Sully's8
Member
Appreciate the Sarcasm. I have 8 kids and I'm doing all the research I can for lots of reasons. Good idea about walking or biking if the alternative is a Prius.
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Appreciate the Sarcasm. I have 8 kids and I'm doing all the research I can for lots of reasons. Good idea about walking or biking if the alternative is a Prius.
UberEV1
Member
Thank you for posting - this is an important distinction. I grew up hearing that two cars heading toward each other at 50 mph is just as bad as a car driving 100 mph into a brick wall, which is not true (as @stopcrazypp points out). This comparison test was shown on one of the Mythbusters episodes and it clearly shows the life-saving benefits of cars with well-designed crush zones (energy absorption). In short, two similar cars colliding have twice as much energy absorption (crush zone length) vs. one car against a brick wall (or sturdy tree). If you get a chance to see that Mythbusters episode, it will make you appreciate the Tesla's frunk all the more. Be safe out there!
EarlyAdopter
Active Member
For a point of comparison, here's what happened when a Mercedes, a very safe car, hit a tree at a high rate of speed:
Tragic accidents, all of these (plus the aforementioned Paul Walker accident). High rate of speed plus a tank full of energy, be it gas or electricity, can result in fire. It just happens so often in gas cars they only get reported when it's someone famous.
Quick bit of physics to think about:
Gasoline contains 33kWh of energy per gallon. A average sized 16 gallon gas tank thus contains 528kWh of energy that is extremely fast burning, and if it vaporizes is highly explosive.
A Tesla battery contains up to 100kWh of energy. That's 1/5 the energy available for a fire. Plus, that energy is divided up into 16 separate modules. You need to seriously compromise the battery to release multiple modules of energy at once. If you do that much damage to the car (remember, that battery is seriously reinforced) odds are you'd be dead from the collision first. And in a gas car, you'd really be as you don't have that extra reinforcement under the passenger compartment.
Please drive safely everyone, and do not drink and drive.
Tragic accidents, all of these (plus the aforementioned Paul Walker accident). High rate of speed plus a tank full of energy, be it gas or electricity, can result in fire. It just happens so often in gas cars they only get reported when it's someone famous.
Quick bit of physics to think about:
Gasoline contains 33kWh of energy per gallon. A average sized 16 gallon gas tank thus contains 528kWh of energy that is extremely fast burning, and if it vaporizes is highly explosive.
A Tesla battery contains up to 100kWh of energy. That's 1/5 the energy available for a fire. Plus, that energy is divided up into 16 separate modules. You need to seriously compromise the battery to release multiple modules of energy at once. If you do that much damage to the car (remember, that battery is seriously reinforced) odds are you'd be dead from the collision first. And in a gas car, you'd really be as you don't have that extra reinforcement under the passenger compartment.
Please drive safely everyone, and do not drink and drive.
I recall an early story about this crash said an eye witness reported seeing the Tesla going significantly over the speed limit, possibly as high as 100 mph just before the crash. The equation for kinetic energy of an object is 1/2 m V^2. The mass of a Model S is high for a car, but the energy goes up by the square of the speed. Taking any car moving at 100 mph and ram it into an object that slows most of it down to a stop in a very short distance and the car is going to disintegrate. About the only thing that wouldn't is a fully armored vehicle.
One of the first fatal Model S crashes was a car stolen from a service center in Los Angeles. The joy rider got up to 100 mph on surface streets avoiding the police when he hit several parked cars. The Model S was torn in half and one part of it ended up embedded in the wall of a synagogue 6 feet off the ground. The crash last year in the Netherlands where a Model S hit a tree at over 90 mph also spread parts everywhere.
But this isn't unique to Tesla. There are websites dedicated to pictures of wrecks of expensive sports cars. They all end up in similar conditions. Several ICE super cars have caught fire from the engine revving too much while sitting still. Take just about any car in the world capable of 100 mph, get it going that fast and ram it into a tree. The results will be at least as bad as the Teslas that have hit stationary objects at high speeds. The Tesla stories just get more reporting coverage.
It's like news stories about anything that kills people. In the United States more people get killed by vending machines falling on them than foreign terrorists. Lawn mowers kill way more people than both. If vending machine accidents and lawn mowing accidents got the kind of coverage that racially or religiously motivated killings get, nobody would go near a vending machine and every house would have grass 2 feet high.
There was an early Model X crash the driver tried to blame on autopilot that happened on an interstate. I believe the first thing they hit was a jersey barrier in a left side offset hit which badly damaged the front wheel and the car continued on for a ways before stopping. Both occupants walked away from the accident with possibly minor injuries.
