Crash and helmet

For all discussions about this "lively" subject. All topics that are substantially about helmet usage will be moved here.
Jdsk
Posts: 24640
Joined: 5 Mar 2019, 5:42pm

Re: Crash and helmet

Post by Jdsk »

dmrcycle wrote: 23 Sep 2022, 5:17pm
buryman wrote: 23 Sep 2022, 4:19pm Momentum is one of the fundamental conserved quantities no matter what energy conversions are taking place.
No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.

Confusion between conservation of momentum and conservation of energy is what is the issue here. I can appreciate it might have been some time from school level physics but the information can be looked up. In a collision energy is conserved momentum is not unless its a perfect situation (similar to two billiard balls). If there is friction, deformation, heat, sound etc then conservation of momentum does not apply. The momentum (mass x velocity) is not the same after the interaction. Think about it, otherwise we would all be bumping around never stopping transferring 100% of our momentum between things we touch. Nothing slowing down. When energy is absorbed and converted from kinetic energy to heat (aka braking) it cant be conserved as you slow down and have less momentum. This is basic school level stuff.
Please could you each describe the limits of the system that you are considering.

Thanks

Jonathan
buryman
Posts: 105
Joined: 16 Mar 2009, 8:30pm
Location: Bury Lancashire

Re: Crash and helmet

Post by buryman »

Jdsk wrote: 23 Sep 2022, 5:33pm
dmrcycle wrote: 23 Sep 2022, 5:17pm
buryman wrote: 23 Sep 2022, 4:19pm Momentum is one of the fundamental conserved quantities no matter what energy conversions are taking place.
No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.

Confusion between conservation of momentum and conservation of energy is what is the issue here. I can appreciate it might have been some time from school level physics but the information can be looked up. In a collision energy is conserved momentum is not unless its a perfect situation (similar to two billiard balls). If there is friction, deformation, heat, sound etc then conservation of momentum does not apply. The momentum (mass x velocity) is not the same after the interaction. Think about it, otherwise we would all be bumping around never stopping transferring 100% of our momentum between things we touch. Nothing slowing down. When energy is absorbed and converted from kinetic energy to heat (aka braking) it cant be conserved as you slow down and have less momentum. This is basic school level stuff.
Please could you each describe the limits of the system that you are considering.

Thanks

Jonathan
The system is the reference frame which includes all the forces and their interactions
Jdsk
Posts: 24640
Joined: 5 Mar 2019, 5:42pm

Re: Crash and helmet

Post by Jdsk »

buryman wrote: 23 Sep 2022, 6:01pm
Jdsk wrote: 23 Sep 2022, 5:33pm
dmrcycle wrote: 23 Sep 2022, 5:17pm
No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.

Confusion between conservation of momentum and conservation of energy is what is the issue here. I can appreciate it might have been some time from school level physics but the information can be looked up. In a collision energy is conserved momentum is not unless its a perfect situation (similar to two billiard balls). If there is friction, deformation, heat, sound etc then conservation of momentum does not apply. The momentum (mass x velocity) is not the same after the interaction. Think about it, otherwise we would all be bumping around never stopping transferring 100% of our momentum between things we touch. Nothing slowing down. When energy is absorbed and converted from kinetic energy to heat (aka braking) it cant be conserved as you slow down and have less momentum. This is basic school level stuff.
Please could you each describe the limits of the system that you are considering.
The system is the reference frame which includes all the forces and their interactions
If that's a closed system then momentum is conserved.

Billiard balls on a table are not a closed system because of their interactions with the surface, cushions, air etc, as above. So the momentum of the balls alone does not need to be conserved. The momentum of the whole system does.

Jonathan
buryman
Posts: 105
Joined: 16 Mar 2009, 8:30pm
Location: Bury Lancashire

Re: Crash and helmet

Post by buryman »

Jdsk wrote: 23 Sep 2022, 6:09pm
buryman wrote: 23 Sep 2022, 6:01pm
Jdsk wrote: 23 Sep 2022, 5:33pm
Please could you each describe the limits of the system that you are considering.
The system is the reference frame which includes all the forces and their interactions
If that's a closed system then momentum is conserved.

Billiard balls on a table are not a closed system because of their interactions with the surface, cushions, air etc, as above. So the momentum of the balls alone does not need to be conserved. The momentum of the whole system does.

