The Physics of a Car Wreck


Drive safe.

It’s easy to forget the forces at work when we commute at a mile a minute every day…

For over a century, people have been driving automobiles. America in particular has a longstanding love affair with its cars, and it’s easy to forget that the machine that you take everywhere can be deadly, if not handled responsibly. Complacency builds in even the most cautious drivers as we race from here to there without incident. Here are a few facts and figures about what happens when two cars make a meaningful connection at speed, to remind us all how vital safe driving really is.

Cars go by converting the potential energy (usually) in gasoline into kinetic energy. The exploding gasoline inside your engine produces 56 megajoules of energy per liter, which is more than you get from exploding the same amount of TNT. About 80% of that becomes waste heat and is lost. It takes about 5 teaspoons of gasoline to accelerate a 2 ton car to 35 mph, and another third of a cup for every minute of travel at that rate.

35 mph may be way too slow for the highway, but to get a sense of the energy at work, imagine an elephant tripping and falling off a 3 story building- you wouldn’t want to be under it when it lands. For a car to stop, all that energy has to go somewhere. When you hit the brakes, it gets turned into heat. When you hit another object, it goes into deforming & reshaping the vehicles involved, and everything inside, including us.

Modern cars are designed to absorb as much kinetic energy as possible in a crash, with crumple zones & airbags. They work by extending the time of the collision, because the more time that’s taken to change speeds, the less severe the crash will be for the occupants. It’s like the difference between walking into a screen door and a glass one: the screen door gives, and you’re embarrassed, while the glass one does not, and you’re bruised. By designing cars to crumple on impact the G forces at work are reduced, making injuries less serious.

One G is equivalent to the gravity of the earth’s pull under us. So 2 Gs is twice the pressure that makes your feet hurt at the end of an active day. Jet pilots endure as much as 9 Gs while they take off and maneuver, and astronauts train for years to sit through an 11G ride as they leave the planet behind. In a car crash, the energies at work are often stronger than that.

A car moving at 30 mph that’s brought to an immediate stop by a brick wall will face 30 Gs of deceleration, if it has a 1 foot crumple zone. A 2 foot crumple zone, likewise, halves the destructive force to 15 Gs. If the car hitting the wall was a classic, from before cars were engineered with passenger safety in mind, the driver would suffer 60 Gs of thrust and probably some serious injuries from the same brick wall. With modern cars, ideally, all that kinetic energy is absorbed by safety features so nobody inside is hurt but that’s not always the case.

Part of the reason injuries are so frequent in a car crash is that when you collide with an object and have a rapid stop from speed, there are actually three impacts: the vehicle hitting the object, your body hitting restraints (like a seatbelt and airbags) and another impact as your organs are brought to a stop inside your body. Just like when you’re belted into a car around your waist, and you feel a stop in your head neck and shoulders, much of human anatomy isn’t fixed to the skeleton or tissue. Many vital organs have “wiggle room” which can lead to serious, even catastrophic, internal injuries from their inertia as one bit of your spleen or heart stops and another bit of it keeps going. The damage from this third impact is especially telling in brain and spine injuries because the central nervous system is so complex and delicate, and made up from materials of different densities. It’s so complicated, in fact, that exactly what happens to the brain in a high speed collision is the subject of much debate in the medical community, though the dangers of whiplash are well known.

These examples assume the people involved are belted in. Being fettered to the vehicle means that you stop with it, instead of increasing the severity of that second impact: delivering the full force of the car’s momentum directly to the passengers against the interior of the car instead of the restraints. The G forces from a wreck are dangerous enough to humans’ delicate innards without adding an impact with glass and metal to the equation. A few people still insist that it’s safer not to wear a seatbelt than to do so, but it’s simply not true. A crash with a seatbelt may feel like a gut-punch from an angry demigod, but going without has the potential to break windows with a person’s head, smash bones against familiar armrests, or to eject people from the car (and into oncoming traffic, or a brick wall, or to roll over them, or just to scrape them painfully against the road for yards and yards.) The simple fact is that seatbelts save lives, and should be used anytime & every time you’re traveling by car.

There are other factors that can make a wreck even more dangerous; the examples above have been at street speeds, not highway, and assume the driver is hitting an unmoving wall. Another moving vehicle adds more energy to the hit, and the kinetic energy of a crash is multiplied by the SQUARE of the speeds involved- so if you double the example velocity to 70mph, the forces at work aren’t doubled- they’re quadrupled. All this math takes on a tangible quality when brakes screech and your airbags go “pop” in a moment of surprise, destruction, and pain that brings your potential energy and your plans for the day to an immediate stop.

So if you had a crash, even a crash at 30mph or less, don’t be surprised if you discover that you’re injured in the moments or days after a wreck. Collisions have been studied much longer than we’ve had cars, and we learn more about how our bodies are affected by them as medicine advances. Car wrecks are not fully understood, and unpredictable things take place during them. So don’t be dismissive of your experience; any car wreck can damage the occupants, and it often takes days or more for the injuries to manifest. So don’t hesitate to get medical help, and to quickly contact a personal injury lawyer if you want to make the best of your unfortunate situation.

-Written by D.M. Eaton