# Which is worse, hitting a brick wall at 60 MPH, or having a head on collision with each car going 30 MPH?

Asked by

Charles (

4799)
April 21st, 2012

Is it the same mechanically? Assume a perfect collision, straight line, cars are exactly the same. Simple model. Would the effect of the collision (on the driver) be the same in both cases?

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## 16 Answers

I’m thinking the two-car scenario, since the momentum of the two cars would be equal, and devastating for both (and there would be two drivers, too).

You’d probably go through many brick walls at 60mph. Total the car (and risk the damage that @ragingloli has noted), but you’d be on the other side, with the car (probably) more or less intact.

Hitting a brick wall at 60 mph will cause a more severe crash. The kinetic energy of a moving object grows exponentially (*k* = ½ *mv*^2, where *k* = kinetic energy of the moving object, *m* = mass of the moving object, and *v* = velocity of the moving object), so hitting the wall at a faster speed will cause much more damage than two cars hitting one another at a slower speed.

If you would prefer it in non-mathematical terms, consider the following: would you rather experience a sudden stop while going 30 mph or while going 60 mph? Presumably, you would prefer the former to the latter. The strength of the wall does matter, of course; but assuming the car does not just crash right through the wall and continue moving, crashing into it will be worse than crashing into another car at the same combined speed.

P.S. You might enjoy this related Mythbusters video.

Speaking from personal experience I did hit a tree (head on) at about 50 mph, and I’ve also collided with another car at around 35 mph. The collision with the tree did much more damage to my vehicle, and the injuries were more severe. I would take the head on car collision (at lower speeds) any day over the tree. My guess would be with the brick wall doing more damage.

This question was dealt with on the Mythbusters TV show. The force of two cars colliding at 30 mph each was the same as a car traveling at 30 mph colliding with a solid wall. The results surprised me and them.

The Brick wall, simple physics, Newton’s Third Law

As others have stated, the brick wall will wreck your world. It takes a car 30 MPH to seriously injure a human but 45 to kill them (If you were to run them over). But in this situation if two cars were to collide at a slow rate of speed the most the drivers would suffer is minor trauma and inertia sickness. Now if you were to drive into a wall at 60 MPH there is a lot of gravity forces at play, adding to that if you’re wearing a seatbelt that could possibly snap cartilage (This is where the rumor of it being more dangerous to wear a seatbelt comes from) and a more forceful “snap” action will occur.

A great example would be to take a soda bottle and move it through the air as fast as you can and stop in one motion. Then open it up and watch how the soda will fizzle violently. Now throw the soda against a wall as hard as you can, and this time when you open it up it will try to bubble out of the bottle (if you let it :P).

Each car has designs to disperse the energy in a crash (crumple zones). Having 2 cars means there are 2 crumple zones vs. the one crumple zone when hitting the wall. I think the wall would be much worse.

A car hitting a brick wall at speed X is equivalent to a head-on collision between two cars of equal mass each traveling at speed X, in agreement with @Bill1939‘s answer. What matters in terms of severity of damage and injury to the occupants is the *deceleration*. In both cases the car goes from speed X to speed zero in the same brief instant of time.

Exactly how brief, which determines the magnitude of deceleration, depends in a detailed way on how the car (and the wall or other car) is constructed. We assume the crumpling is the same in either scenario; otherwise they can’t be compared. For instance, one can imagine a brick wall made of foam rubber so thick and compliant that the car comes to a stop gently. Apples & oranges.

Hitting a brick wall or having a head-on collision at 60 mph is, in any case, worse than hitting the wall or having a head-on collision at 30.

@SavoirFaire You have the right formula for kinetic energy, which says that KE grows as the square (i.e., 2nd power) of velocity. Not exponentially. You probably just meant that it grows faster than proportionally, but mathematically the terminology is worlds apart.

@SavoirFaire is correct the answer has to do with the speed => kinetic energy.

“m” is the mass of the cars.

The total kinetic energy dissipated by two cars going 30 mph is:

2 * ½ * m * 30^2 = 900m

Each car in this case can absorb half of that in its crumple zones => 450m

The total kinetic energy dissipated by one car going 60 mph is:

½ * m * 60^2 = 1800m

The one car has to deal with all of that kinetic energy.

So 450 vs 1800.

@gasman Yeah, I just meant that there was an exponent in the formula that tripped up assumptions of proportionality. Thank you for the correction!

@SavoirFaire is correct IF both cars are of similar masses. “Conservation of Momentum” dictates the collision. In an elastic collision, kinetic energy is conserved. This tends to be true for small particles. In the macroscopic world, collisions tend to be inelastic. In both collision types, momentum is conserved. So, assume it is a collision between a semi and a car: assume the semi has 10x the mass of the car, and we get m(1)v(1) + m(2)v(2) or m(1)v(1) + 10m(1)v(2). After the collision, we get 11m(1)v(3)—since v(2) = -v(1), we get v(3) = -9/11 v(1). So, the final result is a movement in the direction of the semi.

Now, from the viewpoint of the car, it has gone from +v(1) to -9/11 v(1). The delta is 20/11 v(1). So, it would be as if the car was going 54 MPH into a wall. [That is, the force required to do this would be similar to the force of stopping a 54 MPH car.] As the weights of the items get further and further apart, you get closer and closer to a 60 MPH collision into a brick wall. However, you will never get there. (The limit of this would be a 60 MPH collision).

So, the answer is the 60 MPH collision into a brick wall is worse.

I searched for the video for the following myth without success: “Two cars crashing into each other at 50 mph will result in the same damage (for each car) as a single car hitting a wall at 50 mph.”

However, http://mythbustersresults.com/mythssion-control show the myth and the MythBusters result:

“In their small scale tests, the Mythbusters compressed clay at 1x and 2x speeds. Their results showed that two objects hitting each other at 1x speed will cause 1x damage. In their full scale tests, the Mythbusters crashed two cars into a wall at 50 and 100 mph as references. They then had two cars going at 50 mph collide into each other. After surveying the results, it was clear that the two cars suffered damage identical to the car that crashed into the wall at 50 mph. The Mythbusters explained that was possible through Newtonâ€™s third law of motion. Although the total force was doubled by having two cars, that force also had to be divided between both cars during the crash.” >confirmed<

Episode 143: Mythssion Control – Season 8 Episode 6 – Air Date: May 5, 2010

Right you are, SavoirFaire. I failed to notice your link. It points to the show I had seen on TV and the results I posted. I don’t know why my search didn’t find the YouTube video provided by your link. I found that episode for sale on Amazon (which I didn’t want to buy) and a link that claimed to have it, but instead was a trap that my anti-virus program alerted me about (fortunately I seem to have escaped uncontaminated).

Here’s the root of the question: Two similar solid objects smashing into each other expends the same amount of energy as a solid traveling twice as fast hitting the same solid object held still. Ignore that cars are collapsible and that brick walls aren’t.

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