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 »

Are there physics forums out there where this style of "debate" rages 24x7?

What is their equivalent of the Helmet Ghetto?
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Cugel
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Re: Crash and helmet

Post by Cugel »

mattheus wrote: 28 Sep 2022, 1:44pm Are there physics forums out there where this style of "debate" rages 24x7?

What is their equivalent of the Helmet Ghetto?
I don't know where the ghetto is but I imagine there are several raging debates to be had in it. Some might be .....

Whether looking for theoretically-posited particles is a waste of time or will lead to the single theory of everything; perhaps other fantastical revelations.

Whether the single theory of everything is a meaningful reality in the universe or merely the conceptual wish of humans who like there to be some form of absolute truth providing a complete certainty about everything (the religious branch of physics).

There might be one that could be translated into, "Should we open Pandora's box or not"? But the notters wouldn't really be physicists then, eh?

Cugel
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mattheus
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Re: Crash and helmet

Post by mattheus »

Cugel wrote: 28 Sep 2022, 2:26pm
mattheus wrote: 28 Sep 2022, 1:44pm Are there physics forums out there where this style of "debate" rages 24x7?

What is their equivalent of the Helmet Ghetto?
I don't know where the ghetto is but I imagine there are several raging debates to be had in it. Some might be .....

Whether looking for theoretically-posited particles is a waste of time or will lead to the single theory of everything; perhaps other fantastical revelations.

Whether the single theory of everything is a meaningful reality in the universe or merely the conceptual wish of humans who like there to be some form of absolute truth providing a complete certainty about everything (the religious branch of physics).

There might be one that could be translated into, "Should we open Pandora's box or not"? But the notters wouldn't really be physicists then, eh?

Cugel
I suspect none of those will hold a candle to our Tubeless debates.
tim-b
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Re: Crash and helmet

Post by tim-b »

What is their equivalent of the Helmet Ghetto?
The Mars Beagle 2 lander thread; it discusses putting a probe where the sun doesn't shine :)
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Stevek76
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Re: Crash and helmet

Post by Stevek76 »

Disc brakes surely worse than tubeless? :D

Regarding actual physics, string theory, like quite a bit of deeper theoretical physics is relatively contentious, mostly as it's, by definition, presently unmeasurable.

Probably plenty in the metaphysics side of things (e.g. 'mass (and everything else) is an illusion' sort of stuff) though I've always thought that stuff is mostly best left as whimsical curiosity otherwise it gets very meaning of life that isn't going to get answered very quickly. Tends to make it on to telly fairly well though where it can be taken a little seriously.
The contents of this post, unless otherwise stated, are opinions of the author and may actually be complete codswallop
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 28 Sep 2022, 9:53am
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?
Question 1 - If the collision is elastic with no friction momentum is conserved
Question 2 - Momentum is not conserved of the 3 bodies as an external force (friction from deformation) is present.

You will agree that if the object deforms there is a reduction in the momentum ( or speed as mass is constant here). The kinetic energy is lost and the body slows to some extent.

If there was no loss in momentum when objects collided there would be no loss in speed and we would all be bouncing around at constant speed never slowing or speeding up just hitting each other in a never ending perpetual motion. Duh!
mattheus
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Re: Crash and helmet

Post by mattheus »

I'm gonna be kind, give you a hint, and a chance to slightly adjust your answers (if you so wish):
Note, the question states "TOTAL momentum" of the 3 objects.

I'll pop back later ...
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

mattheus wrote: 30 Sep 2022, 8:18am I'm gonna be kind, give you a hint, and a chance to slightly adjust your answers (if you so wish):
Note, the question states "TOTAL momentum" of the 3 objects.

I'll pop back later ...
Ok I think we are talking at cross purposes. If your talking about total momentum of the system and momentum of the 3 objects thats different. I am talking about heat being lost during the deformation which is very likely. That energy leaves the system of the 3 objects and its no longer isolated and conservation of momentum of the 3 objects is not conserved.
mattheus
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Re: Crash and helmet

Post by mattheus »

Clarification is good, thankyou. (and I agree that in most collisions there will be some deformation and heat).

So let's look at the key statements here:
dmrcycle wrote: 30 Sep 2022, 9:29am If your talking about total momentum of the system and momentum of the 3 objects thats different. I am talking about heat being lost during the deformation which is very likely. That energy leaves the system of the 3 objects and its no longer isolated and conservation of momentum of the 3 objects is not conserved.
So you're saying that total momentum - of a closed, 3 object system - is not a conserved quantity if some heat is generated within that system?

Do we need any more clarification here? There's no rush!
Jdsk
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Re: Crash and helmet

Post by Jdsk »

dmrcycles mentioned heat leaving the system, not heat being generated in the system.

Jonathan
mattheus
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Re: Crash and helmet

Post by mattheus »

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 ...
dmrcycle
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Re: Crash and helmet

Post by dmrcycle »

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.
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Re: Crash and helmet

Post by Vorpal »

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.
Very, very few systems conserve momentum (or anything else) perfectly. If nothing else, there is our good friend, entropy. But the energy dissipated through heat due to deformation must be tiny compared to the other elements involved.
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mattheus
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Re: Crash and helmet

Post by mattheus »

Vorpal wrote: 30 Sep 2022, 1:56pm
Very, very few systems conserve momentum (or anything else) perfectly. If nothing else, there is our good friend, entropy.
If you start talking that like, you can't predict any physical process. That's not really a good use of Physics.

The Scene: Offices of a wealthy government space agency ...

- How fast will this rocket accelerate?
- we have no idea; you can never reduce frictional losses to zero in a real-world system!
- well okay ... so what will the orbital period of our new satellite be? Can you give me a number for that?
- I just told you - it's impossible to calculate!
Vorpal
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Re: Crash and helmet

Post by Vorpal »

mattheus wrote: 30 Sep 2022, 2:37pm
Vorpal wrote: 30 Sep 2022, 1:56pm
Very, very few systems conserve momentum (or anything else) perfectly. If nothing else, there is our good friend, entropy.
If you start talking that like, you can't predict any physical process. That's not really a good use of Physics.

The Scene: Offices of a wealthy government space agency ...

- How fast will this rocket accelerate?
- we have no idea; you can never reduce frictional losses to zero in a real-world system!
- well okay ... so what will the orbital period of our new satellite be? Can you give me a number for that?
- I just told you - it's impossible to calculate!
:lol: I am an engineer. My job is basically applied physics. I predict physical processes by making a bunch of assumptions.

Although I could, in theory, calculate the loss due to heat generated by deformation, I can't see how it could be significant in terms of loss of momentum or reduced energy, so I would normally just state that it is not included (on the assumption that it is so small as to have no influence on the outcome).

If we are speaking generally about the system being closed or open, well there are lots of losses. We can calculate some & ignore some & arrive at an estimate. We might be able to get some confirmation of the estimate through testing, or an alternate calculation method, such as simulation.
“In some ways, it is easier to be a dissident, for then one is without responsibility.”
― Nelson Mandela, Long Walk to Freedom
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