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Road Myths: Physics of a head-on collision

Posted Nov 21st 2010 10:05AM

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Shaun de Jager
, Founder of RoadAwareness.org speaks out about road safety and road related issues.

The myth is that if two cars travelling at the same speed, say 50km/hr and collide head-on, the effect and damage is the same as one car colliding with a solid, immovable object at 100km/hr. I've heard this several times but it makes physics buffs cringe because it's false.
At first it seems to make sense since the closing speed would be double at the moment of impact but what's being overlooked is Newton's "Third Law of Motion", which states "The mutual forces of action and reaction between two bodies are equal, opposite and collinear." In its more common form it's known as "For every action there is an equal and opposite reaction". With that in mind, if two objects traveling at the same speed are collided head-on, the energy released would be evenly and oppositely distributed between both objects. Therefore if both are traveling at X, the closing speed would be X times 2 but since equal and opposite forces will apply (both forward and back), the energy released will still equal to a single vehicle colliding with a solid object at speed X.

This was recently explored on the popular TV show "Mythbusters". They started off with small scale tests using clay and then moved to full scale. They got six identical cars in make and model for the various speed tests. First they collided one car traveling 50mph into a solid, immovable slab of cement and observed the damage. Of course the car was a total write-off. Then they collided the second car into the same slab at 100mph and the results were nothing short of catastrophic. The car was barely recognizable as a vehicle at all anymore. Keep in mind that although they used miles per hour, the units of measure aren't important, just that the speeds are doubled.

*Physics side note: There is four times more energy released on impact for every time speed doubles.

The third test was to collide two of the remaining vehicles into each other head-on, with each traveling at 50mph (closing speed of 100mph) and the damage suffered to each car was the same as the single car slammed into the cement slab at 50mph. Even the hosts of the show were surprised at the results because they were believers of the myth too...or at least up until this point anyway.

The forth and final test was to collide the last two cars into each other head-on, each traveling at 100mph, and although the closing speed was a whopping 200mph, the damage observed to each car was the same as in test #2 with the single vehicle colliding into the cement block at 100mph.

This was a very dramatic display of Newton's Third Law and clearly demonstrates that the myth is false. Of course, just because the myth is false, the injuries that would be sustained from a head-on collision (or a solid and immovable object) would likely still be very severe, even at 50mph (80km/hr).

Class dismissed. Drive safely!

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