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February 2026

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[Rct33]

Is one car hitting a wall at 120 equivalent to two cars crashing into each other at 120? Surely it's equivalent to two cars hitting each other at 60.

Not quite - whilst the relative motion is the same, because the accident is with another car, which will crumple, the force of impact on each car is much less.

But the idea is that it is the same as when you hit a brick wall, which does not crumple.

[Zarf]

Is one car hitting a wall at 120 equivalent to two cars crashing into each other at 120? Surely it's equivalent to two cars hitting each other at 60.

Not quite - whilst the relative motion is the same, because the accident is with another car, which will crumple, the force of impact on each car is much less.

But the idea is that it is the same as when you hit a brick wall, which does not crumple.

Im pretty sure that idea is wrong. The same energy is involved but the effective crumple zone for the collision would be twice as big, therefore the accident would take longer to complete, therefore the deceleration is less intense.
Get Chug or another Physics grad in here to provide the right terms.

[DeadlyAvenger]

Yeah - there are obviously going to be other effects that come into play - but, for example it shouldn't matter if the concrete block was travelling at 120mph and the car was stationary, the car was travelling at 120mph and the block was stationary or if both were travelling at 60mph - the relative velocity between the two is the same - 120mph.

Sorry, I was referring to the objects at hand in this situation (car & concrete block) - if you have 2 cars colliding then the duration of the impact is longer due to the crumpling so the deceleration is less as the energy has longer to dissipate. Still - it wouldn't have mattered (unless there are other effects to do with static as opposed to dynamic friction in the tyres when the vehicle is stationary as opposed to moving or gyroscopic effects in the wheels) if the two cars were travelling at 60mph or one collided into the other that was stationary at 120mph.

[CruelCow]

Thanks to you guys I had a dream about cars colliding head on.

(Nobody was hurt and the cars weren't even dent for some reason though )


I assume it is also pretty rare to collide directly head on, but the end message is clear: Don't do it, best case scenario you're gonna get hurt badly.

[Brahms]

It's interesting that Euro NCAAP don't do crash testing above 40mph. is it the case that at motorway speeds it doesn't really matter a good goddamn what safety features your car has? This would be a good justification for driving a piece of trash car like mine as I am always driving at motorway speeds. At all times. Everywhere.

[Jebus]

Yeah - there are obviously going to be other effects that come into play - but, for example it shouldn't matter if the concrete block was travelling at 120mph and the car was stationary, the car was travelling at 120mph and the block was stationary or if both were travelling at 60mph - the relative velocity between the two is the same - 120mph.

Heh I like that there was a big old debate about this in the YouTube comments on exactly the same point. I did think at first they were wrong about it being cars hitting each other at 120mph but after thinking about it I agree with RCT and Aquila. As I understand it the wall will provide an equal and opposite force. Therefore the car hitting the wall at 120mph will have an equal force exerted on it by the wall - which therefore is exactly the same as if it were hit by another car travelling at 120mph (off the same mass). I.e. 2 cars hitting each other at 60mph is the same as a car hitting a wall at 60mph.

[Brahms]

I'm not certain on either answer, but would you rather drive your car into a stationary wall, or a wall travelling towards you at 120 mph?

[DeadlyAvenger]

I'm not certain on either answer, but would you rather drive your car into a stationary wall, or a wall travelling towards you at 120 mph?

Wall coming towards you, then you're not liable

[Brahms]

Thinking about it, I agree with Aquila too. It's a bit nebulous to just say "equivalent", but I thought about it like this: Hitting the solid wall can be thought of as instantaneous decelleration from 120mph. So if you have your car driving at 120mph and you're going to hit another car, for your car to instantly decellerate to 0, the other car must be also going at 120 (assuming it weighs the same) or you'll collide and continue moving either forward (the other car was going <120) or backwards (the other car was going >120) either way you won't be instantly decellerating, so it wouldn't be the same as hitting the wall at 120.

Weird one though.

[Millicent Bystander]

Back from the kitchen design day.  The M25 wasn't as bad as I feared (fantastic sunset coming back over the Dartford Crossing), the course was actually interesting and they gave me a free template and scale ruler.

Not the wasted day I feared.

[Berath]

Yay..freebies 

[Mr Chug]

CHUG IS HERE FOR PHYSICS

Short version: two cars at 60 isn't same as one car + wall at 120 for crumple zone/energy reason listed above.

Long version: too many factors at play here really. For one, are we assuming the wall counts as an unstoppable force/immovable object? A car will almost certainly roll backwards if hit, whereas we're assuming a solid wall won't break. Therefore better to have the wall coming at you, since your car will turn some of that energy into flying away from the impact.

NCAP tests are fairly low compared to speed limits for two reasons: 1) most accidents happen at less than 40mph (officially, front impact tests are designed to simulate two cars colliding at 55 km/h, which covers something like 80% of collisions) and 2) if you made it any higher everyone would probably fail, because it's beyond available technology at the moment.

[Rct33]

Chug, it was pretty clear when they tested it on mythbusters (yes its not very scientific, but it was pretty conclusive!). My understanding of why it is true is that I would treat the situation by looking at the change in momentum P over time t (which is the force F). Let's assume, both crashes take the same amount of time t (which is legit) and both cars have the same momentum P (e.g. same mass and speed). If one car now decelerates from a finite momentum to one of zero in that time, the applied force to that car is F = P/t. Since both scenarios happen at the same timescale, the same force is applied.

[Tommy the Cat]

Say for example your car literally cannot do 120mph, will I ever die?

[Yer man oer yonder]

I think people are getting confused between matters of relative velocities and forces.

CHUG IS HERE FOR PHYSICS

Short version: two cars at 60 isn't same as one car + wall at 120 for crumple zone/energy reason listed above.

2 cars at 60 will be the same as a car and a wall at 60. F = ma. The acceleration will be 60 (whatever units of velocity ye use)/(time over which the car stops) for both cars. It's the same for the one car hitting a wall. Assuming everything comes to a stop upon impact the acceleration is the same in both cases.
By this argument 1 car + wall at 120 <=> 2 cars head on at 120. again, in both cases the cars go from 120 to 0 in the same amount of time, mass is constant therefore the forces are equal (and opposite).


In other news, I'm currently doing a research project for my final year of my physics degree. Today we managed to drop a sample inside an ultra high vaccuum chamber. I won't be getting that sample back until they open the chamber this weekend. Turns out the sample holder is broken but no-one told the chap who was measuring the sample for me. Funnily enough it was broken a while ago when they were trying to fix a different problem.