Car crash physics/mechanics

coylemj

In a head-on collision, the car with the greater kinetic energy (same as momentum) will push the other car backwards. Kinetic energy as already stated is 1/2 * mass * velocity squared. People here are talking about acceleration which has nothing to do with it, all that matters is the actual speed at the moment of impact and the weight (mass) of the vehicle.

Only if both cars have the same KE can you compare it to a single car hitting an immovable object, otherwise one car will push the other backwards and there will be significant differences in deceleration.

Say two cars hit head-on, both travelling at 50 kph but one is bigger than the other. Let's say that a fraction of a second after the impact, the two cars are travelling in the original direction of the bigger car at 20 kph. The bigger car has experienced an almost instantaneous deceleration of 30 kph but the smaller car has gone from +50 to -20 i.e. a deceleration of 70 kph. Leaving out the issue of how much damage has been caused to each car (which depends on how they were built), the occupants of the smaller car in this case will be much more likely to suffer internal organ damage due to the rapid deceleration.

Seatbelts and airbags will protect you from a direct impact with metal and other hard parts of the car, what does people in in a lot of head-on crashes is the damage to internal organs caused by the sudden stop i.e. the rapid deceleration, that's what killed Princess Diana.

Ronnie Beck

Anan1 wrote: »

For the purposes of this discussion the wall is immovable.

'wall', not wall

Ronnie Beck

coylemj wrote: »

all that matters is the actual speed at the moment of impact and the weight (mass) of the vehicle.

Angles, lots of angles involved. Different materials too.

Is this not a discussion about a perfect head on collision of identical cars, neither car swerving or braking. In this case, in theory, the occupants of both cars would experience the exact same decelleration and g-forces on internal organs. m1v1+m2v2before =m2v1+m1v2 after. Simple.

In reality I'd rather be in the slower car. A better chance of avoiding the gobsh;te driving towards you, meanwhile he fly's off the road and hits a tree.

Ronnie Beck

From OP

Car A 30kph. Car B 50kph.

mass 1500kg or m

x = velocity of faster car after collision

30m -50m = 0m + xm
-20m=(0+x)m
-20=x

both car experienced a change of velocity of 30kph.

Car A 30 to 0
Car B 50 to 20 or -50 to -20

Ronnie Beck

Ronnie Beck wrote: »

From OP

Car A 30kph. Car B 50kph.

mass 1500kg or m

x = velocity of slower car after collision

30m -50m = xm + 0m
-20m=(0+x)m
-20=x

both car experienced a change of velocity of 50kph.

Car A 30 to -20
Car B 50 to 0 or -50 to 0

FYP

or 50 to 10 and -30 to 10. How elastic or inelastic is the collision in this situation??? I suppose they would just blend together if there was no angles involved.

coylemj

Ronnie Beck wrote: »

Car A 30 to 0
Car B 50 to 20 or -50 to -20

How can one car go to zero and the other car go to -20? They are going to be embedded in one another a fraction of a second after the impact at which point there will be deceleration on both sides. The dissipated energy will be absorbed by crushing metal and the destruction of various other parts.

In the case you're quoting, if one car goes from 50 to 20, that implies that it continues in the same direction at 20 kph which means the other car goes from +30 to -20 i.e. it will experience a more or less instantaneous deceleration of 50 kph.

If both cars are the same mass, the faster car will push the slower car backwards purely because it has more kinetic energy, there is no way that they will both come to rest at the point of impact unless they both have identical kinetic energy in which case the effect of the collision will be the same as either of them hitting an immovable object.

Ronnie Beck

I was thinking out loud. I just realised at the end that the two cars would merge into one mass.

BrianD3

MrDerp wrote: »

Neither here nor there. I said nothing about damage, I was merely talking about the equivalent speed of impact

The OP asked about the risk of injury. While you seem to be correct about Newton's third law, in the context of this thread, making the statement:

The 'effective speed' (car to wall equivalent) of the crash is the speed of the faster car.

Is very likely to be misinterpreted as relating to injury potential.

BrianD3

Posters are now talking about cars of different masses and the KE. This confuses matters.

Same masses, different speeds:
Car A is travelling at 30
Car A weighs 10

Car B is travelling at -50
Car B weighs 10

((30*10)+(-50*10))20 = -10. That's the speed of the wreckage after impact. Both cars slow down by 40. Car A is pushed backwards but the human body doesn't distinguish between going from -50 to -10 or going from 30 to -10.


Different masses, same speeds:
Car C is travelling At 50
Car C weighs 20

Car D is travelling at -50
Car D weighs 10

((50*20)+(-50*10))/30 = 16.66. That's the speed of the wreckage after impact. Car C slows down by 33.33 whereas car D slows by 66.66 and is pushed backwards. The human body certainly does distinguish between slowing down by 33.33 and slowing down by 66.66.

