Gasoline has approximately 56 Megajoules of chemical energy per liter , which is more
energy than you get from exploding the same of amount of TNT, and is enough to power a
toaster for a full day.
Cars work by burning gasoline to convert that chemical energy into the kinetic energy of
motion of the car, though almost 80% of it is lost as heat in the engine.
Still, 20% of 56 million joules is a lot of joules…
To give a direct sense of gas-to-car conversion, it takes about five teaspoons of of gas to
accelerate a 2 ton car to 60kph, and about a third of a cup more for every additional
minute you want to keep it going at that speed.
That might not sound like a lot of fuel, but the energy of a car moving 60kph is equivalent
to dropping an elephant – or stegosaurus – from the top of a three-story building.
And in order for the car to stop, all that energy has to go somewhere.
If the brakes do the stopping, they dissipate the energy by heating up.
In the case of a collision, energy is dissipated by the bending and crumpling of metal in the
outer areas of the car.
And just like how smooth braking is nicer than a quick jerky stop, cars are carefully
designed to crumple - when they crash - in a way that lengthens the duration of the impact
so that stopping requires less intense acceleration.
Lots of acceleration over a very short time is not good for soft human brains and organs.
However, people don’t like driving cars with Pinocchio-length noses, so most cars
only have around 50 cm of crushable space in which to dissipate the energy equivalent
of our falling stegosaur.
That means that, while crumpling, they need to maintain a resistive force of about a quarter
the thrust of the space shuttle main engine.
Over half of the controlled-crumpling work is done by a pair of steel rails connecting
the front bumper to the body, which bend and deform to absorb energy and slow the car.
Most of the rest of the energy is absorbed by the deformation of other pieces of structural
metal throughout the front of the car.
This meticulously engineered destruction allows a crashing car to decelerate at a high but
reasonable rate: just slightly over the acceleration experienced by fighter pilots or astronauts
in centrifuge training.
As comparison, if cars were super rigid (like they were before the 1950s) and didn’t crumple,
they would stop so fast that they would undergo acceleration 15 times what fighter pilots
experience in training.
Thankfully engineers have learned to make cars with crunchy crumple zones surrounding
their rigid safety cell, because fully rigid cars are not good for fighter pilots or anyone
else.
Except, maybe, robots.
This MinutePhysics video was made possible by Ford - I was able to talk to an awesome
crash test safety engineer there who told me all about the complex physics and engineering
that goes into vehicle development and improving how cars perform in a crash.
Ford gave me this opportunity because they want you to know how important and carefully
designed all the parts involved are, and in particular that the only parts developed and
tested to work with their vehicles are original Ford parts.
If you want to learn more about why the right parts matter, you can head to takeagoodlook.com.
And I personally want to say that making this video has just reinforced to me that regardless
of what kind of car you have, big dents and deformations in the body aren’t just aesthetic
problems – they can be safety hazards, too.