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If we could put Earth in a weighing machine

  1. Apr 12, 2014 #1
    ... regardless of any possible technical problems, and then, on earth:

    a) someone throws an object in the air and it falls

    b) someone puts an object to float in a magnetic field generated by an artificial magnet that lays on earth

    In situation A I think the weight measured would increase as the person throws, decrease as the object falls, because of the gravitational pull that the object exerts on earth, and increase again as it touches earth and then return to the same as it was before while everything was at rest, right?

    But in the second situation would the weight measured decrease after the object is put to float or stay the same? Or who knows what, increase?!

    P.S.: this is mere curiosity, it's not about homework or anything.

    Thanks in advance for the answers.
  2. jcsd
  3. Apr 12, 2014 #2


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    If I'm understanding the thought experiment properly, this is the same problem as a standing on a scale, throwing an object into the air and catching it; and using a magnet to support an object. The earth is just a whole lot bigger, but the principle is the same.

    You are right about case A.
    In case B, the weight of the object will register: the magnet is pushing the object up, so there will be an equal and opposite downwards force on the magnet.
  4. Apr 12, 2014 #3
    What if the so called artificial magnet generated an electromagnetic wave strong enough to make the object float instead (if it is even possible to make something float with an eletromagnetic wave), would the result be the same? Because it sounds a little bit strange for me. I mean, how could the magnet "know" that there is an object that was hit by it's eletromagnetic wave and then "feel" it's weight?
    Last edited: Apr 12, 2014
  5. Apr 12, 2014 #4


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    That makes no sense - objects don't float on electromagnetic waves.

    It is possible to make an object "float" by supporting it with a magnetic field, and that's the case I was describing in the previous post: The magnetic field pushes the object up and the magnet down.

    In fact, that's what's really going on even when you think you're holding the object in your hands. When one solid object is pressing against another (a weight and your hand, for example), the force comes from the electromagnetic interactions of the individual atoms.
  6. Apr 12, 2014 #5
    An electromagnetic wave is nothing more than a lot of perpendicular magnetic and eletric fields that propagate in space, right? If you can create an stationary eletromagnetic wave of high frequency wouldn't it be able to make an object float?

    In that case it's not a magnet that creates the magnetic field, but the variation of the eletric field that creates the magnetic field. So in that case if the object does float where is the reaction force?
  7. Apr 13, 2014 #6


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    What do you mean by the "weight" of the earth? The earth is in orbit around the sun so it's "weightless" just like an astronaut in orbit around the earth.

    Do you mean the apparent weight of the man throwing the object? That would increase/change due to Newtons law.

    How is this any different to using a spring or a metal post to make the object "float" in the air?
  8. Apr 13, 2014 #7
    I said regardless any technical difficulties... obviously it doesn't make any sense talking about earth's weight, once any weight we experience is due to earth's gravity. But that's not the point. It's a mental exercise.

    Well, it is different because, in these cases, the object is not floating but being hanged/supported.

    Anyway... do you maybe know the answer for my last question, about the stationary electromagnetic wave? I'm really curious.
  9. Apr 13, 2014 #8
    The weight you would measure for your transmitter would be it's own weight plus the reaction force from the energy it is emitting. The presence or absence of an object in this scenario is irrelevant.

    google "radiation pressure"
  10. Apr 13, 2014 #9


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    They are both being supported, just in different ways. Is there really so much difference between:

    a) Something being supported by a magnetic field.
    b) Something being supported by an electrostatic field (that binds atoms into molecules in the spring).
  11. Apr 13, 2014 #10


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    No, an electromagnetic wave is a wave in an electro-magnetic field.

    No, it wouldn't. An electromagnetic wave is NOT a magnet nor a magnetic field.

  12. Apr 14, 2014 #11


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    I think I see where you're coming from. The point is that the object wouldn't suddenly be 'there'. It would have been moved into place, from a distance ('infinity'). During the movement, there would be a change in the magnetic fields. The electrons in the electromagnet and object would be mutually affected, due to induction and energy would be transferred, in or out, depending upon the nature of the object. The field must be alternating to achieve repulsion between an electromagnet and an unmagnetised object. A spring doesn't "know" that the weight has been hung on it - there must have been some movement to get the weight on the hook.
    Alternatively, the electromagnet could be just switched on, in which case the energy in its field would be influenced by the presence of the obect - possibly lifting the object.

    My point is that the energy in the magnetic field is different, with or without the object being there.
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