Crash and helmet

[dmrcycle]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Jonathan

Yep. The issue seems to around the understanding of closed vs open systems.

[Jdsk]

Thankyou

Jonathan

[Chris Jeggo]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Jonathan

Yes.

Shall we now move on to discussing the conservation of mass-energy in relativistic systems?

Just joking!

There was a young lady named Bright,
Who cycled much faster than light;
She started one day
In a relative way,
And returned on the previous night.
- misquotation of a well known limerick.

[Jdsk]

Thankyou

Jonathan

[mattheus]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Jonathan

Terribly fuzzy wording, can't agree with that list of statements.
1) Momentum is always conserved. As stated a few posts ago, if you think you see some has gone missing, you have failed to consider one or more objects.
2) classical and open are not two different situations. You can add energy to an object or system, classical physics describes that very well indeed!

[Bike]

I crashed last Friday - wheels went out from under me on a tight/wet curve - and banged up my knee, arm and thigh (all mendable), but for the first time crashing - I have done it a few times (mostly by dogs) my head hit the pavement hard and set my head pounding.

The reason I am posting this thread is that my helmet broke along the side receiving the blow. If I had not been wearing the helmet, I would not be here today to write this. I am not going to preach, but I cannot imagine for one second riding without a helmet and I wouldn't allow anyone to ride with me who did not wear one.

I broke a Kask helmet in December last year; it compressed and cracked. I went down hard at over 35mph after my chain snapped. Internal injuries mostly in my case but fully recovered 2 months later. I was thankful for the helmet; I'm convinced it prevented further injury.

I recall my first racing crash though back in the 1980's; we only had those foam strip helmets! Fat lot of good they did! I had a serious bump and grazing from an accident back then. Over the years I've had a fair few accidents albeit mostly mountain biking (racing) rather than road.

I race and train with a helmet doing circa 300mi per week at 18-25mph averages and 50-60mph descents. It is no fuss at all for me and it's my own personal risk-assessment based upon my own experiences as a cyclist for many decades but I've yet to ride with anyone who doesn't wear one when on Club rides, training, racing (compulsory in this case).

I can continue to train with road-rash but a bump to the head can force time off the bike so I'd prefer to mitigate that risk.

I didn't wear one when commuting while living in Amsterdam for 3 years though; for typical slow city commutes/tourist rides I generally don't actually. My approach is that for cycling as a sport, I use a helmet, but otherwise not always.

[Chris Jeggo]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Jonathan

Terribly fuzzy wording, can't agree with that list of statements.
1) Momentum is always conserved. As stated a few posts ago, if you think you see some has gone missing, you have failed to consider one or more objects.
2) classical and open are not two different situations. You can add energy to an object or system, classical physics describes that very well indeed!

OK, so we haven't precisely defined 'closed' and 'open', nor 'classical', but this thread is about helmets. The subsidiary disagreement about momentum has resolved itself.

[mattheus]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Jonathan

Terribly fuzzy wording, can't agree with that list of statements.
1) Momentum is always conserved. As stated a few posts ago, if you think you see some has gone missing, you have failed to consider one or more objects.
2) classical and open are not two different situations. You can add energy to an object or system, classical physics describes that very well indeed!

OK, so we haven't precisely defined 'closed' and 'open', nor 'classical', but this thread is about helmets. The subsidiary disagreement about momentum has resolved itself.

Yes, the theory has been clarified - but we need clairty of how it applies to helmets (otherwise we may as well just ignore all the physics!). May I ask my own questions?

If we're looking at a system of 3 things: head, helmet, hard/heavy object [the latter could be a car, or a wall, or the ground];
- is that a closed system?
- if not, what other elements are involved?
- have I correctly defined the system in question?

[Jdsk]

And we all agree that:

1 Momentum is conserved in closed systems, but not necessarily in open systems.

2 Energy is conserved in closed classical systems, but not necessarily in open systems.

3 Newton's Laws of Motion apply in classical systems.

?

