Thank you all, it is all soooooo clear now

oceanfire said...

Guys, I reckon it's a pretty safe bet to take the word of the engineers I work with who state that the force of impact is doubled when two cars travelling at 30km/h collide head on, compared with one car travelling at 30km/h collides head on with a stationary wall, than what some of you kind of remember seeing on a TV show.

I'm an engineer and this is Year 11/12 Stuff. I'm going to now prove to you that two vehicles impacting each other at 100km/hr is the same as a single car impacting a wall at 100 km/hr.

Assumptions:

- Both Vehicles weigh 2000kg and are identical
- The wall does not deflect.
- Crumple Zone impact/crush length is 1m
- Assume linear change in velocity on impact

Case 1: Impacting Wall

Car strikes wall and changes velocity from 100km/hr (27.8 m/s) to 0 m/s

Crumple zone means car travels 1m from point of first impact to being 100% at rest.

Given linear acceleration from 27.8 m/s to rest, average velocity over time of impact is 13.9 m/s

Car travels 1m during impact

1m/13.9 = 0.07s impact duration

Acceleration = 27.8/0.07 = 397.14 m/s/s ~ 40g (close your eyes or you will lose your eyeballs)

F = mass x acceleration

mass of car = 2000 x 9.8 = 19.6 kN

F = 19.6 kN x 397.14
= 7783 kN = 7 783 000 N

OK Case 2, two cars in a head on.

Both cars have same momentum as they have the same mass and the same velocity (but in opposite vectors) A perfect head on collision will result in both cars coming to a complete rest.

Both cars will crumple by the same amount. Energy absorbed by each car will be the same. There will be 2 x 7783 kN = 15566 kN dissipated by the crash but it is split evenly between the two cars so the force exerted on each car is the same as one car hitting a brick wall.

QED in an idealised world, there is no difference in hitting a wall at 100km/hr or having a head on with a car travelling in the opposite direction at 100km/hr.

Of course, in the real world, the head on will be slightly off axis, car masses different, velocities different but the cars are likely to continue travelling further past the point of impact, resulting in a longer duration of negative acceleration, hence less Force on the vehicles.

Case closed.

of course you would harry, doing 30 as opposed to 60 would mean half the dissipating force. i would too.
and i love a good debate, and am quite impressed by londons figures.
so surely all who believe the speeds are added are convinced by now?
if the cars collide head on and are identical, at 30, 40, 50, 60.....
it will be the exact same as hitting a solid immovable object.

quote]harry potter said...

Next time you pass a mate on opposite tacks at speed .......flick the tail of your board and throw some extra spray in his direction........ if it hits them in the groin ...it hurts...if it hits them in the face it hurts... but it can be fun...particularly when the water is 9deg

A high five is fun but remember your actual speed when kiting can be deceptive and you could end up smacking someone in the face ....quite hard.

As for the cars........... I would rather be in a car doing 30kph and have a head on with another car doing 30kph than hit a stationary wall at 60kph, if you have even seen car hit an immovable object at 60kph you would realise that the chance of survival is minimal. All learner / probationary drivers should witness a vehicle hitting an immovable object at 60kph if they did there would be far less mucking around on the roads....ok im off topic now

It just doesn't seem right.

Hitting a stationary wall; which presents no opposing force (other than it's integral strength) to overcome as you crash into it.

Yet hitting a vehicle coming towards you; you have that extra opposing force crashing head on into you as you hit it, as opposed to a wall just standing still.

I guess the only way that the two situations can be the same is if the strength in the wall presents a resistance (opposing) force equal to what is exerted by the other car crashing head on into your car.

oceanfire said...




I guess the only way that the two situations can be the same is if the strength in the wall presents a resistance (opposing) force equal to what is exerted by the other car crashing head on into your car.

If the wall does not move then that is exactly the case.

the way i look at it is.... a dead stop is a dead stop... hit a wall at 60kpm you are slowing from 60 -0kph have a head on at 60kph and the dead stop is the same 60kph down to 0kph.....

if you attached a 15mm cable to the rear of a vehicle chassis and to an anchor point drove at 60kph reached the end of the cable and came to a complete stop the force would again be the same 60kph to 0 kph.

I guess vehicle mass plays a huge part in a head on though as you could easily go from 60kph to minus 10kph ( reverse ) if you had a headon with a truck and were pushed backwards at speed.

what an exiting thread

oceanfire said...

It just doesn't seem right.

Hitting a stationary wall; which presents no opposing force (other than it's integral strength) to overcome as you crash into it.

Yet hitting a vehicle coming towards you; you have that extra opposing force crashing head on into you as you hit it, as opposed to a wall just standing still.

I guess the only way that the two situations can be the same is if the strength in the wall presents a resistance (opposing) force equal to what is exerted by the other car crashing head on into your car.

It's known as a counterintuitive solution ie. your intuition is not confirmed by the calculations.

The vehicle might be coming towards you but your car has to apply force to it to stop it, just as that car has to apply force to you to stop you.

The wall is not free to move, (we are assuming a solid wall here, say 1m thick)

If the wall was deformable then the deformation of the wall would required some energy input. Then we start talking about modulus of elasticity and lots of awesome "I'm going to make your eyes glaze over in boredom" concepts.

What forum are we on again?