Vehicle mass: What effet does it have in an accident?

[RAPID_BA]

Which car will end up better off? A stationary f150 pick up truck or a fiesta hitting it at 150km/h

Clearly the f150 is double its mass?

[vztrt]

Originally Posted by Jim Goose

Show me a 4star rated FWD?.... Dont know if things have changed, but FWDs are considered under commercial vehicles and are not given a crash rating.

5star 4x4's.
- Prado
- Merc M-class
- Kluger
- Toureg

While not real ones they are in the same class
- Territory
- Santa Fe
- Sorento
- BMW X5
- Volvo XC 90
- Audi Q5

Funny enough the Nissan Patrol is the only one under 4 stars (even the great wall is 4 stars).

[GasoLane]

Originally Posted by prydey

just because your buzz box is 5 star, doesn't make it safer than a 4 star 4wd.

Prydy is on the money. I drive a no star Volvo, but I'll take on any 5 star car

[sudszy]

Originally Posted by tik-4d

, then we can find the unknown variables in the eqaution.

from my calculations, just after the impact the 3ton 4wd is still travelling at 20.85 m/s while the 1ton hatchback is travelling at 13.9 m/s in the opposite direction. during the impact, the 4wd will exert a net result of twice the momentum of the hatchback.

i could be wrong though i'm pretty sure i'm right, don't hold me to it.

Wont hold you to it, but it isnt correct.

if you do the numbers on yours, the smaller vehicle has suffered a great momentum change than the larger vehicle, which isnt in accordance with laws of physics which predicts that the momentum changes should be equal in magnitude and opposite in direction.
Its also impossible to predict the speeds of the vehicles after the collision unless you know the elasticity of the materials involved, the characteristics of the crumple zones of the two vehicles etc.

[jpd80]

In the worst case, a head on between two vehicles travelling at 100 kph will only ever reduce
the forward velocity to 0 kph, this is the maximum deceleration that can be applied to either vehicle,
any remaining momentum is then expressed as either a forward or negative velocity after impact.

[sudszy]

Originally Posted by tik-4d

i clearly state the total force of the impact, as in the total force of the system is equal to a crash at 200 kph as both cars have a crash, both doing 100 kph.

No, apply Newton's 3rd law to the situation.
Consider car A going to the right versus car B going to the left each doing 100km/h

Car A is brought to a halt by a force of magnitude F acting on it in the opposite direction to its travel, this force to the left comes from car B

Similarly car B's motion to the left is arrested by a force F acting to the right, from car A. Equal and opposite forces, or sum of the magnitude of the two forces = 2F.

Consider car A travelling right into wall at 100km/h
Car A experiences a force to the left of force F to arrrests its motion, the change in speed is exactly the same as above (100 to whatever speed they bounce of each other) hence the force is the same.
The wall experiences a force of F from the car pushing it to the right
Mag of total forces = F + F =2F

the forces are exactly the same, though the amount of damage is clearly double when two vehicles are involved.

[WMD351]

Oh NC Lane, what a can of worms this topic has opened up in the past
There are many things to consider but I think a major factor is that injury to the occupants of the cars is largely a result not of the forces the vehicles are subjected to but how and how much of that force is transferred into the bodies of the occupants.
One way of looking at it is if you put an egg (representing a passenger) into a small box made of inch thick steel (representing a car) and hurl the box into a wall, the box may remain completely unmarked but the egg inside will be smashed.
Safety features such as air bags could be simulated by lining the box with cotton wool, thus diminishing the amount of force transferred into the egg (person).
Occupant safety really comes down to how much of a collisions impact force is absorbed or negated by things such as crumple zones, airbags, pre tensioning seat belts, impact absorbing materials, etc.
That being said there's also the possibility of the 4WDs high center of gravity resulting in a roll over at 100kph which is a situation where I don't think any amount of mass is going to do anything other then carry the vehicle further and possibly through more rolls.
I think it should also be noted however that the star rating is a guide only and isn't the be all and end all of safety, as the difference between 4 and 5 stars has in the past been something as little as having a passenger seat belt warning light on the dash.

Here's a couple of threads that have gone into the question (relating to an older bigger car) in much more detail.
http://www.fordforums.com.au/showthread.php?t=11305690
http://www.fordforums.com.au/showthr...6&page=1&pp=30

[sudszy]

Originally Posted by jpd80

In the worst case, a head on between two vehicles travelling at 100 kph will only ever reduce
the forward velocity to 0 kph, this is the maximum deceleration that can be applied to either vehicle,
any remaining momentum is then expressed as either a forward or negative velocity after impact.

No, if the vehicles bounce back of each other(even with today's crumple zones, they will bounce off each by a few km/h), then the deacceleration will be higher.

