ADUK Archive

Help!

This video shows the crash forces seem to be identical whether a car hits a wall or 2 cars doing the same speed hit head on.

I can't get my head around this. stresseddave or anyone...help!

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Thanks guys - that is a very succinct explanation and has unscrambled my head. I guess it's a common misconception reinforced by the media.

Eugh . . .

But a car travelling at 30 mph and hitting a stationary vehicle (or object which is moveable) is going to be noticeably better off

Of course this also depends on the difference in vehicle mass. I see this a lot in head-on collisions where one vehicle is substantially heavier than the other:

The heavier vehicle takes less of a blow, in direct proportion to it's greater weight, on the expense of the lighter vehicle.

On the other hand, if the heavier vehicle is not as well engineered in terms of secondary safety (which vans and light HGVs often are) it might outweigh their advantage.

I assume, dave, that the impact force would not be quite identical to hitting a brick wall since the other car involved (unlike the brick wall) is crumpling somewhat...

I still think I would rather swerve to crash into something immovable rather than hit an oncoming vehicle, but this comes not only from reducing impact force but rather the amount of casualties.

In my real life I was involved in the examination of collisions between ships, and exactly the same principles hold as those described by Dave. The amount of damage is proportional to the energy of impact.

how does time play a role?
because impact damage is a factor of not just force, but time over which that force is applied...
my first business was R&D / product dev. in the horse safety world and we developed a roll cage that sat inside a rider's jacket to stop them being crushed when a 1/2 tonne horse fell on them from c. 3m the key to success was not just in the design and materials, but in the fact that it slowed down the accident - making it a lower impact accident...

Alasdair

ahh yes...




or I assume:

...a small force applied for a long time produces the same change in momentum—the same impulse—as a larger force applied briefly.

makes sense - but I presume that it does have an affect in accidents - so hitting a wall at 30mph you get the force very quickly applied to you - hitting another car, both at 30mph presumably the crumple zones as well as absorbing energy must slow down the accident...

I am sure it all is obvious to others - just trying to make sense of it!

Alasdair

Here's my amateur way of understanding this....

1. Hit a SOLID and Unmoveable wall and all the energy the car has is bounced back at it.

2. Two cars colliding at the same speed - the energy of each car is taken by the other one. Result is the same in each case.

3. Assuming they are exactly matched in energy then any wall between them wouldn't shift an inch either way..... Nor would sheet of bronco....

I'm sure that this isnt quite scrictly scientific but the modelling works for me

But I've got a feeling there's something else at play in terms of the occupants.

Ian

Just view it as deceleration over a specific distance. That's the cause of the damage and injuries. Whether it's caused by a fixed or moving object, the result is the same - deceleration from speed x to speed y (=0 mph) in distance z (a few feet).

If the crumple zones of the cars are different, or one is moving faster (or more significantly, is much heavier) than the other, then some energy will be transferred from one to the other, and therefore the distance will vary as one is pushed backwards, but your example had two cars (presumably identical) doing the same speed.

HTH

revian wrote: But I've got a feeling there's something else at play in terms of the occupants.

Unfortunately, yes.

Even if your car stops, you won't.
Even if your seatbelt stops your outsides, your insides keep moving forwards.

Why have you started two identical threads? Very confusing ...

Will they both crumple at equal rates if a mod merges them?

Shush, you!