Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Magnets to stop car crashes.

  1. Apr 20, 2010 #1
    No question, I just wanted to share a thought...

    I saw a very old episode of Top Gear last night, where he put magnets onto the front of two model cars, and drove them at each other. Because the magnets were orientated so that they would repel, the cars couldn't crash.

    This was given as a way of preventing accidents. :rolleyes:

    However, if this was done with real cars, although the cars wouldn't touch each other, they would sustain just as much damage as if they had hit.
  2. jcsd
  3. Apr 20, 2010 #2


    User Avatar
    Gold Member

    You are right, but for the wrong reason.

    The concept is seriously flawed. Their magnetic fields do not scale up to the real world.

    But your argument about the amount of damage the car would take even if the magnets worked as intended is wrong:
    1] No they would not. Because
    a] slowing the car over time, even milliseconds (much longer time than an impact) will dramatically lessen the damage (I can push on a wall and it will resist, but if I apply the same force over a shorter time (say, punch it), the wall will simply fail).
    b] the deceleration would be applied to hardpoints on the car (where the magnets are mounted), that are designed to withstand a load. It's equivalent to a shoulder stock for a high-powered rifle.

    2] So what? The goal is to prevent damage to the passengers, not the car. The whole point of crumple zones in a car is to let the car take the force and spread it over time (by crumpling). Spreading the deceleration over many milliseconds would dramatically reduce injuries.
    Last edited: Apr 20, 2010
  4. Apr 20, 2010 #3


    User Avatar
    Science Advisor
    Homework Helper

    [PLAIN]http://www.wired.com/news/images/full/fingerball_f.jpg [Broken]

    There is apparently a fad in the piercings world for implanted rare earth magnets (apart from doing party tricks like picking up ball bearings it supposedly aligns your inner wibble with the cosmic foobar)

    Now if more people had these implanted in their hands and head, strong electromagnets could be fitted into the seat headrest and steering wheel so that when the car was running people would be sitting up and looking forward with their hands on the wheel - rather than drinking coffee, texting, searching under the seat for their phone, putting on makeup or turning round to slap the kids in the back seat.
    Last edited by a moderator: May 4, 2017
  5. Apr 20, 2010 #4
    That would be cool, but it would never work. And then, there's the annoyance of having random metals attaching themselves to your car. That would get real old, real fast.
    But that's a really cool idea.
    And lol to the idea of magnets in the head and hands to stop texting. It would be really useful for teenagers...
  6. Apr 20, 2010 #5
    I had to 'LOL' at this "the wall will simply fail".

    Dave singlehandedly destroying masonry in the noble pursuit of explaining physics. :rofl::rofl::tongue:
  7. Apr 20, 2010 #6


    User Avatar
    Gold Member

    :smile: Maybe, instead of me, I should have used Hulk Hogan or Arnie.

    (BTW, the image in my head was of a drywall wall...)
  8. Apr 20, 2010 #7
    I do not like the idea of using magnets to prevent car crashes, it would just repel the other car forward into another car causing a chain reaction of cars repelling each other...also, this would in no way save the passengers from sustaining damage, it might save the car but based on the laws of inertia the repulsion of the car would have the same effect on the passenger as an actual accident...

    One idea I had about magnets in cars was using them to create "hovercrafts" it would not be all that hard really but it would be expensive, you would need super powered magnets (and a lot of them) and a constant electric field to direct the current...having hovercars through this method would nearly eliminate the need for burning fossil fuels due to the loss of friction and how easily the cars would be propelled...its just an idea that could only happen sometime in the future but I think it could be quite practical
  9. Apr 20, 2010 #8


    User Avatar
    Gold Member

    You must realize the context in which this happening. An accident is already happening. This is where virtually all the damage - and worse, injury - is going to occur, not in any possible subsequent interaction.

    Without exception, the best possible course of action is to
    1] prevent the first collision, and if it can not be prevented,
    2] minimize the speed of impact, maximize the time, and
    3] mitigate the damage in any other ways possible

    This is false.

    A bumper, and the entire crumple zone of a car - indeed any buffer of any sort - allows the car to spread its decleration (from 60 to 0) over as long a time as possible. i.e lower the gs experienced from (say) thousands of gs to hundreds.

    F (the force experienced) is proportional to a (the decleration). Lowering the deceleration (fewer gs) reduces the force to the point where bones don't break and organs don't burst.

    That is what reduces the damage.

    A magnetic repulsion would (in theory) begin this deceleration while the cars are still a non-zero distance from each other i.e. greater than any physical device attached to either car. Take it to a ridiculous extreme. Say the magnets cause decleration to occur one hundred yards from impact. Now you've simply got a hard-brake. It doesn't have to be a hundred yards though - even a couple of feet can mean the difference between life and death.
    Last edited: Apr 20, 2010
  10. Apr 20, 2010 #9
    Not to appear as though everyone here is trying to just contradict you for the sake of it, but...