[jpd80]

Originally Posted by sudszy

No, if the vehicles bounce back of each other(even with today's crumple zones, they will bounce off each by a few km/h), then the deacceleration will be higher.

Vehicles do not bounce back, they are propelled in the opposite direction after forward velocity reaches zero

In order to bounce as you say, the forward velocity must have been zero at one point.

In any case the RTA saying that the impact is the same as a 200kph SVA into a brick wall is incorrect.
You would have to hit a train or a Semi fully laden head on for that to equal a brick wall travelling 100 kph.

[sudszy]

Originally Posted by jpd80

Vehicles do not bounce back, they are propelled in the opposite direction after forward velocity reaches zero

In order to bounce as you say, the forward velocity must have been zero at one point.
.

Propelled in the opposite direction after coming to rest.....that's a fairly good definition of bouncing in the same horizontal plane.....well done!

Yes in order to bounce back the forward velocity must have come to zero at some time, but I didnt dispute that. You claimed that in a head on that the maximum deacceleration occurs when the vehicles velocity is reduced to zero.

If the vehicle goes in the opposite direction the change in velocity is greater, hence the acceleration is greater(assuming all other factors such as the time of collision remains equal), clearly the maximum deaccleration does not occur when the speed goes to zero.

[jpd80]

Originally Posted by sudszy

If the vehicle goes in the opposite direction the change in velocity is greater,

No it is not, the G applied to the driver is the same, it's just a change in direction due to excess momentum being converted into deceleration for a longer period.

If the deceleration in a car crash from 100 kph to 0 happens at XGs,
then any momentum left over propels the car in reverse direction at XGs.

There is no way the Gs approach that of a car hitting a brick wall at 200 kph, do you agree yes or no?

[TC200six]

Since no one has yet mentioned that video of that test between a Fiat 500 (rated Euroncap 5-star) vs an Audi SUV (rated Euroncap 4-star), here it is http://www.youtube.com/watch?v=6pVF1Wr7GLQ

I think that confirms what a few users here have been saying about bigger cars being better off in a head-on with a smaller car.

[WMD351]

Yes passenger cell intrusion is a very nasty thing.
I did point out earlier that the ANCAP system is based on many things and not just structural rigidity.
But consider what's possible through safer design after seeing what happens when one of those itsy bitsy Smart cars goes head on into a Merc with twice its mass here. That they can plough it into a concrete wall like that and still open the door amazes me.
http://www.youtube.com/watch?v=8RAN4gTtc4s

The heavier Volvo in this clip does appear to win the inertia battle in this one but look closer.
http://www.youtube.com/watch?v=emtLLvXrrFs

[prydey]

Originally Posted by geckoGT

Do a search for old v's new car safety. you will find a thread from 12 months ago that ended up huge with a lot of good information and a lot of research involved. I for one would prefer not to go over it again.

that thread was largely about 'old v new' not 'new v new'. if both cars have modern crash protection, an extra 500kg+ will help you out

ancap testing assumes the cars are being hit by a similar mass.

[AussieAV]

Originally Posted by sudszy

Propelled in the opposite direction after coming to rest.....that's a fairly good definition of bouncing in the same horizontal plane.....well done!

Yes in order to bounce back the forward velocity must have come to zero at some time, but I didnt dispute that. You claimed that in a head on that the maximum deacceleration occurs when the vehicles velocity is reduced to zero.

If the vehicle goes in the opposite direction the change in velocity is greater, hence the acceleration is greater(assuming all other factors such as the time of collision remains equal), clearly the maximum deaccleration does not occur when the speed goes to zero.

For acceleration to occur a force must be applied. Please tell us the force that is exerted upon the vehicles once their speed reaches zero. Very hard for a body that is no longer moving toward another to exert a force on it. Its like throwing a ball in the air. It doesn't accelerate more once it gets to the top of the arc and starts coming down. The acceleration in always that of the effect of gravity (9.8 metres per second squared).

Please take the time to look at the link I posted in post #26.

Mythbusters took 4 identical cars.
1 was crashed into a solid wall at 50 mph
1 was crashed into a solid wall at 100 mph
2 were crashed head on into each other travelling at 50 mph each.

It's pretty conclusive as far as this scenario is concerned.

[sudszy]

Originally Posted by jpd80

No it is not, the G applied to the driver is the same, it's just a change in direction due to excess momentum being converted into deceleration for a longer period.
?

