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B Why do orbits not slow down?

  1. Dec 20, 2016 #1
    We see in our solar system the planets orbiting the sun, but why doesn't with all of the other forces in play the perpendicular velocity seemingly not decrease(or does it?). And if the perpendicular velocity of the planets slow down would that result in static planets not moving around the sun?
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  3. Dec 20, 2016 #2


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    Just like an ideal pendulum in a vacuum could go on forever, a planet orbiting can go on forever. A planet in our solar system will be constrained to orbiting the sun, just like a pendulum is constrained by the cable that it hangs from. A planet will speed up (slightly) as it heads toward a far away gravitational body, just like a pendulum will speed up as it heads toward the Earth. The planet will slow down (slightly) as it swings around the Sun and heads away from that gravitational body. Unlike a pendulum, the planets have enough velocity to continue all the way around the sun in an orbit without being slowed to a stop by a far off body. So a planet orbits instead of swinging back and forth like a pendulum. This can continue forever.
  4. Dec 21, 2016 #3


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    All the other forces are either affecting everything in the solar system just as much as they affecting the Earth (gravitational interactions with other stars, galaxies, etc), or don't affect the Earth the because their range is too small or the Earth is neutral (the other 3 forces of nature).
  5. Dec 21, 2016 #4


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    The only way to decrease their total kinetic energy is through very small effects, like converting it into gravitational waves or EM-waves (heating though tidal forces).

    Have you heard of gravity?
    Last edited: Dec 21, 2016
  6. Dec 21, 2016 #5
    Well, I imagine given enough time the combination of friction from interstellar particles/gas and many-body effects, even without counting the possibility of some rogue body entering the system and altering its equilibrium, would slow down orbits and make the planets spiral down and fall into the sun or crash into each other. And by 'enough' I mean many times over the actual time the system will last before the Sun goes red giant and vaporises the Earth anyway.
  7. Dec 21, 2016 #6
    What do you mean by pendicular velocity? If you mean the component of the velocity perpendicular to a line passing through the sun and the planet, it does change. If you're expecting the planets to slow down due to some effect, ask yourself why.
  8. Dec 21, 2016 #7
    A force perpendicular to the direction of motion merely changes the direction, not the speed. Moreover, energy is conserved, so the only way for orbits to slow down is by radiating something, such as gravitational waves, which are really, really, really weak.
  9. Dec 21, 2016 #8
    Right. But it's very rare that the force is perpendicular to the direction of motion.

    When an orbiting planet slows down it's kinetic energy decreases, but the potential energy of the planet-sun system increases. And vice-versa. It's happening in our solar system as we speak.
  10. Dec 25, 2016 #9
    In general can we say that planets are in perpetual motion and as man made perpetual motion devices fail after some time of workout, the planets will also crash down after calculated time depending on all of the forces in action even if sun does not blast?
  11. Dec 25, 2016 #10
    In theory nothing prevents a conservative system to move perpetually as long as nothing extracts energy. In practice of course this is never perfectly true - planets exchange energy with the environment in small ways (including stuff like meteor impacts), so I suppose on the long, long, LONG run they will lose their orbits. Not a "calculated" time because there's nothing strictly predictable about it. Yes, the orbit's stability itself is somewhat of an approximation since the exact solution to the many-body problem isn't known, but that doesn't mean that the planets ought to crash - the energy is there and if nothing takes it away then the orbits' shapes might change but the total energy should be conserved, and therefore something would probably keep orbiting more or less forever.
  12. Dec 25, 2016 #11
    There are tidal forces that generate a conversion of mechanical energy to thermal energy.
  13. Dec 26, 2016 #12
    Right, didn't think of that. So there's that too. Also orbiting bodies radiate gravitational waves I believe? So if we go from Newton to Einstein we have another source of energy loss which prevents orbits from being eternal.
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