The above shows why heavier cars are generally safer than light cars but why speeding up before an impact to "increase momentum and KE of your car and plough through the other car" is not an effective road safety strategy

MrDerp

BrianD3 wrote: »

The above shows why heavier cars are generally safer than light cars but why speeding up before an impact to "increase momentum and KE of your car and plough through the other car" is not an effective road safety strategy

Also, consider that (especially in older cars) if a frontal collision is unavoidable, you might be better off hitting square on than having a partial overlap collision

http://abcnews.go.com/blogs/headlines/2012/08/partial-collisions-prove-more-dangerous-in-new-crash-test/

BrianD3

MrDerp wrote: »

Also, consider that (especially in older cars) if a frontal collision is unavoidable, you might be better off hitting square on than having a partial overlap collision

http://abcnews.go.com/blogs/headlines/2012/08/partial-collisions-prove-more-dangerous-in-new-crash-test/

Yes, the small overlap is a very interesting test. There have been a huge number of interesting crash tests and stuides done in recent years. Full overlap, 40% overlap, 25% overlap, rigid barriers, deformable barriers, oblique angles, big car vs small car, old car vs new car - and those are just the frontal tests

In terms of the deceleration felt by the occupants, Mercedes S class vs Smart Fortwo offset test is a good one. The Smart is flung backwards violently and rolls over. It looks like a terrible collision for the Smart occupants but IIRC the injury risk is not that high and crucially, the very strong Smart passenger compartment remains relatively intact.

Obviously at higher speeds the Smart is at a bigger disadvantage. Increase the speed of both cars by 5 mph and the S class occupants may still walk away while the Smart occupants may be dead.

The Smart test shows the rigidity of modern cars. However they are very reliant on the modern restraint systems working correctly. I don't speak German but this article seems to say that in a full head on crash with an immovable object at 50 km/h, a soft Renault 19 without an airbag is safer than a rigid Renault Megane II with non working airbag
http://www.welt.de/motor/article1244415/Warum-ein-altes-Auto-sicherer-ist-als-ein-neues.html

Yet if the Megane hits the R19 in a full head on or especially a partial overlap test, the old car will be far weaker and will likely fold like tinfoil, crushing the occupants. Not much good having a crumple zone and reduced deceleration if your head, legs and chest are in the crumple zone!

BrianD3

While I'm at it - crash compatibility. We've already seen from conservation of momentum that occupants of light cars are more vulnerable than occupants of heavy cars in a two car collision.

As well as that, the light car will probably have a weaker crumple zone. A light car is designed to crash into a wall at 40 mph and keep the passenger compartment intact and keep deceleration for its occupants at a tolerable level. A heavy car is designed to do the same but due to increased KE of a heavy car, it needs a stronger and/or bigger crumple zone.

If you give the small car the crumple zone from the big car it will either have a very long bonnet or a very rigid crumple sone. The former is undesirable due to packaging and if the the latter is done, in a crash with an immovable object, the crumple zone is too strong meaning occupants are subject to high declerations.

If you give the big car the crumple zone off the small car it will be too weak and the passenger compartment may be crushed in a crash with an immovable object.

So, each car has to have a different crumple zone in case they hit an immovable object. But what happens when they hit each other. The heavy car will likely be more aggressive as well as having more momentum. This is reiterated by the quote below (bold emphasis mine) from "Vehicle Structural Crashworthiness with
respect to Compatibility in Collisions
Frei P, Kaeser R, Muser MH, Niederer PF, Walz FH"

This is why EuroNCAP frontal crash test results can't be compared if the cars are significantly different weights and only claim to approximate a crash with an identical car.

However the EuroNCAP side impact result with the moving sled can be compared among weight classes. Also the pole test can be compared as it's unlikely that you will crash sideways into a pole moving towards you, only a stationary pole.

COLLISION MECHANICS
GENERAL - A small, light car will always experience
a higher deceleration level in a real car versus car
collision compared to its heavier opponent, as the deceleration
for each of the colliding cars can be calculated
with a simplified model from the crush load at the interface
of the two cars. Both cars experience the same
crush load vs. time curve with this simplified model.
deceleration = crush load / car mass
Furthermore, this type of frontal collision will
subject the lighter car to a higher total change of velocity.
The velocity change of both cars can be calculated with
the law of conservation of momentum. In real collisions,
the observed Dv lies 5 - 10 % above the theoretical value
due to residual elasticity of the deformed structures.
Dv1 = m2 * ( v1+ v2) / (m1+ m2)
Dv2 = m1 * ( v1+ v2) / (m1+ m2)
As this paper shows, the main reason for the
higher injury risk for the occupants of currently circulating
small cars in frontal collisions is not intrinsic in these two
physical constraints. Instead, it is given by the lack of
compatibility in frontal collisions among cars of different
size and weight.
The restraint system of any car will only be able
to deploy its effectiveness if the passenger compartment
does not collapse in a collision. Intrusion of structural
components into the cabin and deformation of the passenger
compartment must be kept small. An extremely
important parameter for the development of the restraint
system is the mean deceleration level of the car cabin
with its restraint system interface points and surfaces
impacted by the body of the occupant during the
collision.
Current crash test standards still encourage to
design the car front stiffness for moderate cabin deceleration
pulses, allowing the use of conventional restraint
system components. This approach creates car front
stiffnesses that are more or less proportional to the car
mass. A low mass vehicle, designed in such a way, will
be disadvantaged twofold in case of a collision with a
heavier vehicle. Not only will it experience a higher Dv
due to the mass ratio, but its frontal deformation space,
which is relatively soft (according to the reasons mentioned
above), will be crushed before significant deformation
of the heavier car even starts.
In conclusion, the frontal structural stiffness of a
low mass vehicle must be at least equal or slightly higher
than the stiffness of its heavier counterpart [7] [8].