Crash physics question
Discussion
Perhaps the boffins (cough Mech Eng's) in the group can help me out with this.
There's an ad on tv over here that features a guy sliding his AU Falcon into a power pole, impact LHS front passenger door. The car is wrecked, and the guy theoretically ends up in rehab for 6 months learning to walk and talk again, multiple broken bones, internal haemorrhaging, brain damage, the works. The ad then goes on to state that if he had been doing only 5 km/h less ("at the speed limit", although the limit isn't specified), he would have walked away with cuts and bruises. Now, assuming that the only variable is impact speed ie the contact point is the same, vector is the same, is there any basis to this claim? It just doesn't sit right with me.
Basically I see it as your injuries must be proportional to the force you experience under deceleration. F.dt = m.dv so F = m.dv/dt or the more commonly shown F = m.a
So obviously your dv has increased by at most 10% (for 50 km/h limit, gives the greatest dv possible for 5 km/h difference in impact velocity) and I don't see a 10% change in force can tip a human body from cuts and bruises to absolutely ruined. Does the human body have a 'yield point' above which level any force results in massive destruction, or do we respond closer to linearly with applied force?
My other question is how does your dt change? The car will crumple more as a result of the greater force which will increase the deceleration time but the metal will be stiffer (stiffness being proportional to rate of deformation) which will decrease the deceleration time. Again, for a 10% change in velocity, in an inelastic collision, how much can the time over which the deceleration occurs change by? Normally only stiff materials are strongly affected by a small change in deformation rate and the human body is anything but (barring bones of course but they're already brittle material).
This is all ignoring the fact that the impact is lateral and passenger side (mitigating factors), in a crap car (exasperating factor), assumes no deceleration before impact (mitigating) and could have been avoided by stability control or decent driving... any comments? Flaws in my logic? Obviously I'm not trying to discredit driving at the speed limit, I just hate seeing false justifications used for things.
There's an ad on tv over here that features a guy sliding his AU Falcon into a power pole, impact LHS front passenger door. The car is wrecked, and the guy theoretically ends up in rehab for 6 months learning to walk and talk again, multiple broken bones, internal haemorrhaging, brain damage, the works. The ad then goes on to state that if he had been doing only 5 km/h less ("at the speed limit", although the limit isn't specified), he would have walked away with cuts and bruises. Now, assuming that the only variable is impact speed ie the contact point is the same, vector is the same, is there any basis to this claim? It just doesn't sit right with me.
Basically I see it as your injuries must be proportional to the force you experience under deceleration. F.dt = m.dv so F = m.dv/dt or the more commonly shown F = m.a
So obviously your dv has increased by at most 10% (for 50 km/h limit, gives the greatest dv possible for 5 km/h difference in impact velocity) and I don't see a 10% change in force can tip a human body from cuts and bruises to absolutely ruined. Does the human body have a 'yield point' above which level any force results in massive destruction, or do we respond closer to linearly with applied force?
My other question is how does your dt change? The car will crumple more as a result of the greater force which will increase the deceleration time but the metal will be stiffer (stiffness being proportional to rate of deformation) which will decrease the deceleration time. Again, for a 10% change in velocity, in an inelastic collision, how much can the time over which the deceleration occurs change by? Normally only stiff materials are strongly affected by a small change in deformation rate and the human body is anything but (barring bones of course but they're already brittle material).
This is all ignoring the fact that the impact is lateral and passenger side (mitigating factors), in a crap car (exasperating factor), assumes no deceleration before impact (mitigating) and could have been avoided by stability control or decent driving... any comments? Flaws in my logic? Obviously I'm not trying to discredit driving at the speed limit, I just hate seeing false justifications used for things.
I don't think they'll be relying on Billiard Ball physics 
The first thing will be a decreased deceleration distance, as the same reaction time (at the greater speed) soaks up a greater distance-to-target. And temporarily setting aside George's argument about greater focus and attention when peddling hard.
Then of course assuming uniform energy dissapation in the braking zone the extra 5 kph at the outset will be a lot more than a 5 kph difference at impact.
