rather than a fact. It is rather about making testable predictions than just taking it on faith
The issue comes when you delve in to quantum mechanics and conclude that mass is illusionary. As such photons are no more or less massless than a protons or a neutron or indeed anything made of them
2 I think that you mean momentum rather than moment.
3 The momentum of the photon was measured just about a century ago. And Compton's Nobel Prize followed soon afterwards.
4 Photons are more "massless" than protons or neutrons because protons and neutrons have a non-zero rest mass and photons don't. Because they're travelling at the speed of light or they wouldn't be photons.
Jonathan
PS: And of course quantum mechanics isn't needed in the analysis of the effects of blows to the head. And for that analysis mass is very real.
Re: Crash and helmet
If you are going to be my spell checker you will have a long and thank less jobJdsk wrote: ↑28 Sep 2022, 1:06am1 I think that you mean special relativity rather than general relativity.jois wrote: ↑27 Sep 2022, 12:23pm ...
The maths of general relativity definetly say that photons have moment. I'm less sure that this has ever been measured and could therefore be an artifact of the maths rather than a fact. It is rather about making testable predictions than just taking it on faith
The issue comes when you delve in to quantum mechanics and conclude that mass is illusionary. As such photons are no more or less massless than a protons or a neutron or indeed anything made of them
2 I think that you mean momentum rather than moment.
3 The momentum of the photon was measured just about a century ago. And Compton's Nobel Prize followed soon afterwards.
4 Photons are more "massless" than protons or neutrons because protons and neutrons have a non-zero rest mass and photons don't. Because they're travelling at the speed of light or they wouldn't be photons.
Jonathan
PS: And of course quantum mechanics isn't needed in the analysis of the effects of blows to the head. And for that analysis mass is very real.
I'm quite prepared to be wrong.
But no crompton didn't get his Nobel prize for measuring the momentum of the photon. Something to do with x-rays and electrons and the Crompton effect.
He also did a lot of calculations about photons but no actual measurements of momentum
I've also found a scientific team that says they may be finally able to measure the momentum, that was 2018 no mention that they have as yet.
If you have a link to cromptons achievement measuring momentum of photons IL be pleased to see it
Thanks Joseph
NB something with no mass can't be more mass less than something else with no mass. Just saying
Last edited by jois on 28 Sep 2022, 1:35am, edited 1 time in total.
Re: Crash and helmet
What's the "something else with no mass" to which that refers?
Thanks
Jonathan
Re: Crash and helmet
Re: Crash and helmet
Re: Crash and helmet
They don't have mass at all according to a professor at fermilab, or specifically the mass is an illusion
I can link his vid " why mass is an illusion "if you want to take it up with him
In short nothing they contain ie quarks have mass so..... Where does the mass actually come from ?
Last edited by jois on 28 Sep 2022, 1:52am, edited 1 time in total.
Re: Crash and helmet
A Quantum Theory of the Scattering of X-rays by Light Elementsjois wrote: ↑28 Sep 2022, 1:32amBut no crompton didn't get his Nobel prize for measuring the momentum of the photon. Something to do with x-rays and electrons and the Crompton effect.
He also did a lot of calculations about photons but no actual measurements of momentum
...
If you have a link to cromptons achievement measuring momentum of photons IL be pleased to see it
Arthur H. Compton
Phys. Rev. 21, 483 – Published 1 May 1923
https://journals.aps.org/pr/abstract/10 ... Rev.21.483
Jonathan