Jonathan
For us ultimately the system is the Earth if we assume no interaction with anything o/s our planet.
Stevek76
Posts: 2086
Joined: 28 Jul 2015, 11:23am

Re: Crash and helmet

Post by Stevek76 »

dmrcycle wrote: 23 Sep 2022, 5:17pm
No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.
Not applicable within that limited system being considered, that doesn't mean it doesn't happen altogether. On a universal level it is fundamentally conserved, no exceptions (short of some ground breaking new physics discovery). When you apply the brakes on your bike, most of the difference ends up in the planet. Obviously the planet is vastly more massive than a human so its velocity change is negligible. Similarly loses to heat/sound etc are still transfers of momentum to molecules in the air etc. Momentum is a vector of course and it is only the net momentum that is conserved, the sum of absolute momentum can vary wildly. Two squishy objects of equal mass colliding with equal speed such that they both squish and don't bounce off does not violate conservation of momentum since the momentum started at 0 and ended at 0 whilst the absolute momentum went from some to (nearly) none.
dmrcycle wrote: 23 Sep 2022, 2:24pm But the important factor for me is as go mountain biking as well as road cycling and I don't want to be more careful, I want to protect my head and a helmet is the best way to do so.
I know this is basically an anecdote but this is a lovely example of very conscious risk compensation that some academics seem to like to pretend doesn't exist as a fundamental part of human psychology. Not in anyway a criticism, I do exactly the same thing MTBing, wearing protective gear to lend myself confidence. However I'd not say use of a helmet here is the best way to protect one's head. The best way is not to engage in the risky activity in the first place! We choose not to because it's fun and then use protective gear as partial mitigation.
Olivier!! :roll:

Finland's law is legally unenforceable as the law does not provide for any fine etc so really isn't a useful reference for the impact of helmet laws on cycling rates. Australia is a very good example and that paper simply doesn't even attempt to address the drop in cycling rates that resulted from their law, most estimates that don't come from the fantasy land of Olivier's stable have found that the drop in cycling is greater than the trip in fatalities and the fatality rate (i.e. doom/distance ratio) of the remaining cyclists has actually got worse. Unfortunately data isn't the best in Australia as, being a very progressively minded nation on active travel matters, they don't really make much effort in terms of producing estimates of how much cycling actually happens.
The contents of this post, unless otherwise stated, are opinions of the author and may actually be complete codswallop
Stevek76
Posts: 2086
Joined: 28 Jul 2015, 11:23am

Re: Crash and helmet

Post by Stevek76 »

buryman wrote: 23 Sep 2022, 6:23pm For us ultimately the system is the Earth if we assume no interaction with anything o/s our planet.
Some of it will spill out due to gravitational interactions :)
The contents of this post, unless otherwise stated, are opinions of the author and may actually be complete codswallop
Jdsk
Posts: 24640
Joined: 5 Mar 2019, 5:42pm

Re: Crash and helmet

Post by Jdsk »

Stevek76 wrote: 23 Sep 2022, 6:27pm
dmrcycle wrote: 23 Sep 2022, 5:17pm No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.
Not applicable within that limited system being considered, that doesn't mean it doesn't happen altogether. On a universal level it is fundamentally conserved, no exceptions (short of some ground breaking new physics discovery). When you apply the brakes on your bike, most of the difference ends up in the planet. Obviously the planet is vastly more massive than a human so its velocity change is negligible. Similarly loses to heat/sound etc are still transfers of momentum to molecules in the air etc.
...
Yes.

Jonathan
dmrcycle
Posts: 73
Joined: 20 Sep 2022, 12:16am

Re: Crash and helmet

Post by dmrcycle »

Jonathon

I proposed that during a collision between a head and a car that a helmet converts kinetic energy to heat by deformation and shattering of the hemet thereby reducing brain damage. The response was thats not true because momentum must be conserved so the brain gets the same momentum and the only way to avoid injury was to have some kind of large crumple zones and decelerate the head. Perhaps use a big cushion. :roll:
Momentum is not conserved in the real world otherwise nothing would ever slow down. Shattering of a helmet dissipates kinetic energy reducing impact. Energy is conserved, momentum is not. The polystyrene in a helmet shattering and breaking reduces the impact force as it takes away kinetic energy. The physics of this is not about deceleration and conservation of momentum.
dmrcycle
Posts: 73
Joined: 20 Sep 2022, 12:16am

Re: Crash and helmet

Post by dmrcycle »

