Why do rear end collisions cause so many injuries but 11,000 horsepower dragsters accelerating from a standstill cause no damage to the driver?
In: Physics
I think because the acceleration presses you back into the seat, a solid surface, whereas being hit from behind, you go forward where it’s only the seatbelt catching you and your head flops around and your neck hyper-extends
A dragster accelerates to say 60 mph in about ~~two seconds~~under a second. A crash decelerates from 60 to 0 in a fraction of a second ([this](https://www.wired.com/2011/04/crashing-into-wall/) estimates about the crash takes about 1/20th of a second).
A crash involves changing speed ~~40x~~15xish faster than a dragster, which means the force is ~~40x~~15x higher, and the limit where damage begins can be between those two figures.
A dragster still accelerates at a relatively steady rate…it’s ~4g average for a top fuel dragster, less than a typical rocket launch. The *steadiness* is key, as is the fact that the driver is ready for it and thoroughly strapped into a seat designed for it with a harness designed for it and their head against the headrest. There’s very little motion of the body.
In a rear-end collision the acceleration can be much higher, on the order of 16g, because it’s such a sudden impact, and your head generally isn’t constrained…at the time of impact, it’s free in space because we don’t usually drive with our heads back against the headrest.
A top fuel dragster driver has an uncomfortably tight five point racing harness, HANS restraint, helmet, fire suit, and conforming racing seat. They’re focused on the tree waiting to launch.
A Honda Civic driver has a loose three point harness and is looking at their cell phone.
There’s a lot of whiplash that can occur when the car suddenly launches forward unexpectedly and your head isn’t supported.
Collisions cause a sudden change of speed, meaning that there is a short-term but extremely high acceleration (acceleration is change of speed divided by the time in which that change takes place). However high an acceleration a dragster achieves it still falls short of the collision acceleration by a long shot.
Difference forces at play.
A top fuel dragster accelerates to 200 mph in less than 4 seconds. Those forces push the driver into the seat.
In a rear end collision one goes from speed to stopped or vise versa in a much shorter amount of time and as a result, experiences higher forces.
Now, comparing top fuel dragsters to passenger vehicles is a bit of apples to oranges (they are both fruit and they are both relatively round). A more accurate comparison would be that of the stock cars used in NASCAR (50 years ago they were racing actual production vehicles). So let’s take a look at some of the differences in a stock car versus a production vehicle.
1. Crumple zones. Both modern stock cars and passenger vehicles are built with crumple zones to help absorb energy in a collision. There is some additional strucral engineering which redirects force around the cabin (and away from the occupants).
2. Stock cars feature a 5 point harness made up of an anti-submarine belt (crotch belt), lap belt, and two shoulder belts connected at 5 points to the “cabin” (roll cage). This helps keep the occupant’s back engaged with the seat. Passenger vehicles utilize Volvo’s 3 point design made up of a lap belt and torso/shoulder belt. This design is simple, provides relative freedom of movement to the occupant but isn’t perfect as it doesn’t ensure the occupant’s back is fully engaged to the seat (many drivers don’t have their seats positioned properly to begin with), ensure the occupant remains in the vehicle (if the seatbelt fails to lock, it’s totally possible to be ejected in a rollover), nor does it keep the torso from rotating (dangerous to the spine).
3. Airbags. Stock cars don’t have any due to how restricted the occupant is in the harness but it’s been a requirement in passenger vehicles for over 25 years now. The airbag is designed to decelerate the torso and head (and also helps keep the torso squared up and from rotating).
4. Ever since Dale Earnhardt’s death at the Daytona 500 in 2001, the HANs device has been required (Earnhardt was the fourth driver to be killed due to a head/neck related crash injury in a four month span of the 2000/2001 NASCAR season). The HANs device limits the over articulation of the head and neck in a crash by keeping the helmet restrained to a collar that is secured by the shoulder belts. Passenger vehicles have side curtain airbags to help protect the head/neck from injury.
If you have not yet noticed the pattern, race car restraint systems are designed to support the occupants body by restricting free movement. Passenger vehicles do the best they can to restrain the occupant without limiting comfort because if you eliminate the comfort…people will just bypass the restraint system.
In addition to the other good information here, keep in mind that drag racing is hard on the driver’s body.
The g forces they experience -both accelerating and decelerating – take a toll. When the parachute deploys, they slow down so quickly that it can lead to retinal detachment over time.
Think of it like this. When you sit on a train, in your perspective, you’re not moving, but in reality you’re moving with the train so you’re actually traveling with the speed of the train too. Now if the train suddenly stops immediately, you are still travelling at the same speed of the train prior to the crash but since the train isn’t travelling with you, you’ll be propelled forward. And as you can imagine, flying forwards at the speed of a train won’t end well.
The same theory applies to a car. It is also why its so important to wear seatbelts. It prevents you from flying through the window. It is not the crash that kills you, its the sudden change in motion.
**ELI5:**
It’s about how quickly your velocity changes. If you could accelerate from 0 to 100 mph in 1 second, you’d experience ~4.5x the force of Earth’s gravity (4.5g) for that one second.
Impacts from collisions happen on time scales measured in fractions of a second. If you accelerate from 0 to 20 mph in 0.1 second, you would experience 9.1g. If that collision happens in 0.05s, the force would be 18.2g.
That brief pulse of 18.2g will do physical damage that no amount of time at 4.5g could ever replicate (though there may be other physiological effects from prolonged exposure to 4.5g).
If you were to go from 100 mph to 0 in 0.1s, you would experience almost 46g.
**Non-ELI5:**
It’s all about that ΔV/ΔT. Acceleration is change in velocity divided by change in time (a = ΔV/ΔT). Force is mass * acceleration (F = ma) which means F = m * ΔV/ΔT.
The higher the change in velocity and the smaller the change time, the greater the force. However, the ΔT, being in the denominator will have a much larger impact on the final value when the value for ΔT gets small.
Incidentally, this is why you secure human larvae in special seats. If you just tried to hold onto a 17lb larval human (~6 months old), and your car is suddenly accelerated from 0 to 20 mph (or from 20 to 0 mph) in 0.05 second by a collision, holding onto that baby would feel like trying to lift a 309lb object off the floor.
And this is what crumple zones on cars are all about. All modern cars are built with the unibody/frame forming a rigid cage around the passenger compartment that is strong and resists deformation. Everything outside of that cage is intentionally designed to just crumple and deform. Those regions crumpling during the collision increases the amount of time the collision takes and absorbs energy before it gets to the cage (or redirects it). ***If you can double the amount of time it takes to actually crash, you halve the amount of force applied to the squishy humans inside.***
Imagine the difference between breaking into a dead run from standing still, and having someone twice your size shove you as hard as they can.
One of those is a change in movement you are prepared for and largely in control of, the other hits you with more change of speed over a shorter time and gives you no chance to prepare for it.
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