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Why Don't the planets fall into sun.

  1. Aug 26, 2011 #1
    May be this question has been asked multiple times here i was just unable to search it by topic or search term.

    The Question is if every thing is falling towards the sun (i.e planets). Why planets are not falling towards sun in a spiral fashion and ultimately crash into sun ? This may be because of centrifugal force but how is that force maintained ?
  2. jcsd
  3. Aug 26, 2011 #2

    Andrew Mason

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    The planets do not fall into the sun because they are moving too fast in the tangential direction. As they fall toward the sun they travel tangentially just enough that they never get very close to the sun. They fall around it, in effect.

  4. Aug 26, 2011 #3
    Inertia (Newton's first law). Approximating the orbit to be circular for the sake of discussion, a planet feels a gravitational force only towards the sun, but not in the direction tangential to the circular orbit. With no force in this direction, its own inertia carries it forward continually in this direction.
  5. Aug 27, 2011 #4
    Technically, it could be argued that they ARE falling into the sun. However, as pointed above, they are indeed traveling too fast to do so in a spiral fashion. This works in the following way:

    Object A pulls on Object B
    Object B is travelling fast enough to orbit
    As Object B travels, it is pulled towards object A
    As it is pulled towards Object A, it moves forward so that for every meter it is pulled, it moves so the curvature of Object A makes it still the same distance.

    Essentially, it's because it's going fast enough to the point that it can't escape, but it can't fall in all the way in either. By the time the object WOULD have been pulled onto the other's surface, the other curved so that it's still above it. Basically, just swing a spring with a weight attached. If the object is going fast enough, the spring won't stretch out, unless it goes faster. But if it slows down, the spring will pull it in.
  6. Aug 27, 2011 #5
    Remember how one of Newton's basic laws is that an object continues at the same velocity in the same direction unless a force is applied to it, since gravity applies a force upon the mass... the curve that is created is produced by gravity changing the direction of the object, which takes a force, and creates a centrifugal force. When the centrifugal force becomes equal to the force of gravity from the object, the object being pulled travels in an orbit.
  7. Aug 27, 2011 #6


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    On the scale of the solar system, the sun is an incredibly tiny target. To fall in, you either need perfect aim, or something actively deflecting you from a free-fall trajectory, I believe.
  8. Aug 27, 2011 #7


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    It's not that tiny. About 1/100th AU in diameter means it's going to take up a small, but not insignificant portion of the system's total cross-section. That's certainly enough to ensure that if it wasn't for spherical symmetry of the potential, everything would have ended up in the Sun by now.

    In short, saying that things don't fall into the Sun just because Sun is small compared to the size of the system is at least incomplete. That's not the only reason.
  9. Aug 28, 2011 #8


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    Sure; the main thing I was trying to do is distinguish from the case of someone standing on the Earth or in low Earth orbit, for which the Earth makes a rather large target, and you have to go through great lengths to manage to miss it.
  10. Aug 31, 2011 #9
    Yes so simple i just forgot newtons first law. this makes complete sense.
  11. Aug 31, 2011 #10

    Thanks, Awesome Analysis.
  12. Aug 31, 2011 #11
    You can also think of it like this:

    A planetary body has some momentum. In outer space, unless there is a collision or the body explodes, the momentum is conserved.

    Now, if the planetary body begins spiraling towards the sun, it would have to go faster and faster to conserve its momentum. At some point, however, the momentum in the tangential direction becomes big enough that the planetary body can start escaping from the sun's gravity.

    Then, as it moves further, due to the change in the curvature of the orbit, the tangential component becomes smaller and the planetary body is once again captured in orbit.

    If you want to know more about this, the stability of the planetary orbital motion is essentially what made Poincare famous (well, technically it was implementing chaos into the calculations), and awarded him the Oscar award in mathematics. Just google "three body problem".

  13. Oct 23, 2012 #12
    I too had problem imagining why earth shouldn't fall into the sun, and I still am doubtful about this stuff...

    What if the velocity with which earth revolves around the sun were to be suddenly reduced? Or if earth was given a little velocity having some component in the sun's direction?
  14. Oct 23, 2012 #13

    D H

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    This is wrong on many levels.

    First off, what centrifugal force? There is no such thing in an inertial frame. All you need is gravitation and tangential velocity. You don't need to invoke a fictitious force to explain orbits.

    Now if you insist on using centrifugal force to explain orbits, you can't talk about the object as orbiting the Sun. The object isn't moving from the perspective of a frame in which centrifugal force counteracts gravitation.
  15. Oct 23, 2012 #14


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    Let's say we suddenly cut the velocity of the Earth in its orbit in half. The Earth begins to accelerate towards the Sun thanks to gravity and ends up in a very elliptical orbit with a close approach (Periapsis? Perihelion?) much closer to the Sun than it is now. If we were to suddenly increase it, then the Suns gravity would slow it down as the Earth's distance increased until again, it was in an elliptical orbit with it's farthest approach (Apoapsis? Apohelion?) much further out than it is now. If we were to shove the Earth in the direction of the Sun just slightly, we would again see a more elliptical orbit. Of course if we slow it down far enough the Earth would fall into the Sun, however that would require shedding nearly all of the Earth's orbital velocity. It is actually quite difficult to get objects to fall into other objects that they orbit. It takes more fuel to have a space probe go to the Sun than it does to send them out of the Solar System.
  16. Oct 23, 2012 #15
    Planets ARE falling towards the sun.....the loss of energy via gravitational waves opf the earth/sun system by itself would cause them to move ever so slightly closer and closer...but I think the sun will erupt and become a red giant before that happens....

    Actually all bodies in the Solar System orbit the center of mass of the entire Solar System....but viewing it as the earth orbiting the sun is an excellent approximation.
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