A Driving Instructor's Blog

Originally posted in September 2010.

Mythbusters is a show on Discovery Channel (and various others) which looks at various movie stunts and other things to check if they are really possible or just Hollywood licence. They cover things like “is it possible to shoot a gun out of someone’s hand without hurting them?” or one from tonight: “can you knock someone out of their socks?” (the answer was yes – if you virtually dismember them at the same time because of the force needed).

In this particular episode, they also looked at something many ADIs would be familiar with, namely:

If two cars travelling at 50mph collide head-on, the combined speed of impact is 100mph

Everyone automatically assumes that the forces exerted on the occupants of each car in such a crash are equivalent to a single car driving into a solid wall at 100mph. This is not true – as they proved in the show.

Newton’s Third Law states that for every action, there is an equal and opposite reaction – so if a first object exerts a force on second object, the second object exerts an equal but opposite force on the first. The upshot of this is that if those two cars collide at 50mph (yes, it is a combined speed of 100mph), the forces exerted on the occupants of each car are equivalent to those that would be experienced by driving into a solid wall at 50mph.

It was quite interesting how they did it, using crash facilities and data-logging devices with real cars, and models in the lab using the deformation of lumps of clay.

Of course, a crash at any speed is likely to lead to serious injury. But it’s worth knowing the facts if anyone asks (and they sometimes do).

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