Terribly fuzzy wording, can't agree with that list of statements.
1) Momentum is always conserved. As stated a few posts ago, if you think you see some has gone missing, you have failed to consider one or more objects.
2) classical and open are not two different situations. You can add energy to an object or system, classical physics describes that very well indeed!

OK, so we haven't precisely defined 'closed' and 'open', nor 'classical', but this thread is about helmets. The subsidiary disagreement about momentum has resolved itself.

That sounds right to me.

But if a fifth party would like to know why I chose those precise words please just ask.

Jonathan

[Jdsk]

...
If we're looking at a system of 3 things: head, helmet, hard/heavy object [the latter could be a car, or a wall, or the ground];
- is that a closed system?
- if not, what other elements are involved?
- have I correctly defined the system in question?

Limiting this to major interactions that might affect the outcome (there are many other minor interactions):

Do you really want to consider an unconnected head? In the real-world settings of interest where a helmet might affect forces on the brain then the head will be connected to the neck and the neck to the body. But in a testbed situation there might not be any connection at the neck.

A hard/heavy object such as a wall might have external interactions that could affect the energy transfer in a collision.

Even with an unconnected head it isn't a closed system because there is a gravitational field. Any changes in vertical position of any of the components which have mass will have associated changes in potential energy.

Jonathan

[mattheus]

OK, but you have previously made an incorrect statement (my emboldening):

Agreed. But in reality there can be a substantial amount of momentum transferred to vibration of molecules as heat from friction. That’s why you can drop two things of the same mass say one tennis ball and one ball of plasticine and they will bounce different heights. The masses at the start are the same and their velocities are the same (assuming similar air resistance) but after the collision the masss are the same but their velocities are different.

When a lump of plasticine falls to the ground it sticks. Its momentum is transferred to the ground, not to its own atoms. Yes, kinetic energy is lost, and is converted to thermal energy, but the sum of the momenta of all the nuclei and electrons in a solid at rest is zero because momentum is a vector and the summation has to be done vectorially.

Your statement should read "there can be a substantial amount of kinetic energy transferred to vibration of molecules as heat from friction".

The essence of protection is to absorb energy while limiting peak accelerations.

Yes.
This is, IMO, the main problem with Mr dmr's arguments; he has repeatedly stated that "heat from friction" will affect the momentum of the system. This is patently untrue, as your example of the plasticine hitting the ground illustrates perfectly.

He then goes on to suggest - if I may paraphrase his posts - that energy used in cracking his helmet will also "use up momentum" in some way, as a sort of axiom of how cracked helmets must be protecting heads and brains.

I therefore posit - and this is purely my own theory, I must stress this - that Mr dmr either doesn't understand the interactions of momentum and energy, or he is inventing a novel new version of those laws to support his ideas about helmets. The former seems unlikely, if he is indeed a physics graduate with published papers and some specialism in asteroid collisions.

[mattheus]

Even with an unconnected head it isn't a closed system because there is a gravitational field. Any changes in vertical position of any of the components which have mass will have associated changes in potential energy.

Jonathan

You can easily eliminate those effects from the modelling by considering impacts where the head starts and finishes with no vertical velocity components.

Come on Jonathan, that is basic stuff! I do hope you're not just proposing these things to be mischievous?

[Jdsk]

You asked a question about the physics. You received an answer.

Jonathan

[mattheus]

You received an answer.

Yes I did ...

[dmrcycle]

All along you are confusing the conservation of momentum as a theory and momentum of one or two objects in a closed system. After a collision the momentum of the object is less. We are talking about the reduction in the speed/momentum of a head. After a collision you do agree that is loses momentum? It slows down and eventually stops! It’s momentum is then zero. Simple.

Then you chose expand the situation and to argue that I have invented a new branch of physics and discredit me. It’s a perfectly simple objects slow down their momentum gets less. When the system is only considering two objects they don’t conserve their momentum. It’s transferred out of the system we are talking about.