Five dead, head-on Prius and Blazer

Why would you want to slow down before a head on collision? You stand a much greater chance of survival if you accelerate to match or exceed the SUVs force.

 

This is not true. The faster the impact the more damage there will be. The laws of physics apply to even the Prius.

 

Charge up the flux capacitors and accelerate to 88 mph, and your chances improve even more.

 

You're telling me about the laws of physics (and wrongly so)? Let's theoretically say that this is an elastic head on collision, so all momentum is conserved. Since momentum = mass * velocity, the sum of total momentum upon impact will travel in the direction of the SUV. The total momentum will be the same as before the impact, however.

Now in the real world, car collisions are inelastic since the kinetic energy after the collision would be lost due to thermal and sound energy, but the same principles apply and my point still stands: If the Prius (smaller car) increases its head on velocity the force applied by the SUV is decreased.

 

The faster you go, the greater everybody's delta-V in the collision and the greater everybody's injury.

Nothing in your 'physics' description points to lesser injury at higher speed.

 

Think of it this way: After impact, the SUV decelerated only a bit, while the Prius not only lost all forward speed, but was propelled backward. Ouch!

Yes change of force and momentum is shared evenly by both Prius and SUV, but the direction is key. And direction is dictated by momentum. You don't stay in one place after you crash.

 

Think of it this way: after-impact direction is completely irrelevant, sudden delta-V is what kills.

If the SUV weighs twice as much as the Prius, and both are going 60 mph, the SUV will suffer a 40 mph speed change at impact, while the Prius suffers 80 mph speed change.

If the Prius speeds up to 120 mph before impact, it won't be thrust backwards at all. But the SUV will have a speed change of 60 mph, the Prius 120 mph. Both sides lose out, suffering greater injury than at the lower speed.

 

Speeding up in the Prius wouldn't point to lesser injury, but lesser impulse.

Ok theoretically we are both correct. If you slow down in the Prius you're toast. If you speed up, you're toast. The SUV will always kill you because its metal is stronger, heavier. Lol.

 

we are barking up the wrong tree. a hard left at highway speed would not turn the car anyway. it would go into a sideways skid and mostly have a t-bone collision.

i am fairly certain that the critical role power steering plays means that several redundant safety features are built in. i have heard of several accidents where drivers claimed their car did something it was not supposed to do but have yet to hear about one i believe.

fact of the matter; as a race we SUCK at safe driving.

 

we are barking up the wrong tree. a hard left at highway speed would not turn the car anyway. it would go into a sideways skid and mostly have a t-bone collision.

i am fairly certain that the critical role power steering plays means that several redundant safety features are built in. i have heard of several accidents where drivers claimed their car did something it was not supposed to do but have yet to hear about one i believe.

fact of the matter; as a race we SUCK at safe driving.

 

False. Speeding up will increase the impact impulse.

Slowing down will reduce the toasting, speeding up will increase it.

Previous crash postings here have included head-ons where the Prius driver survived just as well as the SUV driver. But be careful about unsecured loads striking the head from behind.

 

Momentum is conserved either way. Both both elastic and inelastic collisions conserve momentum. Energy is conserved only with elastic collisions. Automobile collisions are largely inelastic, due to crumpling.

Assuming a perfectly protected cabin and a true inelastic collision, it is correct that higher speed is safer. In this sort of collision, higher momentum equates to lower changes in velocity. Think of an extreme case to make it easy to understand: You are driving along in your Prius at 70 mph when a bug traveling the other way strikes your windshield. The bug is now traveling 70 mph your way, while you are unhurt although annoyed by the grease spot on your windshield. In this collision momentum was conserved. The Prius, being much faster and much heavier, completely dominated the momentum battle and won.

The same thing is true with collisions between vehicles. Heavier and faster vehicles have more momentum, and as a result, will suffer less delta V during the collision. This is often referred to as "impulse".

Here is where the argument starts to break down: In the real world, no car has a perfectly protected cabin. All of that crumpling tends to push things back into the cabin, at which point some of that non-conserved energy goes into rearranging human body parts. The impulse may be lower, but the higher initial kinetic energy makes up for it by driving the steering wheel through your heart.

Without detailed analysis it's impossible to say which is worse for any given collision situation, but given my choice, I will start with less kinetic energy. In general, I worry more about intrusions into the passenger space than specific impulse. Likewise, lower speeds allow more time for maneuvering. In the event of a collision, I'm hitting the brakes instead of the gas.

Tom

 

Do a little thought experiment, and you can quickly come to the correct conclusion. Assume that your car is hitting a brick wall, in an elastic head on collision you will be going the same velocity in the oposite direction. Double your velocity and you will double the change in velocity. OK, now you can get to the real inelastic case by allowing your brick wall to move.

No not at all. Energy was used to squash the bug, momentum is not conserved. To keep the same momentum power must be applied to the vehicle. This power is little since the bugs mass was small. Hit a bigger animal and what happens?

if you increase velocity of either vehicle, accelleration will increase in the direction of the other vehicle for both vehicles. It is this acceleration not the velocity that is dangerous. If you accelerate your car through the accident it would be the only way to reduce this in an elastic collision. But these collisions are not elastic, and depending on how the crumples to absorb energy, and the seat belt and air bag are deployed to hold occupants in place, and the collection of objects in the car will determine how safe they are. More speed only adds energy to the system that must be absorbed.

