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Five dead, head-on Prius and Blazer

Discussion in 'Prius, Hybrid, EV and Alt-Fuel News' started by bwilson4web, Oct 10, 2010.

  1. qbee42

    qbee42 My other car is a boat

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    It's not just confidence. I have a 4.0 in physics, all the way through atomic and nuclear physics. I'm not being pedantic here, I just want to see correct physics.

    Correct. Since this is classical Newtonian physics we don't have to worry about the reference frame. The Earth's rotation and translation through space is irrelevant, except when it is used as part of the total system. If you confine the total system to the two cars you are okay to ignore the Earth. If you choose to include frictional braking and acceleration, you must also include the Earth in the total system.

    Stop right there. We have your mistake. Both cars will eventually stop from friction, but that happens after the collision. The friction to which you refer is an external influence not related to the impact or the calculation of impulse. In relation to momentum, during an ideal nonelastic collision, the only factors are the mass of car A, the speed of car A, the mass of car B, the speed of car B; in other words the total momentum of the system. This total momentum will be the same going into and out of the collision. Immediately after the collision frictional forces will continue to slow both cars, but at that point the collision is over.

    Obviously, during a real collision some of the frictional forces will start during the collision. This will cause a *small* deviation from the theoretical values, but not enough to worry about for a practical discussion. If we want to include these in the momentum calculation we have to include the Earth.

    Momentum is always conserved in a non-nuclear reaction.

    Yes. The total momentum is conserved. Friction slows the Prius, but each force has an equal and opposite force. The opposing force in this case is applied to the Earth. As your Prius slows down its momentum is transfered to the Earth. Obviously the Earth's mass dwarfs that of the Prius, so the change in velocity is very, very small. The total momentum of the system is the same before and after the frictional slowdown.

    Tom
     
  2. fuzzy1

    fuzzy1 Senior Member

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    I haven't been able to read the whole thread yet, but I hope you meant higher mass (compared to the opponent), not higher speed.
     
  3. qbee42

    qbee42 My other car is a boat

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    Higher mass is always safer, everything else being equal. The higher speed comment was in reference to an earlier poster who made an unsubstantiated claim about it being safer to speed up than slow down. Obviously this is not generally the case. I was pointing out that there are special cases where higher speed reduces the impulse imposed on the occupants in the higher speed vehicle. You aren't going to experience any of these special cases in ordinary automobile accidents, since the participants are well matched in speed and mass.

    Unfortunately, this led to some confusion about elastic and inelastic collisions, conservation of energy, and conservation of momentum. My correcting these errors only led to further confusion and further discussion, none of which directly related to the topic.

    We may need to add another forum on PC: Basic Physics. That way the rest of the community can get on with useful Prius discussions while a few of us physics sticklers beat each other over the head about minor details. It would be a win-win situation, except for the wasted server space. :D

    Tom
     
  4. austingreen

    austingreen Senior Member

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    And yet you have failed to set up the problem right. Maybe I need to use your magic words. In the open system of two colliding cars because of external forces momentum is not conserved. Furthermore useing the conservation of momentum formula adds nothing to our knowledge of how these cars collide. You used conservation of momentum incorrectly as in the first set up of the problem we are considering adding energy and momentum to the collision.


    Well since in the real world collisions take time and there is air resistance, friction, and even flying parts. As the tires lock up and the body crumples friction bleeds away energy from the open system. For most of us observers there is near 0 final speed at the end of the collision. Since you are including the part of the collision when the crumpled prius is being pushed back you must be including enough time for momentum to leave the system in a substantial way. In the open system conservation of momentum is not true, but using the energy content of the bodies, friction, and deformation, a velocity profile can be made. Checking out your math with crash test dummies is also important. The important figure for crash damage is dp/dt and crumple zones definitely help reduce the magnitude.

    Now if you want to model the universe in this crash scenario go ahead. You will not get any more insights into what happens, but you will need to work through a lot of equations that are not easily solved. Let me know the benefit of this. Maybe I just forgot to tell you the magic words "open system" when I said forces and you would have noticed you set up the equations in an improper meaner. I thought giving you a thought experiment would have helped you realize what was going on.

    Finally for your pedantic tone. In relativity momentum is conserved, but the formula for momentum shows that clasical momentum is not conserved. This is because the mass of an object can change in relativistic physics. This is what I was referring to when I commented about being carefull in using the term always.
     
  5. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    wow, lets see if we can split the hair 5 ways then everyone will be happy!!

    speed kills, PERIOD.

    why? well we dont need physics to understand. the slower you drive the more reaction time you have, the better your chance to win the battle against lack of friction, inertia and the direct transfer of power in a 180º vector collision.

    facts are, if going slower, its nice if you can , maneuver enough to glance off instead of head on.

    the faster you go, the less time you have to make any of that happen
     
  6. qbee42

    qbee42 My other car is a boat

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    Exactly. And in any real world car crash, extra speed will also translate into larger impulse and greater dissipated energy. In plain words, that means you are more likely to be crushed or turned into red Jello.

    Tom
     
  7. macmaster05

    macmaster05 Senor Member

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    LOL I cant believe you're all still having this argument. I would speed up no doubt. And then jump out of the car. Tata!
     
  8. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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  9. macmaster05

    macmaster05 Senor Member

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  10. DaveinOlyWA

    DaveinOlyWA 3rd Time was Solariffic!!

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    well there are SEVERAL things i would think that no one would do, but i am constantly proven wrong
     
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  11. nerfer

    nerfer A young senior member

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  12. DetPrius

    DetPrius Active Member

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