Lets see: If the velocity goes from +100km/h to 10km/h in the opposite direction, we have a change in velocity of 110km/h, if it just comes to a stop in then its only a 100km/h change in velocity, that's a higher change in velocity when it "bounces", fairly straight forward isnt it?

the former creates a higher deacceleration(assuming collision time is equal) since acceleration is the rate of change of velocity for both the car and the occupant, assuming of course they are wearing a seat belt.

Claiming "excess" momentum is converted into deacceleration is nonsense. Momentum is not converted into other quantities, it is conserved/transferred shared amongst other bodies in the collision.

Originally Posted by jpd80

There is no way the Gs approach that of a car hitting a brick wall at 200 kph, do you agree yes or no?

Certainly not, but that wasnt what you were claiming

[jpd80]

Originally Posted by sudszy

Certainly not, but that wasnt what you were claiming

It most certainly is, I mentioned that fact several times.

[sudszy]

Originally Posted by AussieAV

For acceleration to occur a force must be applied. Please tell us the force that is exerted upon the vehicles once their speed reaches zero. Very hard for a body that is no longer moving toward another to exert a force on it. Its like throwing a ball in the air. It doesn't accelerate more once it gets to the top of the arc and starts coming down. The acceleration in always that of the effect of gravity (9.8 metres per second squared).

Please take the time to look at the link I posted in post #26.

Mythbusters took 4 identical cars.
1 was crashed into a solid wall at 50 mph
1 was crashed into a solid wall at 100 mph
2 were crashed head on into each other travelling at 50 mph each.

It's pretty conclusive as far as this scenario is concerned.

I perhaps ask yourself how a ball can bounce off a wall when its speed reaches zero at impact?(surely you have seen that) and then you will have your answer.

balls are different to cars? they are masses and both have different levels of elasticity, the same principles of motion apply

[sudszy]

Originally Posted by jpd80

It most certainly is, I mentioned that fact several times.

No you didnt, Here is what you wrote:

Originally Posted by jpd80

In the worst case, a head on between two vehicles travelling at 100 kph will only ever reduce
the forward velocity to 0 kph, this is the maximum deceleration that can be applied to either vehicle,
any remaining momentum is then expressed as either a forward or negative velocity after impact.

reference to a car colliding with a brick wall at 200kph was never made.
My point that the acceleration can be larger when the vehicles rebound after the collision stands.


Sorry to turn this into a he said/she said folks and really its sidetracking the original question of the thread, enough from me, goodnight all.

[AussieAV]

Originally Posted by sudszy

I perhaps ask yourself how a ball can bounce off a wall when its speed reaches zero at impact?(surely you have seen that) and then you will have your answer.

balls are different to cars? they are masses and both have different levels of elasticity, the same principles of motion apply

Bouncing doesn't require an increase in acceleration. You said that after the cars get to zero forward velocity and bounce back that there is an "increase" in their deceleration (-ve acceleration). There can't be an increase without additional force.

The ball doesn't bounce of the wall faster than it hit it, unless of course you thought the movie "Flubber" was actually a documentary.

[last fairlane]

OK what if Im in My Mack truck and dog
fully legally loaded at 45 Tonne gross and I hit
a small hatch with 5 star rating at 1 tonne gross
head on @100Ks
Im maybe gonna get a sore knee or ribs (no airbags at all)
what is going to happen to the passengers in the hatch
I hope it never happens as I wouldnt be able to live with it
but work it out
me doing 100Ks and them doing 100Ks
Im sorry but no amount of hi tech stuff will save them
thats just physics
if an asteroid weighing a thousand tonnes
hit the earth and you were prepared and were
living in a bunker (airbag technology sort of)
what would happen with a direct hit
we all know that weight will always win
try hitting a nail with a hammer
now try hitting a hammer with a nail
I rest my case

[mik]

Originally Posted by WMD351

Yes passenger cell intrusion is a very nasty thing.
I did point out earlier that the ANCAP system is based on many things and not just structural rigidity.
But consider what's possible through safer design after seeing what happens when one of those itsy bitsy Smart cars goes head on into a Merc with twice its mass here. That they can plough it into a concrete wall like that and still open the door amazes me.
http://www.youtube.com/watch?v=8RAN4gTtc4s

The heavier Volvo in this clip does appear to win the inertia battle in this one but look closer.
http://www.youtube.com/watch?v=emtLLvXrrFs

it looked to me like it was not really a head on , as a concequence the small car that hit the volvo and east west motor took a large part of the impact , if it had of been a direct head on the volvo would fared better imo, but either way cars are only folded sheet metal with a few heavy bits bolted in place, if you have a crash fast enough the occupants will end up scrambled eggs regardless what car it is.

