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Bumper Fishing.

  1. Aug 14, 2009 #1
    Hello everyone,

    I just joined the forum because my friends and I have been having an argument for a couple hours now over magnetism and its uses.

    So I will just jump right into our argument...

    I say that an electromagnet attached to the front bumper of a vehicle can be attached to the back of another speeding vehicle and used to bring it to a standstill by applying its brakes to drag the other car to a stop.

    They say that it is impossible for a electromagnet to be strong enough to hold another vehicle hard enough to perform said maneuver over ANY length of time.

    I have tried to prove this through simple math, but they will not accept that and want someone else to prove me wrong (or right).

    The values for everything are... (V1 is the vehicle with the magnet and V2 is the other)

    1. V1 has a magnet core of UP TO 1 meter in width, and 2.5 meter tall. This can have any kind of wire wrapped around it you see fit, and as many wraps as is realistic for an object that size.

    2. V1 can supply a maximum of 350 amps to the magnet coils.

    3. V1 and V2 weigh the same at ~4,000lb.

    4. V1 and V2 are traveling at 70mph.

    5. V2 has an engine that is capable of putting out 300ft/lb of torque.

    6. V1 and V2 both have the same amount of traction.

    7. V1 has brakes strong enough to stop its tires from rotating (Skid).

    8. We will assume the closest that the magnet can get to any materials it can pull on is 0.1524 meters (1/2 a foot) which degrades its capabilities.

    That's it... so if anyone wants to try and prove this situation either way (mathematically) please do.

    Thanks to everyone for reading this.
  2. jcsd
  3. Aug 14, 2009 #2


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    Let me get this picture straight. You have two cars moving together at 70 mph, stuck together with an electromagnet. At some point, the driver in the back puts on the brakes while the driver in the front removes his foot from the gas pedal. Will the two cars keep moving as one until they come to rest? Is that what you wish to know?

    Also, I don't understand the statement

    8. We will assume the closest that the magnet can get to any materials it can pull on is 0.1524 meters (1/2 a foot) which degrades its capabilities.
  4. Aug 14, 2009 #3
    For the most part, the only difference is that the lead car is still trying to accelerate and does not remove his foot from the gas pedal. My friends say that there is no way that a magnet can maintain enough "pull" to get the car to a stop against its will and that the connection would break.

    Newer cars have plastic covers over their bumpers which provides an air-gap that the magnet would have to work over. It can't get any closer than half-a-foot away from any materials that the magnetic field can grab on to.
  5. Aug 14, 2009 #4


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    OK. The short answer is you are right and your friends are wrong. Before I explain why you need to consider a few things

    1. Force is momentum (speed times mass) change over the time it takes for the change to take place. That's Newton's 2nd Law.
    2. The force that stops the two-car system has nothing to do with the magnetic force because the magnetic force is an internal force. Car 1 exerts a magnetic force on Car 2 and Car 2 exerts an equal an opposite magnetic force on Car 1. That's Newton's 3rd Law. The force that reduces the momentum of the two-car system to zero is an external force, e.g. friction between the tires and the road, air resistance, etc.
    3. The force that reduces the momentum of the front car to zero is the magnetic force and the inevitable air resistance.

    Now follow the logic. Since I am allowed ANY length of stopping time, I choose a very long time, say a few hours. This means that the front car needs a relatively small force to stop it over that time interval, certainly within the capabilities of a magnet that is half a foot from the iron in the front car. So how do I get a long stopping time? By having the cars move on an extremely slippery surface (essentially frictionless) and relying on air resistance to bring their momentum down to zero. In this case the magnet between them will be required to produce a very small (if not zero) force. Let the front driver torque his engine all he wants - he will spin his wheels fruitlessly. Let the back driver step on the brakes - he will just slide.

    Your friends may complain that this is not what they had in mind. However, they opened the door by allowing ANY stopping time. :wink:
  6. Aug 14, 2009 #5
    What they are arguing is that on a real road, the amount strain/weight put on the magnetic link by the one car accelerating and one decelerating would overpower the magnetic field... I.E. the cars would disconnect from eachother.

    Managed to bring one of them around to my POV but the other one is dead set on disagreeing.
  7. Aug 15, 2009 #6


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    A very icy road with a film of water on top is no less "real" than a dry concrete surface, but I will not push the point. From some "Back of the envelope" calculations that I did, it seems that who is right and who is wrong depends on details and assumptions. For example the coefficient of friction between rubber and dry concrete is listed as 0.5-0.8. If you assume the more slippery 0.5, you might be able to keep the cars linked. Also, the lifting power of electromagnets is quoted assuming direct contact. At 1/2 foot distance, one has to assume a certain reduction of that power, but exactly what? It depends on the magnet design.

    What I'm trying to say here is that there is no physical principle that says that you friend cannot be right as there is no physical principle that says that you cannot be right. The devil is in the details.
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