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A 100km/h head-on with a similar car going 100km/h should be similar to a 100km/h head-on collision with a solid concrete wall. However a car traveling at 200km

by todd8 7y ago

A 100km/h head-on with a similar car going 100km/h should be similar to a 100km/h head-on collision with a solid concrete wall. However a car traveling at 200km/h has four times the kinetic energy as a car traveling at 100km/h so a collision at that speed is much less likely to be survived than a collision at 100km/h.

matt-attack 7y ago
Wow you are the first person to point out what I’ve been saying for decades. A car hitting another car at the same speed is indistinguishable from a car hitting an immovable wall. Intuition wants to think it’s twice as bad. As an aside can I pose another physics car challenge to you that 100% of people I’ve asked get wrong? You’re the only car at stop light waiting for the light to turn green. You look up in your rear view mirror and see a car bearing down on you presumably with a distracted driver. You know you’re about to get rear-ended. In that split second do you: A) Let up on the brakes and put the car in Neutral or B) slam in the brake as hard as you can.
- deleted 7y ago
  [deleted]
- dzdt 7y ago
  The brake will increase survivability for you; neutral will increase survivability for the other driver. What hurts you is the sudden acceleration from the impact, and the brake will decrease that. What hurts the other driver is the sudden deceleration of impact; your brake will make that a harder stop for him while being in neutral would make it a bit less of a hard stop.
  - gamblor956 7y ago
    This is backwards. Your car will accelerate suddenly from the impact either way, so you want to increase the time-frame over which the impact forces of the crash are dissipated. If your car is in neutral, then the crash is less severe because only part of the other car hits you before your car starts gets pushed forward, so more of the energy of the impact gets transmuted to accelerating your car and less energy needs to be absorbed by your car's crumple zones. If you slam on the brakes, you effectively turn your car into a stationary object, and that means you and the other car must absorb the full energy of the collision. At collision speeds where survivability is a concern, this usually means the crumple zones are inadequate to absorb the energy of the collision and so more of it gets passed on to your body. Ideally, you would want to accelerate your car as much as possible before the impact to minimize the speed differential. Even a 10mph decrease in the differential has a huge impact on survivability. [Note: bigger cars can survive crashes better because they have more mass they can act as crumple zones to absorb the force of the crash. Older, more rigid cars would frequently survive with limited damage crashes that killed their occupants, at lower average speeds than today, because the cars transmitted more of the impact forces to the occupants. Modern cars are designed to crumple to dissipate impact forces.]
- esotericn 7y ago
  Surely the answer there depends on what's in the intersection - whether you want to roll into it or not. The answer may well be c) gun it and hope for the best! WRT the two cars crashing - I would bear in mind that immovable objects are quite uncommon, most highway barriers are compressible. So whilst you might be strictly correct I'd imagine few people crash into completely stationary objects like bridge supports without crash buffers.
dsfyu404ed 7y ago
> However a car traveling at 200km/h has four times the kinetic energy as a car traveling at 100km/h so a collision at that speed is much less likely to be survived than a collision at 100km/h. Kinetic energy by itself does not matter one bit when it comes to how damaging a crash will be for the human occupants. People use it as a proxy for acceleration and jerk. Kinetic energy is only actually useful when you're trying to figure out what acceleration and jerk will be if the crash involves multiple moving vehicles or hitting an object that's light enough to move. Imagine a freight train crashing into an immovable something at low speed. The kinetic energy is going to be higher than almost any car crash but the occupants will be just fine and they have far less crash protection stuff around them than anyone in a car. Alternatively, imagine an occupied small car traveling at some low speed crashing head on into a person who was shot out of a cannon at high enough speed for their kinetic energy to be equal. Both parties are going to decelerate over the same amount of time but the person shot out of the cannon will experience much higher acceleration (and jerk) because their initial velocity was higher.
- randallsquared 7y ago
  The kinetic energy of the human is the important part, and higher speed increases that, unlike higher mass of vehicle as you note.

by todd8 7y ago

matt-attack 7y ago

Wow you are the first person to point out what I’ve been saying for decades. A car hitting another car at the same speed is indistinguishable from a car hitting an immovable wall. Intuition wants to think it’s twice as bad. As an aside can I pose another physics car challenge to you that 100% of people I’ve asked get wrong? You’re the only car at stop light waiting for the light to turn green. You look up in your rear view mirror and see a car bearing down on you presumably with a distracted driver. You know you’re about to get rear-ended. In that split second do you: A) Let up on the brakes and put the car in Neutral or B) slam in the brake as hard as you can.

deleted 7y ago
[deleted]
dzdt 7y ago
The brake will increase survivability for you; neutral will increase survivability for the other driver. What hurts you is the sudden acceleration from the impact, and the brake will decrease that. What hurts the other driver is the sudden deceleration of impact; your brake will make that a harder stop for him while being in neutral would make it a bit less of a hard stop.
- gamblor956 7y ago
  This is backwards. Your car will accelerate suddenly from the impact either way, so you want to increase the time-frame over which the impact forces of the crash are dissipated. If your car is in neutral, then the crash is less severe because only part of the other car hits you before your car starts gets pushed forward, so more of the energy of the impact gets transmuted to accelerating your car and less energy needs to be absorbed by your car's crumple zones. If you slam on the brakes, you effectively turn your car into a stationary object, and that means you and the other car must absorb the full energy of the collision. At collision speeds where survivability is a concern, this usually means the crumple zones are inadequate to absorb the energy of the collision and so more of it gets passed on to your body. Ideally, you would want to accelerate your car as much as possible before the impact to minimize the speed differential. Even a 10mph decrease in the differential has a huge impact on survivability. [Note: bigger cars can survive crashes better because they have more mass they can act as crumple zones to absorb the force of the crash. Older, more rigid cars would frequently survive with limited damage crashes that killed their occupants, at lower average speeds than today, because the cars transmitted more of the impact forces to the occupants. Modern cars are designed to crumple to dissipate impact forces.]
esotericn 7y ago
Surely the answer there depends on what's in the intersection - whether you want to roll into it or not. The answer may well be c) gun it and hope for the best! WRT the two cars crashing - I would bear in mind that immovable objects are quite uncommon, most highway barriers are compressible. So whilst you might be strictly correct I'd imagine few people crash into completely stationary objects like bridge supports without crash buffers.

dsfyu404ed 7y ago

> However a car traveling at 200km/h has four times the kinetic energy as a car traveling at 100km/h so a collision at that speed is much less likely to be survived than a collision at 100km/h. Kinetic energy by itself does not matter one bit when it comes to how damaging a crash will be for the human occupants. People use it as a proxy for acceleration and jerk. Kinetic energy is only actually useful when you're trying to figure out what acceleration and jerk will be if the crash involves multiple moving vehicles or hitting an object that's light enough to move. Imagine a freight train crashing into an immovable something at low speed. The kinetic energy is going to be higher than almost any car crash but the occupants will be just fine and they have far less crash protection stuff around them than anyone in a car. Alternatively, imagine an occupied small car traveling at some low speed crashing head on into a person who was shot out of a cannon at high enough speed for their kinetic energy to be equal. Both parties are going to decelerate over the same amount of time but the person shot out of the cannon will experience much higher acceleration (and jerk) because their initial velocity was higher.

randallsquared 7y ago
The kinetic energy of the human is the important part, and higher speed increases that, unlike higher mass of vehicle as you note.