Do you fasten your seatbelt for every journey?.....really?......I don't

Gloomendoom

stevemcol wrote: »

Gloomendoom, don't take it too badly. We all make mistakes

Explain please... and I'll take it like a man!

bigjl

stevemcol wrote: »

A mature tree (completely immovable) is probably a better example than a wall. Deceleration g will be far greater hitting the tree at 60 than two cars head on at 30. Either way, not a pretty outcome.

Interestingly, the road traffic accident rate actually increased measurably when seat belts became law. A psychologist could explain it.

In the same way that a significant proportion of RTC's occur shortly after the car has some major work done to it, new tyres, new wheels etc.

Possibly proceeded by the fateful phrase "watch this".

Kilty_2

bigjl wrote: »

Possibly proceeded by the fateful phrase "watch this".

:rotfl:

Seatbelt? Would never drive without it. Only exception being when I would get out of my car to open the barriers at the underground car park I was using for a couple of months. Even then my car (Renault Laguna) would go off it's head with the beep... beep... beep........ BEEP BEEP BEEP BEEP OMFG BEEEEEP thing between the time it took to get back in the car and park up :rotfl:

bigjl

Gloomendoom wrote: »

Using my figures and assuming all cars have identical crumple zones each car crumple zone will have to dissipate 900 in the 30mph head on crash.

The car hitting the wall will have to dissipate 3600 through its single crumple zone.

The odds on the wall driver are getting worse.

Why are crumple zones even being considered in this thread.

You say a car has 3600 ?Joules from memory of energy when it hits a wall, that means that each car driving at half the velocity has 1800 ?Joules, when they hit each other head on then the totla force exerted will be 1800 and 1800 added. Surely?

If a car driver hits a stationary object at 60 mph then broadly the same forces will be exerted on his body as there would be in a collision with a car coming the other way at 30 mph if he was doing 30 mph. Assuming both cars has similar mass, I suppose.

All this theory is fine, but it is still all theory, and in real life things are never absolute, one driver hits the brakes, therefore one car goes under the other, one car swerves just at the point of impact.

Theory is great, but it is best kept in the lab.

In real life, and you can take this to the bank, all Ambulance crews, Fire crews and pre hospital Dr's assess the accident in the way I do, worrying about crumple zones and such like is not considered.

Mass of the various vehicles is, as is direction of travel and speed.

HAve you ever seen an X5 after it hits an average size car in an RTC, it is hardly damaged, and these cars are all assessed in the same way by NCAP.

I have seen a 5*NCAP car after a collision with an older X5, I am sure that the older X5 was lower than 5*NCAP rating, though you wouldn't be so sure after looking at the other car involved.

Gloomendoom

bigjl wrote: »

So the, admittedly basic, way I have been taught to assess possible forces in an RTC is correct then?

Which since it is the way it is done world wide I assumed it would be.

We where told to mainly focus on the speed of the vehicles involved, and the direction of travel.

So a vehicle travelling at 30 mph, hit from the rear by a car doing 60mph tends to have a similar "mechanism of injury" as a car that is hit from the rear whilst stationary by a car travelling at 30 mph.

That I can believe, as the impact energy is likely to be dissipated by the car in front being pushed forward in addition to the bodywork crumpling.

Just reread DoomandGloom's comment, yes that is completely wrong, should have looked closer.

His thinking is surely completely flawed as the objects are going in opposite directions and therefore you need to add the two figures together, not average them as he has done surely.

The two figures are added together. The energy stored in the cars increases as the square of the speed. A 60mph car has four times the momentum as the same car doing 30mph.

stevemcol

gloomendoom

say the cars weigh 1000kg
30mph = 13.4 ms-1

1 car + wall 1/2*1000*26.8^2 = 359120J
2 cars 1/2*2000*(13.4*2)^2= 718240 =359120J per vehicle

bigjl

Jesus Steve, you do realise it is twenty odd years since I did any physics etc..................

I need to lie down.

Gloomendoom

stevemcol wrote: »

gloomendoom

say the cars weigh 1000kg
30mph = 13.4 ms-1

1 car + wall 1/2*1000*26.8^2 = 359120J
2 cars 1/2*2000*(13.4*2)^2= 718240 =359120J per vehicle

Wall: 1 car 1/2*1000*26.8^2 = 359120J

Two car collision: 2 cars 2(1/2*1000*13.4^2) = 179560j

stevemcol

your second equation describes two cars crashing seperately into a wall, not a combined collision. The speeds need to be combined before squaring.

Gloomendoom

stevemcol wrote: »

your second equation describes two cars crashing seperately into a wall, not a combined collision. The speeds need to be combined before squaring.

No they don't. Trust me.