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Why don't attracting charges/planets run into each other?

  1. May 5, 2007 #1
    why does an electron orbit a positive nucleus.. if it is attracted to it should'nt it take a straight path and like stick to it? what's keeping it from doing that? same with planets.
  2. jcsd
  3. May 5, 2007 #2
    When you tie a rope to a rock, and swing the rock in a circle around your head, the same thing happens.

    That is, the inward force (you pulling the rope, gravitational, electrostatic, etc) is necessary to make the object travel in a circle. Anytime you see circular motion, there must be an inward force that always points tward the center of the circle.

    If the moon was made to have zero speed at this instatn, it would begin to fall to earth, gradually at first, then gaining speed (just like you tugging an initially stationary rock). It is the net inward force + velocity around the circumfrence (perpendicular to the inward force) that are necessary and sufficient for circular motion.
  4. May 5, 2007 #3
    I think the answer for planets around the Sun and electrons around a nucleus are different. The answer above works for planets.

    Using the same classical mechanical answer, an electron should spiral into the center of an atomic nucleus in a tiny fraction of a second, unlike a planet around the Sun. The answer wasn't understood until quantum mechanics was developed and Heisenberg's uncerainty principle prohibitted that event. Someone better schooled in QM than me can probably explain it.

    Perhaps an interesting extension of the planet/Sun question is this. . . How stable or uniquely designed are planetary orbits? In other words, would a slightly slower speed, or slighly different trajectory, cause a planet to spiral into the Sun, or spin off into a never-returning vector? Or would it just settle into a more or less eliptical orbit and at different average distance from the Sun?
  5. May 5, 2007 #4
    Think I read recently that they (NASA?) noticed that the earth IS spiraling in towards the sun. getting fractionally closer every year.
    Maybe explains the global warning ?

    Apart from that, I'd say the planets we have today are the lucky ones which didn't fall directly into the sun when our solar system was made. And some masses (large or small) with sufficient escape velocity left the solar system altogether.

    -In other words it's really just luck that we sit in a ~circular orbit around a the sun.
    If we weren't so lucky we simply wouldn't be here would we....?
    Last edited: May 5, 2007
  6. May 5, 2007 #5
    i understand the centripetal force concept.. but i'm thinking in more of like yo-yo terms.. like i can have a rock on a string and pull it up to my hand you know? and also if you spin a rock on a string.. you can't get it done just by pulling on it.. you have to like use your fingers and wrist to change the direction of the force by rotating them.
  7. May 5, 2007 #6


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    That's pretty much it in a nutshell. Makes me laugh my guts out when some idiot Creationist says that it's no coincidence that the Earth is perfectly arranged for humans to thrive on. Of course it is, because we evolved to thrive on it. :rolleyes:
  8. May 5, 2007 #7


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    For circular motion, the radial force applied must be perpendicular to the velocity of the object at all times. This is the reason why you have to rotate your wrist in order to change the direction of the force.
  9. May 5, 2007 #8
    I remember reading somewhere that there are fixed orbits in which an object will stay by nature. these fixed orbits are determined by the mass of the object in the center. If I remember correctly, the earth is actually in the fifth orbit, just that 2 of the lower orbits are empty, which makes us the third planet.

    Please don't take my word for it as I am no expert, I just remember reading something about this. Maybe somebody with more knowledge of this subject can explain it and say if my memory isn't too messed up,. :)
  10. May 5, 2007 #9
    Accelerating charges emit waves of energy. In our solar system (orbiting "gravitational charges"/masses), the energy loss to gravitational waves is negligible. Electromagnetism is a stronger force than gravity, so you'd classically expect the analogous orbiting electric charges (composing atoms) to produce light quickly then collapse. Furtunately the physics of very small scales is different to classical physics.
    Last edited: May 6, 2007
  11. May 6, 2007 #10
    How else would it be possible that the other 14 proto-planets just disappeared? alien abductions? :rolleyes:

    a few digrees more off course and we would have been a fried rock a billion years ago
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