    The effect would not be that pronounced. You can think of it as an invisible inflatable cushion in front of your bumper. If you drove into someone they wouldn't ping off into the car infront, it would simply slow the impact. (post above contains details)

    And unfortunately most of the thing stopping the car is not friction (rolling resistance fron the tyres), but drag from air resistance.
  11. Apr 20, 2010 #10


    User Avatar
    Gold Member

    Good point. It would be interesting to compare the power requirements of a hovercraft moving at 60mph with a car moving at 60mph.
  12. Apr 20, 2010 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the magnets were strong enough to effect a high speed collision they would also prevent you from pulling into a nose to nose parking space. So we would have to redesign most supermarket parking lots. Also if there was any off center component during the collision you would end up spinning both cars so the end result would be that cars would collide at an unpredictable angle, making it even harder to protect the people in the car.

    Seems like an overall bad idea.

    I guess it would serve to clear all steel parts from the road surface. I can see sending the kids out every night to remove all the nuts and bolts from the front of the car. A muffler may take some real effort. Not to mention the possibility of having to stop while traveling to remove the bigger objects which you would be dragging along with you.

    Again Bad idea.
  13. Apr 20, 2010 #12


    User Avatar
    Gold Member

    You jest. :wink:

    Nobody was suggesting the magnets would be on all the time.

    There are a million ways of faciliating this, but the most obvious might be to have field strength take into account the vehicle's velocity. (max strength on highways, off below 10mph).
  14. Apr 20, 2010 #13


    User Avatar
    Gold Member

    No, the only real flaw with the idea is the huge power requirement to make a field that extends yards out from both ends of a vehicle.

    Loosely, magnetic field strength falls off as the fourth power of the distance. The strength at one foot distance is 1/16th of the force at six inches.

    Try this with two magnets: put + to - and then try to hold them a couple of millimetres apart by hand. The reason it is extremely difficult to do is that the field strength ramps up very rapidly, meaning the tiniest movements result in a very large change in field strength. You can't compensate fast enough. The corollary of that is that field strrengths also drops off very rapidly as you move just a few millimetres farther away.
  15. Apr 20, 2010 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Oh, I though you all were somewhat serious. Now you explain to me that you are just kidding. There is no way you can pack enough copper wire in the front of a car or generate the current necessary to do what you want. The only possibility would be permanent magnets.

    Do do ANY good you would have to have magnet strength similar similar to a wrecking yard magnet. Look at the current requirements for those.

    This is a bad idea, getting worse by the minute.
  16. Apr 20, 2010 #15
    Wait, wouldn't part of a problem be that your own car parts would, like, get pulled out? The magnet would have to be strong enough to repel cars, so wouldn't that total your car?
  17. Apr 20, 2010 #16


    User Avatar
    Gold Member

    Wait. We have been serious. No one's kidding.

    I thought you were joking because your ideas were (no offense) absurd consequences.

    I see though, that, with your assumption of permanet magnets, your conclusions are not absurd afterall.

    However, I think perm magnets are definitely ruled out, if for no other reasons than the ones you mention.

    Yes, this is what I'm trying to get across.

    It is, but not for the reasons everyone (except you and I) think so.

    The concept is flawed in practice (we simply cannot make a magnet powerful enough), but it is perfectly sound in theory (a magnet, albeit an implausibly compact and powerful one, could indeed measurably reduce accidents while maintaining the safety of the occupants).
    Last edited: Apr 20, 2010
  18. Apr 20, 2010 #17
    quick ?, I don't have the field strength needed for such an application but would not the needed field to dissipate that much energy also tear apart the vehicle? I think of those people who shrink coins with magnetic fields. Then what about the EMF wave?
  19. Apr 20, 2010 #18
    Okay, this is assuming we are trying to PREVENT an accident from ever happening, but if the affect were to be pronounced like bumper to bumper traffic with some drunk guy not noticing that the cars ahead aren't going very fast, the guy sitting in bumper to bumper traffic is going 4.2 meters per second, the other guy is going 25 meters per second, in order to PREVENT this accident over a magnetic field of X distance, the affect would have to be profound (approximately 31,200 Kilograms m/s of force if both cars weigh 1500 KILOGRAMS) and in fact, ALMOST as bad as the original accident...I had a long drawn out equation to show you but physforum logged me off for some reason and wouldn't resend my info...*sigh*

    You also must assume that it would be stopping the car within 3 or so feet, or else you would have to do away with the entire lane system and have it be one lane all the way through because the magnetic field of cars going side by side would repel each other and cause more problems then good...

    we cannot assume we know the situation the other person is referring to, but if we were trying to LESSEN an impact then there are more practical ways of doing it that wont affect every aspect of driving, IE a sensor you can turn on and off at will that, upon having an object enter its sensory area deploys an air bag off of the bumper that slows the "infiltrating" object down before impact...

    having magnets do the job is a BAD idea
    Last edited: Apr 21, 2010
  20. Apr 20, 2010 #19


    User Avatar
    Gold Member

    You know, I hadn't thought of that. In heavy high-speed traffic, cars will be pushing off each other like bumper cars. Anytime there is an imbalance in speeds, so that cars don't stay at a constant distance, you would, at best, be nudging the car ahead of you and/or being nudged by the car behind you.

    This is extremely dangerous.
  21. Apr 20, 2010 #20
    Would it be possible to absorb some of the kinetic energy of before impact by using electromagnetic induction? If the colliding system could act like a transducer, could one car convert some of the kinetic energy into electrical energy using some type of solenoid configuration?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Magnets to stop car crashes.
  1. Three car crash physics (Replies: 17)

  2. Car crash (Replies: 1)