Crash and helmet

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

Post by jois »

Jdsk wrote: 28 Sep 2022, 1:51am
jois 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
A Quantum Theory of the Scattering of X-rays by Light Elements
Arthur H. Compton
Phys. Rev. 21, 483 – Published 1 May 1923

https://journals.aps.org/pr/abstract/10 ... Rev.21.483

Jonathan
That's what I read.. it's the momentum of electrons, says so in the first paragraph. Is there a momentum of photon bit you can refer me to ?

Thanks Joseph

Electrons get mass from the higgs field so they are not at all the same
Jdsk
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Re: Crash and helmet

Post by Jdsk »

Here's the simplest explanation that I know:
https://people.physics.anu.edu.au/~cms1 ... 1923a.html

And one up from that:
https://nigerianscholars.com/tutorials/ ... -momentum/

And here's the history of the debate and how Compton helped to resolve it:
https://en.wikipedia.org/wiki/Photon#Hi ... evelopment

Jonathan
jois
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Re: Crash and helmet

Post by jois »

Jdsk wrote: 28 Sep 2022, 2:05am Here's the simplest explanation that I know:
https://people.physics.anu.edu.au/~cms1 ... 1923a.html

And one up from that:
https://nigerianscholars.com/tutorials/ ... -momentum/

And here's the history of the debate and how Compton helped to resolve it:
https://en.wikipedia.org/wiki/Photon#Hi ... evelopment

Jonathan
I'm not going on a wild goose chase reading endless links

Where in the first link does it say he measured the momentum of the photon. If it didn't please confirm the fact it didn't.

Then give me your link that actually says he did

Thanks Joseph
Jdsk
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Re: Crash and helmet

Post by Jdsk »

Those linked articles give the simplest explanation that I know of how his experiments demonstrated the momentum of the photon.

In his own words in the abstract of the Phys Rev article:
"This remarkable agreement between experiment and theory indicates clearly that scattering is a quantum phenomenon and can be explained without introducing any new hypothesis as to the size of the electron or any new constants; also that a radiation quantum carries with it momentum as well as energy."
My emboldening.

Jonathan
tim-b
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Re: Crash and helmet

Post by tim-b »