[xc4me]

forces are equal on both cars no mater how big the mass (newtons third law - this is the basics guys), its the change of momentum or impulse which is different as the weights are different.

pet hate of mine when people throw around and talk about physics laws in the wrong way but that's probably the engineer in me coming out haha

[xy500]

Originally Posted by xc4me

forces are equal on both cars no mater how big the mass (newtons third law - this is the basics guys), its the change of momentum or impulse which is different as the weights are different.

pet hate of mine when people throw around and talk about physics laws in the wrong way but that's probably the engineer in me coming out haha

well put, best to do some reading first before you preach from the book of physics.
as for all the brick wall talk, are we talking single brick or double brick wall

[sudszy]

Originally Posted by AussieAV

Bouncing doesn't require an increase in acceleration. You said that after the cars get to zero forward velocity and bounce back that there is an "increase" in their deceleration (-ve acceleration). There can't be an increase without additional force.

The ball doesn't bounce of the wall faster than it hit it, unless of course you thought the movie "Flubber" was actually a documentary.

Goodness gracious, talk about building strawman.

I said the acceleration will be more if the body ends up travelling in the opposite direction to its original motion, not that the acceleration entering the wall would be less than when leaving, that would mean some type of explosion on impact.

Do you understand how the force varies as the body is compressed on collision?

There are two things to look for in a collision, the actual amount of deacceleration and how long that acceleration lasted. You can easily survive 3000G's for a nanosec or so, you survive small bumps and knocks everyday that have high G values but dont last long, but not over the average time of a car collision.
Clearly bouncing also causes the acceleration to continue for longer, not desired.

[sudszy]

Originally Posted by xc4me

forces are equal on both cars no mater how big the mass (newtons third law - this is the basics guys), its the change of momentum or impulse which is different as the weights are different.

pet hate of mine when people throw around and talk about physics laws in the wrong way but that's probably the engineer in me coming out haha

you got the forces right, but not the change in momentum/impulse. The change in momentum/impulse(same thing) of the bodies in a 2 body collision is the same in magnitude, opposite direction.

[DBourne]

edit: beaten

[NC_Lane]

Originally Posted by WMD351

Oh NC Lane, what a can of worms this topic has opened up in the past
There are many things to consider but I think a major factor is that injury to the occupants of the cars is largely a result not of the forces the vehicles are subjected to but how and how much of that force is transferred into the bodies of the occupants.
One way of looking at it is if you put an egg (representing a passenger) into a small box made of inch thick steel (representing a car) and hurl the box into a wall, the box may remain completely unmarked but the egg inside will be smashed.
Safety features such as air bags could be simulated by lining the box with cotton wool, thus diminishing the amount of force transferred into the egg (person).
Occupant safety really comes down to how much of a collisions impact force is absorbed or negated by things such as crumple zones, airbags, pre tensioning seat belts, impact absorbing materials, etc.
That being said there's also the possibility of the 4WDs high center of gravity resulting in a roll over at 100kph which is a situation where I don't think any amount of mass is going to do anything other then carry the vehicle further and possibly through more rolls.
I think it should also be noted however that the star rating is a guide only and isn't the be all and end all of safety, as the difference between 4 and 5 stars has in the past been something as little as having a passenger seat belt warning light on the dash.

Here's a couple of threads that have gone into the question (relating to an older bigger car) in much more detail.
http://www.fordforums.com.au/showthread.php?t=11305690
http://www.fordforums.com.au/showthr...6&page=1&pp=30

Thanks, that does make sense.. But would the force applied the passengers in a bigger vehicle be significantly less than to those in a lighter vehicle? Lets assume they both have airbags

[AussieAV]

Originally Posted by sudszy

Goodness gracious, talk about building strawman.

I said the acceleration will be more if the body ends up travelling in the opposite direction to its original motion, not that the acceleration entering the wall would be less than when leaving, that would mean some type of explosion on impact.

Do you understand how the force varies as the body is compressed on collision?

There are two things to look for in a collision, the actual amount of deacceleration and how long that acceleration lasted. You can easily survive 3000G's for a nanosec or so, you survive small bumps and knocks everyday that have high G values but dont last long, but not over the average time of a car collision.
Clearly bouncing also causes the acceleration to continue for longer, not desired.

How about you activate your PM (private message) ability and we'll continue this discussion between us and stop hijacking the thread. You're just not getting it and I've got the feeling it will take a bit of back and forth. I don't mind people having opinions, as there is no right and wrong. This however is physics, so there is. Now one of us is obviously wrong, so how about we work out which so the other can learn.

[burnz]

Originally Posted by HULK_BA

Which car will end up better off? A stationary f150 pick up truck or a fiesta hitting it at 150km/h

Clearly the f150 is double its mass?

both survivable
fesses impact 50kph impact energy
f150 25kph impact energy