No detailed analysis is required faster is less safe in the real and theoretal worlds. Only adding more power to the system would calculations need to be made.

 

In both the elastic and inelastic case the brick wall does move, as momentum is always conserved in any collision. Granted, the wall doesn't move very fast, as it is attached to the earth and the mass of the earth is very large when compared to the mass of the car.

Momentum is conserved. Momentum is *always* conserved. This is very basic physics. If you don't understand that the rest of this discussion is pointless.

When the bug and the car collide, the total momentum of the system is the same before and after the collision. The car is slowed by a tiny amount as it absorbs the momentum of the bug.

The total kinetic energy of the system is not conserved, as this is an inelastic collision. Some of the kinetic energy is used to transform the bug into glob of goo. Obviously the total energy is conserved, so if we add the energy used to transform the bug to the resultant kinetic energy, it will equal the starting kinetic energy.
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This section is very confused. I can only guess at your intention. I suspect you are trying to refute my point about higher speed producing lower impulse for the faster vehicle. This all comes down to the speeds and masses of the involved vehicles, and the degree of elasticity of the collision. As I said in my previous post, for any real world collision I would opt for lower speed.

Tom

 

yes, basic understanding of physics seems to be lacking.

How is this for the second time around? momentum is conserved unless a force is exerted. The original misguided post to this speed up in an accident means apply force (accelerate the car) in increase momentum of your vehicle.

In inelastic collisions momentum may or may not be conserved depending on the work the forces did during the collisions. When I accelerate a car at a wall, I do not need to take into account the way my car moves the earth, but yes you can build your system as large as you want. In more advanced physics you find that even mass is not necessarily conserved. Always and never are luxuries physicists don't often have. When we get to cars the friction of the tires, the spring rates of the bodies and energy absorbing natures all have an impact on modeling.

 

Still wrong. Momentum is always conserved, regardless of "the work the forces did during the collisions." You must be thinking of energy. Work during the collision consumes energy, but not momentum. This is why energy is conserved in elastic collisions, but not inelastic ones.

You are correct that increasing the speed before the collision increases the kinetic energy and momentum. Likewise speed decreases the length of time for crumpling in a car collision, which increases the impulse. Yet another reason to not be going fast in an accident.

I think I made this pretty clear already, but just in case it got lost somewhere along the line I will state it again: In real world garden variety car crashes, I would rather be going slowly. Speed dramatically increases the risks, and not just because of the increase in kinetic energy. All this talk about speeding up before you hit is only academic discussion, pertaining to rather specialized cases.

The physics of collisions is interesting. Someplace along the line you have become confused about conservation of momentum. Other than that, I think you have it.

Tom

 

I'm kind of surprised Bob. Anything that was recorded before the 12V was disconnected can be recovered providing the Flash chip itself is not destroyed, and it's probably in a hard case of some kind. I'm sure they've anticipated this kind of need, so I would bet they also write continuously to Flash, and likely have a cap backup to finish the last write after 12V is lost, so I would expect that they would have full data up to the point of crash.

 

ugh. I've got to admire your confidence. Let me try one more time. How about brakes for a way to use energy, you have to admit that its possible gravity and friction are involved on earth. You also do not need to know the speed of rotation of the earth or if it changes to compute these things. Pick parked car A. You hit it with your car B and N miles of hour. If you are going fast enough you may move car A, but unless you are adding more energy to the system you both will eventually stop, so the system car A + car B momentum will be reduced. If you speed up before hitting the parked car will your impulse be better or worse? OK now lets say the car is moving toward you instead of parked? I'll let you play with the scenarios. In the system of the 2 cars do you think momentum is still always conserved even if forces (braking or acceleration) is applied?

So just for your thought experiment a body in motion will remain in motion. Will your 60 mph prius remain at 60mph in neutral if it doesn't hit anything on a flat road? Why is the momentum not conserved?

 

Mythbusters had an interesting segment on much the same principle. They staged an accident where two vehicles hit each other at speed x, and they stated it was the equivalent of one vehicle hitting a brick wall at speed 2x. Numerous people, more learned than I, wrote in saying that was incorrect, and actually hitting a car moving in the opposite direction was essentially irrelevant (particularly for vehicles of similar mass I assume). The vehicle in question comes to a stop in the same distance regardless of whether it hits a brick wall or a car head on. So they had a follow-up episode to determine experimentally if this was true and it was. Two cars hitting each other at speed x has the same damage to a car as if it hit an immovable brick wall at speed x, not 2x.

So I agree with Tom, going slower is better. To MacMaster/AustinGreen's point: Any change in the distance covered before coming to a stop (measured in feet at best) is minor compared to the increased energy at the start of a crash due to increased speed. Besides, you're creating more force for the people in the oncoming vehicle, and if the police see you're accelerating into an accident, I sure wouldn't want to try explaining that to a jury also listening to the bereaved parents.

Sidebar: I've heard people say that if you're sitting at a red light and you see you're about to get rear-ended, you should take your foot off the brake to reduce the forces involved. This is true for your car, since your car will transfer some of the force into motion instead of damage, but now that force is also transferred into your neck as your head snaps back into the seat rest. It is always better to keep your foot on the brake, and let your car take the brunt of the accident, not the contents of the car (you, passengers, etc).