When a photon goes on holiday does it have luggage, or is it travelling light?
~~~~¯\(ツ)/¯~~~~
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pjclinch
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Re: Crash and helmet

Post by pjclinch »

Jules59 wrote: 27 Sep 2022, 3:12pm
Steady rider wrote: 26 Sep 2022, 8:23pm how much force to crack a helmet?
how much force to crack a skull?
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 resulted
I would say a readier inference from Steady rider's rhetorical question is that those people who equate a cracked helmet with a certain fracture of their skull had they not been wearing the helmet (and there really appears to be no shortage of such people) are extrapolating much further than they really ought to.

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

Post by pjclinch »

dmrcycle wrote: 27 Sep 2022, 11:01pm 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.
You are very limited in your application of physics to those situations where the outcome is better with a helmet on.

How about those occasions where wearing a helmet makes things worse, such as providing a bigger lever to twist things further, or making a head heavier or bigger such that it might make the difference between having an impact and having no impact?

If I only think of simple scenarios like banging a protected part of my head against a hard surface then certainly a helmet will help, but the reality of the world isn't that simple. And modelling human safety using nothing other than energy absorption in a direct impact is ridiculously simplistic.

Of course one can argue the toss about which of these scenarios is more or less likely, but it turns out the best way to decide whether people would be safer is with epidemiology, not mechanics. And epidemiology is really none too sure.

Meanwhile, given that the biggest causes of serious head trauma are being in a car crash and trips and falls, and given "you cannot argue with the physics that wearing a helmet reduces injury", I take it you always wear a crash helmet in a car and when using the stairs? If not, why not? Until you can demonstrate clearly that cyclists in any situation are clearly safer wearing helmets than not, and back that up with reality in a real cycling population, it's not much use looking at simple stuff and saying it's some sort of proof.

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

Post by jois »

Jdsk wrote: 28 Sep 2022, 2:18am Those linked articles give the simplest explanation that I know of how his experiments demonstrated the momentum of the photon.

In his own words in the abstract of the Phys Rev article:
"This remarkable agreement between experiment and theory indicates clearly that scattering is a quantum phenomenon and can be explained without introducing any new hypothesis as to the size of the electron or any new constants; also that a radiation quantum carries with it momentum as well as energy."
My emboldening.

Jonathan
If it's a fact as you claim. Just post a link to a statement of fact that he measure the momentum of a photon. Not an electron.

As you have now had 4 goes at this and come up empty I'm begining to suspect you've realised your wrong, some what late and are obscuring the issue
Jdsk
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Re: Crash and helmet

Post by Jdsk »

jois wrote: 28 Sep 2022, 9:28am
Jdsk wrote: 28 Sep 2022, 2:18am Those linked articles give the simplest explanation that I know of how his experiments demonstrated the momentum of the photon.

In his own words in the abstract of the Phys Rev article:
"This remarkable agreement between experiment and theory indicates clearly that scattering is a quantum phenomenon and can be explained without introducing any new hypothesis as to the size of the electron or any new constants; also that a radiation quantum carries with it momentum as well as energy."
My emboldening.
If it's a fact as you claim. Just post a link to a statement of fact that he measure the momentum of a photon. Not an electron.
...
You've just quoted that fact in the emboldened words taken from the 1923 paper, already linked above.

What Compton describes there as "a radiation quantum" is what we now call a photon, not an electron.

(That article was published in 1923. The word photon wasn't used for this particle until after Lewis's letter of 1926 and Compton's own adoption of it in 1928.)

Jonathan
mattheus
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Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 27 Sep 2022, 11:01pm 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.
Let's simpify things (for brevity as much as anything else):

We'll use your 3 things. Let's be precise - imagine the "Impact surface" is something like your asteroids, but simpler: a smooth 2000kg adamantium block, with nothing holding it in place. Simple so far, yes?

They collide. Now:

Q1 (5 marks): Would the total momentum of these 3 objects always be conserved? [It's a closed system, as Newton referred to].

Q2 (10 marks) - Would one (or more) of the objects deforming change this conservation?
jois
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Re: Crash and helmet

Post by jois »

Jdsk wrote: 28 Sep 2022, 9:39am
jois wrote: 28 Sep 2022, 9:28am
Jdsk wrote: 28 Sep 2022, 2:18am Those linked articles give the simplest explanation that I know of how his experiments demonstrated the momentum of the photon.

In his own words in the abstract of the Phys Rev article:
"This remarkable agreement between experiment and theory indicates clearly that scattering is a quantum phenomenon and can be explained without introducing any new hypothesis as to the size of the electron or any new constants; also that a radiation quantum carries with it momentum as well as energy."
My emboldening.
If it's a fact as you claim. Just post a link to a statement of fact that he measure the momentum of a photon. Not an electron.
...
You've just quoted that fact in the emboldened words taken from the 1923 paper, already linked above.

What Compton describes there as "a radiation quantum" is what we now call a photon, not an electron.

(That article was published in 1923. The word photon wasn't used for this particle until after Lewis's letter of 1926 and Compton's own adoption of it in 1928.)

Jonathan
It doesn't explicitly say he measured the momentum of radiation quantum though does it. It's just a tagged on bit under " also" it doesn't say also what

I'm not playing duck and hide with you.

Last chance to Post an explicit statement to back up your claim or I'm just going to conclude you cant. And move on
Jdsk
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Re: Crash and helmet

Post by Jdsk »

Direct measurement of radiation pressure only became possible later. Compton measured the momentum of the photon by a completely different method.

And it was a major part of his contribution to physics for which he received the Nobel Prize. The key paper is linked to and quoted from above.

Jonathan
jois
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Re: Crash and helmet

Post by jois »

Jdsk wrote: 28 Sep 2022, 10:23am Direct measurement of radiation pressure only became possible later. Compton measured the momentum of the photon by a completely different method.

And it was a major part of his contribution to physics for which he received the Nobel Prize. The key paper is linked to and quoted from above.

Jonathan
Ok il take that as a concession. He didn't "measure " it now by your own admission

Can you provided an explicit link to anyone measuring it ever ?
Jdsk
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Re: Crash and helmet

Post by Jdsk »

jois wrote: 28 Sep 2022, 10:30am
Jdsk wrote: 28 Sep 2022, 10:23am Direct measurement of radiation pressure only became possible later. Compton measured the momentum of the photon by a completely different method.

And it was a major part of his contribution to physics for which he received the Nobel Prize. The key paper is linked to and quoted from above.
Ok il take that as a concession. He didn't "measure " it now by your own admission

Can you provided an explicit link to anyone measuring it ever ?
He didn't measure radiation pressure. He did measure the momentum of the photon by a different method. The experimental details and the results are in the 1923 paper.

I suggest choosing a physics textbook and seeing what that says.

Jonathan
jois
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Re: Crash and helmet

Post by jois »

Jdsk wrote: 28 Sep 2022, 10:37am
jois wrote: 28 Sep 2022, 10:30am
Jdsk wrote: 28 Sep 2022, 10:23am Direct measurement of radiation pressure only became possible later. Compton measured the momentum of the photon by a completely different method.

And it was a major part of his contribution to physics for which he received the Nobel Prize. The key paper is linked to and quoted from above.
Ok il take that as a concession. He didn't "measure " it now by your own admission

Can you provided an explicit link to anyone measuring it ever ?
He didn't measure radiation pressure. He did measure the momentum of the photon by a different method. The experimental details and the results are in the 1923 paper.

I suggest choosing a physics textbook and seeing what that says.

Jonathan
If he didn't measure it he calculated it. There is no other options. I've not disputed at any point it can be calculated
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