Jdsk wrote: 23 Sep 2022, 6:31pm
Stevek76 wrote: 23 Sep 2022, 6:27pm
dmrcycle wrote: 23 Sep 2022, 5:17pm No absolutely not the case, thats conservation of energy. Momentum is only conserved if there are no other forces acting. Newton stated that is the case.
See
http://teacher.pas.rochester.edu/PHY_LA ... _Laws.html

Quote
Friction between moving bodies and their surroundings means there is an external force acting on them, so that conservation of momentum is not applicable.
Not applicable within that limited system being considered, that doesn't mean it doesn't happen altogether. On a universal level it is fundamentally conserved, no exceptions (short of some ground breaking new physics discovery). When you apply the brakes on your bike, most of the difference ends up in the planet. Obviously the planet is vastly more massive than a human so its velocity change is negligible. Similarly loses to heat/sound etc are still transfers of momentum to molecules in the air etc.
...
Yes.

Jonathan
No, Its energy thats universally conserved not momentum within the reference field we are looking at. Yes on a molecular level there will be movement of molecules and if you consider the whole universe as the system then its conserved. But what about chemical energy. Charge up a battery with a bike dynamo, energy is conserved not mechanical momentum. Only in a closed system does it apply. There are many cases where momentum is not conserved but never a case with energy. Momentum is simply mass of an object multiplied by its velocity.
mattheus
Posts: 5044
Joined: 29 Dec 2008, 12:57pm
Location: Western Europe

Re: Crash and helmet

Post by mattheus »

dmrcycle wrote: 23 Sep 2022, 8:21pm No, Its energy thats universally conserved not momentum within the reference field we are looking at. Yes on a molecular level there will be movement of molecules and if you consider the whole universe as the system then its conserved. But what about chemical energy. Charge up a battery with a bike dynamo, energy is conserved not mechanical momentum. Only in a closed system does it apply. There are many cases where momentum is not conserved but never a case with energy. Momentum is simply mass of an object multiplied by its velocity.
KINETIC energy is not universally conserved. Not sure how the "K" word got missed out at the beginning of all this, might be my fault, if so, sorry everyone ...

You keep saying "closed system" - but clearly energy is only conserved in a closed system. Just like momentum. You're not adding much there.

Anyway ..... my initial point was that creating some sound energy doesn't help you much if your brain is still going to get a shove around your skull. It's that brain rattling that does all the significant damage - the rest is just cuts, bruises, and sometimes a skull fracture.
(The latter isn't fun, but not a disaster - you will see professional footballers with those natty mask things holding theirs together on the pitch, and pro cyclists often compete with pinned shoulders, splinted fingers etc etc.)
Stevek76
Posts: 2086
Joined: 28 Jul 2015, 11:23am

Re: Crash and helmet

Post by Stevek76 »

dmrcycle wrote: 23 Sep 2022, 8:21pm But what about chemical energy. Charge up a battery with a bike dynamo, energy is conserved not mechanical momentum.
What 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.
Momentum is simply mass of an object multiplied by its velocity.
Well... not entirely. Classically yes it is, once relativity gets involved then not so much. Hence photons, which are massless, still having some momentum.
mattheus wrote: 26 Sep 2022, 8:49am and sometimes a skull fracture.
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 contents of this post, unless otherwise stated, are opinions of the author and may actually be complete codswallop
Steady rider
Posts: 2749
Joined: 4 Jan 2009, 4:31pm

Re: Crash and helmet

Post by Steady rider »

how much force to crack a helmet?
how much force to crack a skull?
Steady rider
Posts: 2749
Joined: 4 Jan 2009, 4:31pm

Re: Crash and helmet

Post by Steady rider »

sorry, posted twice
mattheus
Posts: 5044
Joined: 29 Dec 2008, 12:57pm
Location: Western Europe

Re: Crash and helmet

Post by mattheus »

Stevek76 wrote: 26 Sep 2022, 12:01pm I think people can be OTT when misquoting others just to make themselves seem cleverer.
Yes, I think they can Steve. Well done on joining that bandwagon!
Jdsk
Posts: 24640
Joined: 5 Mar 2019, 5:42pm

Re: Crash and helmet

Post by Jdsk »

Stevek76 wrote: 26 Sep 2022, 12:01pmA 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.
Yes, skull fractures can have very nasty effects even without immediate brain injury.

And yes, we'd be much wiser to discuss any possible protective effects as being graded and any predictions as probabilistic. Absolute statements may be attractive in web forums but it's a messy old world out there, especially when biology is involved.

Jonathan
Post Reply