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Magnets or EMF in car bumpers to protect from fender bender

  1. May 26, 2012 #1
    Hi I am new to the forums. I am sure I have found the right place for help with an idea I have been working on for a while. First off please forgive my lack of physics knowledge. What amount of magnetic force would I need to stop or dramaticallyjust deccelarate a standard size 5000lbs car going about 15mph? If there is any other information needed let me know. One I know the magnetic power I can begin designing what type and size of magnets I will need
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  3. May 26, 2012 #2


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    You need to define what you mean by "dramatically" and "just" decelerate. After you have done that, force = mass x acceleration.

    The maximum deceleration you get from conventional brakes (without locking up the wheels) is about 1g, and the force for that would be about 5000 lbf.

    But this isn't going to work unless there is another magnet NOT attached to the car, to repel or attract the magnet in the bumper.
  4. May 26, 2012 #3
    sorry that looks like a typo from my phone :( I ment to say "dramatically decelerate"...in any event...dramatically means slow the car down to about 5mph.

    I have the force of the car , using wolframp alpha, i came up with 30,848 newtons of force. so I guess my next question would be. How do i find out the dimensions or stats of a magnet with that much force? I've been going on magnet websites and trying to find one but i've had no luck because they don't list the power of the magnet.

    I don't understand your second sentence. I'm not really factoring in the brakes in this equation because I want ot focus on the power of the magnets in the car bumper.

    Yes i'm somewhat aware of your third statement. I'm assuming there is a magnet in the opposing car. Would it be possible to attach a device to a car that emitts concentrated EMF's that would slow down an incoming car?
  5. May 26, 2012 #4


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    I was jsut trying to give you some idea of how much force you would need for a a quick (but controlled) deceleration.

  6. May 26, 2012 #5


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    Yay! I've in the past, wondered the same thing; "Why don't they just put magnets on bumpers?"

    And thank you for that link Aleph. An equation from that link


    gave me the answer.

    I :!!) PF!

  7. May 27, 2012 #6
    Can u all put the last couple posts in words I can understand. I don't speak physics D:
  8. May 27, 2012 #7


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    From the equation, and an $800 magnet you can buy * 2 = $1600:

    1.25 B0 is the magnetic flux density very close to each pole, in T
    0.0182 A is the area of each pole, in m^2
    0.0508 L is the length of each magnet, in m (2 inches thick)
    0.0762 R is the radius of each magnet, in m (6 inch diameter)
    1.0000 x is the separation between the two magnets, in m
    3.1416 pi
    0.00000126 μ0 is the permeability of space, which equals 4π×10−7*T·m/A
    5.457 Newtons of force
    0.002 acc = m/sec^2

    So with your vehicle traveling at 15mph, when it is within 1 meter of the stopped car, the force between them would be equivalent to that exerted by me to hold up my cellphone against the force of gravity.

    At 1/2 meter, it's still not looking good:
    73 Newtons of force
    0.032 acc = m/sec^2
    (13.3 cell phones)

    At 1/4 meter, it's looking a bit better:
    849.6 Newtons of force
    0.375 acc = m/sec^2
    (the force required to lift a 190 lb person)

    At 1/8 meter, we're finally talking about some stopping power:
    8,030 Newtons of force
    3.541 acc = m/sec^2
    but it still yields an acceleration of less than a "g", and you've only got 5" left.

    At 1/16 meter, or ~2.5 inches, we get our result:
    58,782 Newtons of force
    25.918 acc = m/sec^2
    Since the acceleration here is greater than 1 g, I would assume that the lateral forces inherent in pushing together two magnets would cause the two vehicles to shift, misaligning the magnets, causing the two vehicles to accelerate towards each other at a high rate.

    And then you will have to get two tow trucks to pull the two vehicles apart.

    Not to mention that you will have to keep children with braces away from your vehicle when shopping. And don't get the shopping cart near it, or we'll have to call the fire department to get it loose.

    Ps. Thank you for the topic. I've been meaning to solve this for years, but never got around to it.
  9. May 27, 2012 #8
    Haha thanks for naming everything for me! I definitely understand now. Man I feel so satisfied I have been working on this idea for months but i hadn't found the right people or books to help me. Would have taken me more months to solve this equation myself!
  10. May 27, 2012 #9


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    You're welcome. And when it comes down to it, if I don't have the supplies lying around my house, and it's going to cost more than $20 to build, I always do the math first.

    Also, the equation is obviously not valid for magnets that are extremely close to each other. As when the distance approaches zero, the repulsion tends towards infinity. And I know that is not correct, as I can with just a bit of effort, push two of my neodymium magnets together.
    (1/8" length, 5/16" diameter)

    At one micron separation, my magnets, according to the equation, should have a repulsion of about 190,000,000 Newtons*, which is a bit more than my thumb and index finger are capable of I'm sure.

    *What is 190,000,000 newtons equivalent to? The first stage of the Saturn V rocket had a thrust of only 34,000,000 newtons. So a 190 meganewtons is a lot!
    Last edited: May 27, 2012
  11. May 27, 2012 #10


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    These are approximate formulas assuming the "magnetic poles" are geometrical points, so they are only accurate a reasonable distance away from the magnet. (Choose your own interpretation for "accurate" and "reasonable").
  12. May 27, 2012 #11
    The second most tricky thing about a magnetic bumper, after scraping kids with braces off the front of your car is:

    Magnets in bumpers would act like a big spring (bear in mind you have to be lucky enough for someone to have a like pole in their bumper). Whatever you slow down by, the car in front would speed up.

    I cant wait for the time that you come to the back of traffic and end up playing car pinball, and punting the car at the front into the middle of a crossroads/intersection.
  13. May 27, 2012 #12


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