I believe in the case of two cars going 45 mph having a head on collision, the energy involved would be the equivalent of one car going 90 and the other stationary. As speed goes up the energy released when it stops suddenly gets frighteningly huge. It's why airliner crashes usually result in a field full of barely recognizable bits of things and occupants.
The unmovable and very solid object in the wall or tree scenario makes a big difference, but it would be valid to compare two cars going to the same speed head on vs one car going twice the speed vs the other standing still in the same head on. The dynamics of the crash would be a bit different but the energy involved and the energy distribution would be most similar than a car vs an object.
RogerHScott
Active Member
Here's a kind of amazing bit of physics to contemplate: if you swing it hard enough you can cut a 4500 lb Tesla in half with a 4" diameter
tree (the relativistic equivalent of driving the car into a stationary tree fast enough). Maybe the display should include a little "vine"-style
video loop showing what will happen to the car if you hit a stationary object at the current speed?
tree (the relativistic equivalent of driving the car into a stationary tree fast enough). Maybe the display should include a little "vine"-style
video loop showing what will happen to the car if you hit a stationary object at the current speed?
SomeJoe7777
Marginally-Known Member
No, this is not correct.
There are two factors at work here:
1) Total energy that must be dissipated. This is 0.5 * m * v^2 for each moving object.
2) Available mass to deform and dissipate that energy once the collision occurs.
Let's do actual calculations with reasonable numbers:
Car #1 (Telsa): m1: 2000 kg
Car #2 (2nd Tesla): m2: 2000 kg
Wall: Immovable, infinitely strong, i.e. no deformation or ability to dissipate energy.
Now we'll look at 5 different scenarios, in order of crash severity (using the dissipation factor as the crash severity indicator):
A. Car #1 @ 100 km/hr vs. Car #2 stationary:
Total Energy = 0.5 * m1 * v1^2 = 0.5 * 2000 kg * 100 km/hr ^ 2 = 772 kJ.
Dissipation factor = Total Energy / Total Mass = 772 kJ / 4000 kg = 0.193 J/g.
B. Car #1 @ 100 km/hr vs. Car #2 @ 100 km/hr:
Total Energy = 0.5 * m1 * v1^2 + 0.5 * m2 * v2^2 = 0.5 * 2000 kg * 100 km/hr ^ 2 + 0.5 * 2000 kg * 100 km/hr ^ 2= 1543 kJ.
Dissipation factor = Total Energy / Total Mass = 1543 kJ / 4000 kg = 0.386 J/g.
C. Car #1 @ 100 km/hr vs Wall:
Total Energy = 0.5 * m1 * v1^2 = 0.5 * 2000 kg * 100 km/hr ^ 2 = 772 kJ.
Dissipation factor = Total Energy / Total Mass = 772 kJ / 2000 kg = 0.386 J/g.
D. Car #1 @ 200 km/hr vs. Car #2 stationary:
Total Energy = 0.5 * m1 * v1^2 = 0.5 * 2000 kg * 200 km/hr ^ 2 = 3086 kJ.
Dissipation factor = Total Energy / Total Mass = 3086 kJ / 4000 kg = 0.772 J/g.
E. Car #1 @ 200 km/hr vs. Wall:
Total Energy = 0.5 * m1 * v1^2 = 0.5 * 2000 kg * 200 km/hr ^ 2 = 3086 kJ.
Dissipation factor = Total Energy / Total Mass = 3086 kJ / 2000 kg = 1.543 J/g.
You can see how much the speed affects the outcome. Physics doesn't lie, and that squared velocity will always catch up to you. Compare scenarios B and D: Same closing speed of 200 km/hr, but with all the speed in one car, the crash is twice as severe as the same collision with the speed evenly split between the two cars.
But the available mass to dissipate the crash energy is the other forgotten factor. Compare scenarios D and E: Same closing speed of 200 km/hr, but without the extra 2000 kg of the other car to dissipate the crash energy, the collision with the wall is twice as severe as the collision with another car.
UberEV1
Member
Very consistent with the Mythbusters outcome (Physics doesn't lie, nor do the results from the actual experiment)
So, another way to look at it, if there is a choice between hitting a car or a wall/tree, choose the car. Choosing to ht the car protects the driver and passengers better than hitting the wall, but has the moral issue of hurting people in the other car needs to be considered vs just hurting people in your own car. All this decision in a split second, if there is an option.
