Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Question about the planets

  1. May 14, 2006 #1
    If the planets are constantly orbiting the Sun won't they eventually run out of energy, succumb to the gravitational pull and fall into the Sun?
    They don't seem to be doing... why not?
  2. jcsd
  3. May 14, 2006 #2
    Because gravity doesn't require energy for it to work. The gravity is always them same for the planets so that the work does not chage if there's no chage in work there's no energy required.
    I think your going to need some other member to explain it better to you.
  4. May 14, 2006 #3

    Stop and think.

    What causes a system to lose energy? Things like friction, cooling losses of thermal energy, etc. But how is an object in orbit around the sun going to lose kinetic energy to friction (granted, objects in low earth orbit do, but they're not really beyond the atmosphere, there is still a lot of matter there, and those objects are very light compared to say a planet. A system doesn't just 'run out' of energy, the energy has to go somewhere, thats the first law of thermodynamics. And in the case of planets in stable orbits around a star, there isn't a significant way to lose any energy.
  5. May 22, 2006 #4
    What is in motion will stay in motion until acted upon by an outside force.
    Earth is in motion, there is no outside force.

    Summary: Earth won't fall.
  6. May 23, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    But wait...dust in the solar system is a kind of friction to slow down the Earth. (Granted, it's a small effect...but acting over billions of years...plus, there's the increase in mass (again, small) of the planet which affects the gravitational interaction.)

    Anyway, planets' orbits can and do change due to gravitational nudges from other planets, passing stars, etc. The interaction is complex...lose some energy here...gain some there...over and over throughout its existence. The current planets in the solar system are in what turned out to be the stable/safer orbits.

    Unless something wild happens (rouge interstellar body messing up our solar system in the distant future?), the net energy loss should be negligible such that the end of the Earth won't be it spiraling into the sun but rather the sun expanding out to the Earth's orbit when it becomes a red giant.
  7. May 24, 2006 #6


    User Avatar
    Science Advisor
    Gold Member

    Simple orbital systems [e.g., binary neutron stars] do bleed off momentum via gravitational waves and can eventually collide. Absent external influences, as mentioned by other posters, it is theoretically possible the earth could inspiral and eventually merge with the sun: albeit would take many billions of years [emphasis on many].
  8. May 24, 2006 #7
    I'm not disputing that orbits can change under various influences. I'm disputing the form of the original question "Won't they run out of energy?".

    That said, I would be surprised to find that any drag force on the earth from the gases in the inner solar system was significant, even over the entire lifespan of the sun (with the possible exceptions of the planetary nebual phase, and the planetary formation phase, but even then I would still be surprised).
  9. May 24, 2006 #8
    The Earth is moving under the force of gravity. An object in motion continues in a straight line, unless acted on by a force. The Earth is orbiting in an ellipse.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook