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Why doesn't the moon fall down?

  1. Dec 21, 2011 #1
    I understand that the moon doesn't fall down because it is in an orbit around the Earth. Is this orbit created by warped space due to the mass of the earth?
  2. jcsd
  3. Dec 21, 2011 #2
    The velocity of the moon allows it to overcome (well, not completely overcome) the gravitational pull towards the earth. The gravitational pull itself is a centripital acceleration (take a look at any circular motion physics problem). Eventually it will fall into the earth, just like we will eventually fall into the sun. At least, that's my understanding, which could be wrong.

    A quick google yields that the moon is a few cm further from the earth each year, indicating that the velocity is exceeding the centripital (gravity), and thus, over time, the moon should leave earth instead of crashing into it.
  4. Dec 21, 2011 #3
    Why is the velocity overcoming the centripetal force caused by gravity? Shouldn't it be vice versa, that the velocity is slowing down over time?
  5. Dec 21, 2011 #4
    let's do a little though experiment. (let's also ignore air resistance since it should be negligible in space)

    first, imagine standing on a hill and dropping a rock. it should fall straight down, no surprises.

    now, imagine throwing it forward. it no longer falls straight down, it goes forward and curves down.

    now, imagine throwing the rock even faster. the rock goes further. if you keep throwing the rock faster, it keeps going further.

    on an infinite, flat world we could do this forever with greater speeds/distances. the earth, however, is curved.

    if we zoom out some on our experiment so we can actually see the curvature of the earth, and we continue throwing our rock farther, it starts going farther and farther around the planet.

    now, imagine throwing the rock so fast that it goes all the way around the earth back to you. at this point it should be right where you first threw it and have the same velocity it initially had. that means it will continue around again... and again, and again, and again, etc.

    this is essentially what orbit is. the object moves so fast that it falls around the planet.

    edit: so, in fact, the moon does fall down, it just has such a huge velocity in a different direction that the direction of the gravitational force on it (and therefore where "down" is) changes really fast as well.
    Last edited: Dec 21, 2011
  6. Dec 21, 2011 #5


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    JPL gives a pedagogical presentation on orbits at http://www2.jpl.nasa.gov/basics/bsf3-4.php. The moon is slowly receeding from earth due to tidal friction, which is also slowing the rotation of earth.
  7. Dec 21, 2011 #6


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    And a little pic to go with the explanation...
  8. Dec 21, 2011 #7
    This makes much more sense now,
  9. Dec 21, 2011 #8


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    Actually, that's the part that's inaccurate. The moon will neither fall into Earth nor escape from it.

    These effects are not unstable i.e. the fact that it's moving away does not mean it will just continue to move away. The Moon's distance from Earth is fraught with subtleties having to do with rotational rates and tides. The upshot is that the distance has changed over very long periods but it is not a runaway process.
  10. Dec 22, 2011 #9


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    Actually, the moon probably can potentially escape earth orbit, it will just take billions of years longer than for the sun to go red giant. All bets are off when that happens.
  11. Dec 22, 2011 #10
    One could argue that the moon does fall down, it just keeps missing.
  12. Dec 22, 2011 #11
    Perhaps we should rename Luna to Arthur Dent?
  13. Dec 26, 2011 #12
    If the Sun, Earth and Moon remained as they are, the Earth's oceans remaining much the same, then the Earth will slow until it matches the Moon's orbital period and the Moon will recede to match the net angular momentum of the system. This state is reached when the Earth's day is 48.5 times the present day and the Moon is at a radius of 552,000 km. However the tidal braking from the Sun still occurs - it's 1/3 the Moon's tidal effect on the Earth and will be much stronger relatively as the Moon backs away. Since the Sun is still slowing the Earth down, eventually the Earth will be turning slower than the Moon and create a net drag on the Moon, causing it to begin a slow approach to the Earth. After about 15 billion years the two will be so close, that the Moon will break up into a ring of debris.

    However the Earth, Sun and Moon are not alone. The other planets jostle the three with their gravity, introducing chaotic behaviour into their orbital evolution. Mercury especially could go wandering into the path of the other planets. Thus the Moon might escape, but not via tidal evolution alone.
  14. Dec 26, 2011 #13


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    Darn! I was about to make that reference myself!:tongue:
  15. Mar 4, 2013 #14
    Thank you all, what great responses. I guess this would clarify my confusion (???) a bit: The work expended due to a Force active over a distance is described as W= ∫F°dL. So a force acting on an object over a certain length L is W. It seems that this doesn't apply with the force of Gravity because there is no real Work involved in the moon rotating around the earth (if you discount the friction due to tides, which was an excellent point).

    I think the answer to my question has to do with what Einstein said about gravity: that it is not a force per se, but a curve in Space-time, and it takes Relativity to explain it fully. From what I gather: from the perspective of the moon, it is really traveling in a straight line at a constant velocity (weird). Therefore Newton's first law of motion would apply and it will go on traveling at the same velocity for eternity (in the absence of another force). It just wigs me out to think of this.
  16. Mar 6, 2013 #15
    The tidal friction thing is interesting. Would tidal friction not decrease during periods of heavy glaciation? If this were to be the case would the moons regression from earth not slow down, cease or even reverse? Just a thought.
  17. Mar 6, 2013 #16
  18. Mar 7, 2013 #17
    It is the tidal effects on the solid Earth that have the major effect, not just the effects on the oceans.
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