[Petrochemicals]

Because momentum is related to duration where as energy is just a total regardless of variables.

How should we interpret this statement? Is total volume becomes energy? Or total intensity?
Energy stored in a battery is often expressed in kWh. It means that energy is also related to duration.

Joules is also expressed as time dependant. But it is a total. Momentum is a quality.

[Bored chemist]

Because momentum is related to duration where as energy is just a total regardless of variables.

How should we interpret this statement?

I wouldn't try to interpret it.
It's nonsense.

[yor_on]

I not entierly sure Mr cat but it seems the culprit is in this " Why is it that if a car doing 100K crashes into a stationary car, it's a gonner, and yet if I have a head-on with another car doing 50 each, I would expect to walk away, unscathed, in my big car "

It depends on how one read it possibly but are you assuming that there is a difference between those two scenarios, in the kinetic energy experienced by the driver? As if one could assume one of those cars, regardless of which one you choose, as representing something unmovable, as a rock? In which case the ´kinetic energy' experienced then will be 'halved', represented by just one side, no matter which side you choose?

[yor_on]

Heh, missed the second page. It seems that you do? Assume that the the momentum of something isn't there if it is of a speed, equal to what it collides with?

Is that correct?
=

Both cars use earth as their relative frame of no motion. and both cars are moving at a same speed relative earth. You put up a rock in front of any of those cars and the kinetic energy will exist, being the same for both occasions, assuming the cars to be identical clones :) as well as the speed etc.

So that kinetic energy must exist as a momentum for both cars and will transform into kinetic energy at a collision. So it should be the same as if you collide with that rock at a hundred, or crash head-on, doing fifty, into that other car, it too doing fifty although in the opposite direction.

But it is weird, momentum and kinetic energy. You could define something else than earth as your 'stationary point', some patch of space that earth swish by in its 'relative motion' through the universe. At least in relativistic terms. Then vectors and speeds relative that motion will come in and play a role for how that collision will be interpreted etc. But it won't change the kinetic energy involved in the 'system' colliding. So suddenly it's not relative anymore, or is it?

[Bored chemist]

So that kinetic energy must exist as a momentum

That can't make sense; it has the wrong units.

So it should be the same as if you collide with that rock at a hundred, or crash head-on, doing fifty, into that other car, it too doing fifty although in the opposite direction.

No.
See above.
Hitting an oncoming car which is doing 50 is the same "experience" as hitting a wall at 50.

[yor_on]

How can that be BC?
What about the momentum?

[yor_on]

And how do would I get to a kinetic energy without a momentum involved BC?

Hmm, okay, depends on definitions. https://www.sarthaks.com/454279/a-can-kinetic-energy-of-a-system-be-changed-without-changing-its-momentum

[yor_on]

I will need to ponder that one.
=

Not here though. It will take too much place.

[Just thinking]

I will need to ponder that one.

HI Yor-on you see a hard surface like a concrete wall or a 100 ton rock has no give in it so a car hitting it at 50 is the same as two cars colliding with a closing speed of 100 as both cars will share the impact fealing only there Owen deceleration. The two cars combined have double the amount of crumple zone so double the amount of give and double the distance to stop all amounting to halving the damage.

[yor_on]

You're thinking of inelastic collisions relative elastic, if I get you right JT?
But that's not what I got stuck on :)

Practically it may come out that way, but using definitions as kinetic energy and momentum I don't find those two situations equivalent, and that's my headache.

[Bored chemist]

How can that be BC?

It hasn't changed.
https://www.thenakedscientists.com/forum/index.php?topic=83180.msg656048#msg656048

[Just thinking]

You're thinking of inelastic collisions relative elastic,

Maybe if you think of it like this if you roll a rubber ball at a hard wall it will bounce back if you roll two rubber balls at the same speed at each other they will bounce back the same amount even though the closing speed is doubled both balls equally share the collision. The hard wall did not share.

[yor_on]

That one was very nice JT, intuitive. But what I don't find equivalent are the initial conditions. In the first case both kinetic energy and momentum differs from the other.

[alancalverd]

The paradox is due to the fact that the driver is not part of the car, but essentially a floating body travelling at the same speed as the car.

Ignoring crumple time, the damage caused to the driver is due to the fact that he continues to move when the car has stopped. So if he was initially moving at 100 kph, the energy dissipated when he hits the dashboard will be four times the energy at 50 kph. Crumple zones are really only effective if you are tightly belted to the chassis, so you hit the belt as the car deforms, not a hard surface when it has stopped.