Finally, wonder how these intelligence can be programmed to FSD, and then convince the regularity authorities with consistent results.
Finally, wonder how these intelligence can be programmed to FSD, and then convince the regularity authorities with consistent results.
Super_Popular
Well-Known Member
ped·ant·ry
ˈped(ə)ntrē/
noun
ˈped(ə)ntrē/
noun
- excessive concern with minor details and rules.
"to object to this is not mere pedantry"
RogerHScott
Active Member
This analysis seems to ignore time. You can dissipate all the energy you want, harmlessly, if you have enough time. Is the whole
point of a crush zone to spread the absorption of energy out of a longer time, even if it is only a handful of milliseconds rather than
just a couple? One car hitting a wall has only its own crush zone, while two cars hitting each other have two crush zones. Yes,
the total dissipated energy is the same, but in one case it must be dissipated in a much shorter period of time, typically doing a lot
more harm in the process.
stopcrazypp
Well-Known Member
That's true, but misleading. There is a significant difference between crashing into an immovable wall and a stationary car.
For ease of explanation, I will say KE = kinetic energy of a car moving at 45mph.
In the immovable wall case at 90mph, you are dissipating 4KE into 1 car. In the stationary car case, by conservation of momentum, a simple physics model will have the two cars still moving at 45mph immediately after the cars crash, so total energy dissipated is 2KE into 2 cars (in real life the rest of the energy would eventually be dissipated by friction such that both will eventually become stationary, but that is besides the analysis).
Using reference frames can make this more clear:
For the 45mph head on crash, let's assume a reference frame where car 1 appears stationary (assume you are viewing the accident in another car going 45mph in the same direction as car 1). Before the start of the crash, it would appear as if car 2 is moving at 90 mph toward a stationary car 1. Car 2 will have a kinetic energy of 4KE.
However, examine what happens after the crash: from your point of view, it would appear that there are still 2 cars moving at 45mph in opposite direction to you (remember you are in a car moving at 45mph). That means the system still has 2KE left in it (each car has 1KE times 2 cars = 2KE). That means the amount of energy dissipated is 4KE - 2KE = 2KE. Notice this is exactly the same result as when you stand stationary in the middle of where the crash occurs and see two cars head on crash at 45mph.
In the wall case however, the car would be stationary after the crash and all 4KE would be dissipated into that 1 car.
I'm not sure this is correct. Fuel (petrol) is burnt and in effect it's physical state is converted to energy after which there is nothing left. A battery releases the power you mention but still had a physical state that can also burn. A battery fire therefore releases much more energy than the energy we can take out and put back in during normal useage as it's the combustion of the physical battery that also needs to be considered.
The Mythbusters episode mentioned up thread did a great job of explaining what happens when two cars hit each other vs hitting a stationary object. After reading this thread I had to go back and watch that episode, as I had always thought you added the speed of the two cars together when in fact that is not the case.
SomeJoe7777
Marginally-Known Member
No, the time is not an initial condition, it is a result.
The amount of time to dissipate the energy is not directly controllable. The time that the collision will take is dependent on the distance the vehicle moves during the collision, initial velocity of the vehicle, and the amount of deceleration that the crush zone can provide.
Take the simplified example of a point mass, moving at initial velocity Vo, and final velocity Vf = 0:
Vf = at + Vo
or
t = - Vo/a
If the crush zone can only supply a constant deceleration, then higher initial velocities will result in more time to dissipate the energy. More time means more of the crush zone will crush, and thus more damage to the car. But the time here is the result, not the initial condition.
Note: The car and its crush zone are actually not idealized like this simple point mass example. The manner in which the crush zone dissipates the energy (deformation, fracture, or a combination) as well as the elasticity of the collision will affect the deceleration and therefore the time.
Sure, you can theoretically say OK, I'm going to bleed the energy away very slowly and therefore make the crash much more gentle. But how are you going to do that without changing the physical makeup of the car?
Yes, if the car is 500 feet long and made out of balsa wood, then the impact would be very slow and controlled as the first 485 feet of balsa wood takes several seconds to fracture away and slowly bring the 15' passenger compartment to a stop. However, Tesla does not make a Model B.
There-in lies the rub with full autonomy. The choice of lesser evil if a collision is unavoidable and how the software determines which loss of life/property is acceptable. It is the uncanny valley of self-driving.
I agree. If I may predict, the FSD-and-driverless will take a very long time, but most all of the features of FSD might be functional with the condition that driver has one hand on steering - to satisfy regulations. Musk can claim to have done "his" part and wait his time with winning the legal battle.
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