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How Orbits are Maintained

  1. May 6, 2009 #1
    Why doesn't the force of the sun's gravity, cause a drag on the earth and cause it to slow down and fall into the sun?

    For, example, if I have a North Pole magnet, and project(throw) a South Pole magnet around it, no matter what speed and direction of my initial throw, will not cause the South Pole magnet to circle the North Pole magnet perpetually. If thrown too fast, it goes past the North Pole magnet, If thrown too slow it immediately crashes into the NP magnet.

    I don't think there is a speed and direction which would cause the two magnets to orbit.

    Therefore, the idea that the Earth was given a initial velocity, that would put it into perpetual orbit, is not feasible to me.
  2. jcsd
  3. May 6, 2009 #2
    Where are you getting your magnetic monopoles?

    Your experiment is completely invalid because there is no such thing as you think you have.

    You need to look at the concept of conservation of angular momentum.
  4. May 6, 2009 #3


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    You don't need a monopole, you just orient the magnet perpendicular to the object obiting it.

    I don't see any reason why it wouldn't be possible in theory to have a magnetic orbit, but there are a couple of reasons why such a thing would be difficult and not very stable:

    -As you imply, DruidArmy, the region of stability is very small for magnets. This is because the magnetic force is so much stronger than the gravitational force, the distances involved are compressed, yet the laws of motion remain the same. In other words, the inertia of the object in orbit of a magnet has a tougher time keeping it in orbit than for objects in space.

    -I don't know what you envision, but a ball bearing orbiting a magnet (for example) would be rolling and would therefore be subject to friction.

    I don't have any idea what you mean by drag - gravity and drag are completely unrelated concepts. For example, in a circular orbit, the force is perpendicular to the motion, so it can't even temporarily (as in an eliptical orbit) slow down the object in orbit.

    The universe doesn't really care what you believe - it is what it is. But if you don't believe in orbits, you can always pick up an introductory physics textbook and learn the math to prove to yourself that they work. It is high school level math at worst....and thousands if not millions of physicists have done the derivations themselves to prove orbits mathematically possible. And, of course, Newton first did that derivation more than 300 years ago. It works and you'll see it if you choose to do it.
    Last edited: May 6, 2009
  5. May 7, 2009 #4


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    The real question is why you think that magnetic monopoles (which do not exist) have anything to do with gravity!
  6. May 7, 2009 #5
    I never said anything about magnetic monopoles.

    the basic question is doesn't the perpendicular force of gravity on an object cause a slowing down of the object. I was trying to give an analogy, using say a steel ball thrown around a magnet.

    For instance, if you drive a car in a straight line at 80 mph and you have a cross-wind (perpendicular) at 40 mph, doesn't the cross-wind force cause the car to slow down?

    Perhaps this is related to "conservation of angular momentum" but I don't know.

    It seems this pull of gravity would work to slow an object down that was travelling perpendicular to the pull.
  7. May 7, 2009 #6
    DruidArmy, you spoke of having a northpole magnet; that is where the magnetic monopole came into the discussion.

    russ watters, you say that this can be done without a magnetic monopole by just orienting the magnet perpendicular to the object orbiting it. I presume you mean perpendicular to the plane of the orbit. This cannot be sustained without some sort of support because the magnetic field will pull the "planet" out of the orbit planet if it is not supported by attraction to the other pole. If you postulate such a support, then you immediately introduce friction and all of the associated problems. That is why I asked about the magnetic monopole.
  8. May 7, 2009 #7


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    Yes, you did: a "North Pole magnet" is a "magnetic monopole".

    No, it doesn't. A force perpendicular to the direction of motion doesn't change speed at all.

    Again, a force perpendicular to the direction of motion doesn't change the speed.
    Last edited by a moderator: May 7, 2009
  9. May 7, 2009 #8


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    No. A force can only slow an object if it is applied in the direction of motion.

    The only example I can think of where there is a relationship at all is with friction, but in friction, it isn't the normal (perpendicular) force that slows a sliding object, but the resulting friction force: which is opposite the direction of motion.

    This can easily be seen in Newton's laws of motion. Acceleration and velocity are vector quantities, so the equation f=ma requires the "f" and "a" to act in the same direction.
  10. May 7, 2009 #9


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    I did, I did, and I did. I addressed all of that, at least implicitly, when I said the object would need to be rolling (ie, supported by a surface) and therefore subject to friction.
  11. May 8, 2009 #10


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    The compelling evidence against magnetic monopoles is permanent magnets. If monopoles were floating around all over the place, they would be attracted to the poles of permanent magnets, dilluting their attraction to each other. This is not observed.
  12. May 11, 2009 #11

    Interesting, the terms "friction" and "rolling" suggest that the small sphere is in physical contact with the larger sphere ( ie if the ball was in a fluid the common expression would be "drag"). However, you clearly state this is not an "orbit" in the traditional sense, as the magnetic force is not perpendicular to the small sphere's tangential velocity, put parallel with the radial vector ...

    There is a case where the magnetic force is perpendicular and the motion is circular; the common electric motor! In one configuration of a PMDC motor permanent magnets are mounted on the rotor and the armature is a "moving" electro-magnetic field. But the "moving" electro-magnetic field is moving relative to the observer, not the rotor! What is missing from this discussion is Galileo's relativity ( though special and general are involved here too, eg general relativity explains the planets orbit as a geodesic in curved space time -- thus eliminating the mysterious gravity, special relativity the magnetic field as an accelerated electrostatic field)

    Incidentally, in a motor there are radial magnetic forces as well, and they can be larger[tex] ^1[/tex] than the tangential "motoring" forces, but balanced by symmetry of the rotor poles. To convince yourself of this, remember that magnetic field lines are closed loops ( ie no "mono-poles, or [tex]\nabla \cdot \overset{\rightharpoonup }{H} = 0[/tex]). In special cases this "radial" force can be used as "magnetic bearings" ( ie the rotor is truly making no physical contact with the motor housing such that there is no contact friction, it is "floating" in the field, or "orbiting" if you will ... ).

    Also, has anyone ever wondered what happens to all of these "magnetic motoring" forces in the common transformer?

    Another interesting case is a moving electron in a "static" magnetic field. The electron will follow a nearly closed orbit ( eg, the cyclotron).

    [1] actually larger flux, more concentrated in space, denser field lines ...
    Last edited: May 11, 2009
  13. Jun 22, 2009 #12

    D H

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    Why do people teach this nonsense?

    You do not need to invoke centrifugal force to explain the stability of orbits.
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