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The Physics of Car Crashes

The Physics of Car Crashes

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 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.

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100 thoughts on “The Physics of Car Crashes

  1. When I asked a friend of mine, who is a firefighter, abour car crashes, he told me about an observation he did.

    More accidents with low speed cars (like 30-40 km/h) were fatal for the driver in relation to accidents with arround double the speed.

    Our explanation was that the engineblock gets crushed at arround 70 km/h but is pushed into the driving compartment when on lower speed consequently killing the driver.

    Needless to say that there is an upper Border of speed to make this work, since there is only so kuch energy the engineblock can take until it is completly crushed.

  2. I'd still rather have a rigid car from before the 1950s. At least they're safer in collisions with lighter, smaller cars, or pedestrians.

  3. Nice video! It would be also interesting to have one about speed, related to traffic safety. About the reaction of a human body when hit by a car, the importance to drive and to transit safely.

  4. 56000000 Joules of energy powers a toaster for only 22 hrs? sounds like it should power it for a week or something.

  5. it doesn't matter if you drop a stegosaurus or a bowling ball from a building they will both fall at the same time so their speed will be the same and you didn't say that the dino wasn't in space. 🙂 so next time be more specific ok?

  6. Why don't we make future cars out of rubber or harder carbon fabric materials for safety reasons?
    And why the engine needs to be at the front of a car?

  7. No way it takes 1/3 of a cup of gas every minute to maintain 60km/h….
    Otherwise you'd spend 20 cups of gas an hour… That's not right! 🙂

  8. dont forget seatbelts. they also let you move a bit which extends the length during which your body has to deccelerate from hollyshit to 0.

    And also this is why Smart (and equivalents) is a really stupid car to use outside cities (50kph zone)

  9. Dear MinutePhysics,
    Has the technology in cars improved as much as it should be?
    Or it has gone so far, but the manufacturers are still keeping it to themselves for marketing purposes?

  10. How the hell didn't i find this video whilst doing a physics assignment on Car Safety. Application being crumple zones?????

  11. so a car crash disipates the energy by crumpling, but can we make a car that disipates the energy in another way?

  12. From 1:38 – What if the beam/the front part is replaced with Spring/coil? which can be absorbed energy and divert it to ground.

  13. I still don't understand why we don't build all cars with a 50 cm sacrificial crush zone. Probably because they're designed to destroy everything in the quest for 50cm. Engine block shattering included.

  14. If you crash at a slower speed
    will you experience a higher G-Force
    since it's not fast enough to crush the car?
    And the crushing is what lengthens your impact.
    (though it will end a lot sooner)

  15. soooo in the video the Stegosaurus has 2.8 metric tons but a Stegosaurus had a maximum of 2.4 tones 🤔🤔🤔

  16. The other thing that's improved since the 50s, and even since the 90s is that in addition to bending more where you want it, they also bend less where you don't – ie, the passenger compartment. Those crumple zones are great but only if they surround a rigid structure that you remain inside of, since crumpling up the person as well defeats the purpose.

  17. 1:04 so that is why real life cars break so much easier that toy cars! i thought it was because they didn't scale well

  18. 56 MJ/l?? It seems to be a stupid mistake or dubious fancy. You should better start MinuteArithmetics channel.)

  19. Those early cars also lacked seatbelts, which were forced on Fords by Robert MacNamara, who had observed their value during ww2, but he still screwed up the Vietnam war

  20. Did you edit your voice to speed it up? Wish the background sound can be much softer, so your voice is clearer for listeners

  21. I'm a car modder, but not like scraping the pavement and loud exhaust on a small Japanese car haha. My current project is making a 83 vw to be the most crash resistant. I'm trying to find a blend between avoiding permanent damage to the vehicle and absorbing impact. The read bumper mounts which are normally designed to crush have a large coil spring to help reduce permanent damage while the fronts have a fault zone at a cheap, easy to replace and easy to crush bracket that cost $4 to make/replace per side. My goal is to be able to get in an accident rear ending a car at 20mph then to be rear ended at 20mph and to only have the two front bracket to replace. Of course in testing it out on a large tree and not on the freeway haha

  22. I like these videos. I feel like the more I watch the dumber I feel like I am being talked to. Does that mean I’m getting smarter?

  23. what about military vehicles, like tanks travel at a high speed as well or those offroad vehicles where they do tricks like flying and jumping….just curious

  24. 1. wrong. its about 60
    2. a car with a thermal efficiency of 20 % is stone age technology. Most cars these days do over 30%, some even 50% and mercedes made a test F1 engine that should do 60%
    3. Cars are not 100 meters long because its inconvinient, not because people dont want it. It would also be bad for traffic, weigh a lot, make for terrible handling because of weight distribution. it is not only the front of the car which crumples, its the entire car.
    4. And finally, dont trust car designers unless you have proof in YOUR OWN HANDS. We saw how the diesels took a massive turn, and that was not only VW, but mercedes, ford, chevy, mitsubishi, subaru, etc. etc.

  25. Thanks!! I love your videos ! You can explain so good! I like your concept of drowning by hand the graphics and your funny animations!

  26. My car sucks it waste my gas and had spend $80 for gasoline so I’m gonna save money for battery car like Tesla

  27. Can you guys explain the physics of "Ghost Vehicle Crash" where car crashes happens when it apparently hit nothing, what happens there?

  28. Cars were not "super rigid…before the 1950s". What was different was that there was no engineering for the safety of occupants. Cars were bent and deformed in various ways that might very easily intrude into the passenger compartment, but if so, that was just the tough luck of anyone inside. And without airbags or belts, you got tossed around inside – or tossed completely out – to your detriment.

  29. can you please explain how to calculate the force absorbed by vehicle having certain velocity and mass hitting the object. it is very important for my project please respond. [email protected]

  30. Am I fucked, or is he saying 'acceleration' when he should be saying 'deceleration'??? That's what you do when you stop suddenly. You decelerate. Check ya facts.

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