Then this extra energy will result in greater deformation, and the jagged bit do you more damage, blood loss, oxygen deprivation, brain damage, . . . then you'll volunteer for a lobotomy, vote Labour, believe everything that LTNZ says
The first thing will be a decreased deceleration distance, as the same reaction time (at the greater speed) soaks up a greater distance-to-target. And temporarily setting aside George's argument about greater focus and attention when peddling hard.
Then of course assuming uniform energy dissapation in the braking zone the extra 5 kph at the outset will be a lot more than a 5 kph difference at impact.
Then this extra energy will result in greater deformation, and the jagged bit do you more damage, blood loss, oxygen deprivation, brain damage, . . . then you'll volunteer for a lobotomy, vote Labour, believe everything that LTNZ says
Must be time for bed
I'm guessing that this will be a similar equation to the ad we had here a year ago with the two identical Falcons hitting the truck.... one at 65km/h and one at 60... the difference at impact was 35km/h or some such.
You'll see in the other thread about this that they used an unrealistically long reaction time (that's consistent with someone being not very alert at the wheel). The longer the reaction time, the bigger the speed difference at impact will be (generally). I did a kinematic equation to show that if you were alert you could have been doing another 10km/h again and not had an impact at all.
These ads atre generally bullsh1t dressed up to look like scientific fact. You can't complain about them either because what they show is "in the public interest"
You'll see in the other thread about this that they used an unrealistically long reaction time (that's consistent with someone being not very alert at the wheel). The longer the reaction time, the bigger the speed difference at impact will be (generally). I did a kinematic equation to show that if you were alert you could have been doing another 10km/h again and not had an impact at all.
These ads atre generally bullsh1t dressed up to look like scientific fact. You can't complain about them either because what they show is "in the public interest"
The horrifying thing is that the links provided by Dan show ACC and LTSA statistics show a 90kph crash into a solid object has an over 90% fatality rate. So we have the nimrods lieing that 105kph is dead and 90kph is safe as houses. The thread that covered the ad where the car drops from 1 and 9 stories covers this.
As far as another political party making a difference to LTSA policy. hahahahahahah! Oh dear how quaint.
The same useless room temperature IQ nitwits don't change. LTSa and it's policy is driven by burecrats and having met corresponded and seen these idiots and their the total inability to do anything with real live in your face evidence of criminally dangerous vehicle problems I long ago gave up hoping for change.
Cheers
Mark Stacey
As far as another political party making a difference to LTSA policy. hahahahahahah! Oh dear how quaint.
The same useless room temperature IQ nitwits don't change. LTSa and it's policy is driven by burecrats and having met corresponded and seen these idiots and their the total inability to do anything with real live in your face evidence of criminally dangerous vehicle problems I long ago gave up hoping for change.
Cheers
Mark Stacey
marksteamnz said:
The horrifying thing is that the links provided by Dan show ACC and LTSA statistics show a 90kph crash into a solid object has an over 90% fatality rate. So we have the nimrods lieing that 105kph is dead and 90kph is safe as houses. The thread that covered the ad where the car drops from 1 and 9 stories covers this.
As far as another political party making a difference to LTSA policy. hahahahahahah! Oh dear how quaint.
The same useless room temperature IQ nitwits don't change. LTSa and it's policy is driven by burecrats and having met corresponded and seen these idiots and their the total inability to do anything with real live in your face evidence of criminally dangerous vehicle problems I long ago gave up hoping for change.
Cheers
Mark Stacey
As far as another political party making a difference to LTSA policy. hahahahahahah! Oh dear how quaint.
The same useless room temperature IQ nitwits don't change. LTSa and it's policy is driven by burecrats and having met corresponded and seen these idiots and their the total inability to do anything with real live in your face evidence of criminally dangerous vehicle problems I long ago gave up hoping for change.
Cheers
Mark Stacey
Yeah what he said..................totally agree !
Gassing Station | New Zealand | Top of Page | What's New | My Stuff