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Forces in Levitation HELP

  1. Dec 16, 2011 #1
    Forces in Levitation HELP!!!!

    If you have a magnet suspended by an electromagnet, and the force exerted by the electromagnet on the magnet is greater then the weight force of suspended magnet, would there be a weight force? . Looking at it using vector forces it would seem that the resulting vector would be acting up which would mean that the weight force wouldn't exists. Im not sure if this is correct but i would appreciate responses.
  2. jcsd
  3. Dec 16, 2011 #2
    Re: Forces in Levitation HELP!!!!

    All objects in Earth's sphere of influence that have mass have a force due to gravity acting toward the center of the Earth. Your magnet does too. The electromagnet creates a magnetic field that, acting on the magnetic field of the magnet you are levitating, creates an upward force.

    Since magenetism is dependent on distance, the closer you get, the stronger the force. Obviously, if you have a light magnet and you want to move it close to the electromagnet that is repelling it, you will have to apply a force to the magnet to overcome the stronger force that the EM is pushing with.

    So you have: EM --------> <--M + <------Finger
    Otherwise, the EM will push the magnet outward until the system reaches a state of equilibrium. That is, until a point where the EM is pushing on the magnet with a force equal to the weight of the magent due to graviy.
  4. Dec 16, 2011 #3
    Re: Forces in Levitation HELP!!!!

    So is there any way to have something levitating where there is no weight force acting downwards. In a state of equilibrium would there be a force acting downwards, or no forces acting at all. If you had an electro magnet set up on a set of scales, levitating a magnet and you let it reach equilibrium, would the weight on the scale increase.
    Last edited: Dec 16, 2011
  5. Dec 16, 2011 #4
    Re: Forces in Levitation HELP!!!!

    I'll answer in bullet points, I find that's usually more clear.

    1) No. There will always be a "downward" force due to gravity. Unless you find a way to make an object have no mass, you will have "weight". A levitating magnet is not weightless, it is simply being pushed up with a force equal to it's weight.

    2) F=m*a is really the sum of the forces acting on an object. So an object with mass, m, that is not moving (having a=0 m/s^2), you would think it has zero forces acting on it right? Well that isn't the way it is. It's the sum of the forces acting on it. So if an object isn't moving that means that there is a force acting opposite the force of gravity, with the same magnitude.

    If you have a system that looks like this:

    EM-------> <---M

    You'll notice that the EM is pushing with a force much greater than the weight of the magnet (indicated by the arrows) so the magnet will see a free body diagram like this:


    The resulting vector looks like this:


    And so the magnet will move outward until the forces balance.

    So when they balance, the resulting, or the sum, vector will behave as though you have:


    When in fact you are really seeing:


    They are equivalent, and both result in zero movement. However it is important to note that the object still has mass, and therefore still has a downward force due to gravity. It is just that that force is being balanced by the EM.

    3) That's a great question, I'm actually not sure. I would imagine so, but because we are using an electromagnet and not simply two regular magnets, I'm not sure. Suffice it to say that at no point will the object lose the downward force.
  6. Dec 16, 2011 #5
    Re: Forces in Levitation HELP!!!!

    Yes, having a magnet levitate another magnet on a scale is no different then having both magnets on the scale itself. In fact its the same forces involved in either case (EM and gravity). What makes a scale possible is the electromagnetic force (largely electrostatic) between the scale pan and the object you are weighing preventing the object from falling through the scale under the force of gravity.

    You are levitating right now standing on the ground, with the electrons in the atoms of your feet repelling the electrons in the atoms of the floor. We call it "contact" instead of "levitation" but its really just a matter of scale.
  7. Dec 16, 2011 #6
    Re: Forces in Levitation HELP!!!!

    So no matter how much power you add to the electromagnet to increase the magnetic field, the scale is always going to read the weight of the EM and levitating magnet.
  8. Dec 16, 2011 #7
    Re: Forces in Levitation HELP!!!!

    Unless the EM is not on the scale as well. if the Em is suspended above the levitating magnet(LM) (by say a cable) and the LM is on a scale the scale can read zero.(if the Em pulls with the exact correct force) What would happen then is the total weight of the Em and LM would be registered on the cable. Still the same rules as above but the scale could read zero. mag-lev trains work similarly. (though kinda opposite:)
  9. Dec 17, 2011 #8
    Re: Forces in Levitation HELP!!!!

    I was thinking of using levitation in trucks, use electromagnets to levitate the load that it is carrying. Increase the magnetic field strength on the EM until the force is greater than the weight and repulsion force, then you wouldn't have a weight force acting down, thus resulting in less wear in tyres etc, but know it seems as this is not possible and the weight would be the same as if the load was sitting on the truck bed itself as they do know.
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