[Just thinking]

That one was very nice JT, intuitive. But what I don't find equivalent are the initial conditions. In the first case both kinetic energy and momentum differs from the other.

I think I see where you may be confused say each car is travelling at 50k if we double the speed of the cars the kinetic energy will go up to x4 but in the event of the two cars colliding even when the speed is doubled the kinetic energy is not changed as each car is a separate energy to its self the two cars are as 1x1=1 not 1+1=4 each car has its Owen momentum to deal with and it is not shared as double. The impact stays as one, not 2x and not 4x Energy only goes up for the individual object when the speed is incrested not by combining. However, by combining two objects travelling in the same direction we will increase the energy of both combined.

[Just thinking]

An example of a different kind the car is travelling at 50k a truck with a big solid bumper is travelling at 50k a head on in this scenario is very different for the car as the truck will force the car backwards that will increase the level of energy to the car but not by 4x certainly not equal but possibly the force on the car could be in the order of 3x why not equal because the car is experiencing a force that is past its internal inertia by being reversed in motion only slightly reduced by the bumper of the truck collapsing a small amount. If you were travelling down the road at 50k and a 100 ton concrete block was travelling at 50k and it was a head on impact then yes the force of the collision on the car will be 4x as the concrete block will have no give and the cars direction will be subjected to a force that is equal to that of doubling the speed this is the big difference concrete block has no give and the car is not only stopped but forced in the reversed direction at 50k. This gives the car a total of 100k impact with no shared absorption. This is x4 impact.

[Colin2B]

That one was very nice JT, intuitive. But what I don't find equivalent are the initial conditions. In the first case both kinetic energy and momentum differs from the other.

The ke and momentum are saying the same thing.
If you look at the eg I gave of the person jumping from a building, keeping ke, momentum & speed constant the killer is acceleration/deceleration. Alan & BC have said the same thing in different ways. Accident investigators reckon  that the crash that killed Diana put 70-100g on her body and that tore a major artery from her heart.

KE is easier to see the relationship with speed v but less so with deceleration. However, as BC says ke is the work done accelerating/decelerating an object to v:
F x d = 1/2 mv²
If we have a crumple zone of d that stops the car then the force is proportional to v²

Now look at momentum. Although it is only mv the force is due to the rate of change of momentum, usually expressed as F = ma.
To see relationship between speed and a take an eg of car hitting a child (light so car won’t slow). The child will be accelerated to v in the width of its body and in time t and at 2v the time will be halved. If you look at acceleration units = m/s² so if you halve the time (double the speed) you see the force is square law against speed just as in KE. It’s all saying the same thing.

As KT says, momentum also plays a part if car hits juggernaut head on. Car occupants are accelerated forward by their own momentum, but the juggernaut’s greater momentum pushes the car back increasing the acceleration on the occupants.

Survivability is interesting. Most researchers say the turning point is 43mph and above that the chance of survival drops exponentially. The reason is that the materials used to decrease acceleration- crumple zones, seatbelts, airbags -  have limited ability to absorb energy. If you look at the fireman’s blanket eg I gave, a person from the 10th floor is likely to tear the material.

[Halc]

Like I forgot Empirical

Gotta remember that one, because if a theory doesn't match empirical observations, it's wrong, and if it doesn't make empirical predictions, it isn't a scientific theory.

Should I have put this question in the Tech section!?

No, that section is for 'how do I get my gadget to do X?'. You can talk about repairing the car after the crash in that section, but the crash itself goes here.

[hamdani yusuf]

If you look at the eg I gave of the person jumping from a building, keeping ke, momentum & speed constant the killer is acceleration/deceleration. Alan & BC have said the same thing in different ways. Accident investigators reckon  that the crash that killed Diana put 70-100g on her body and that tore a major artery from her heart.

How the force/acceleration is distributed matters. You can accelerate in 100g free fall while not feeling anything.

In general, objects get deformed when their parts receive non-homogeneous force. A water baloon gets deformed when laid down on the floor or hung to the ceiling because the floor or the string only put force to some parts of the baloon.
In a free falling balloon, every part of it experiences the same gravitational force, hence it doesn't get deformed.

[Colin2B]

How the force/acceleration is distributed matters. You can accelerate in 100g free fall while not feeling anything.
........
In a free falling balloon, every part of it experiences the same gravitational force, hence it doesn't get deformed.

Yes, this the big difference I try explain to people.
In an acelerating car the back of the seat pushes the atoms of the back forward, but the rest of the atoms eg stomach want to go as they where and the back nerves feel a similar sensation to lying on the ground. As you say, freefall and car acceleration are very different - important difference when discussing car crashes!