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

How does the moon move ?

  1. Jul 5, 2012 #1
    as we know that Newton's Motion of First Law state that everything will stay at stationary or uniform velocity unless an force act on it ...., the moon moves, as earth gravitational field act as centripetal force on it , but how does the moon moves start from stationary ??? unless it is start moving already ??
  2. jcsd
  3. Jul 5, 2012 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Welcome to PF,
    you are correct, the Earth and Moon did not start out as mutually stationary.

    In general, any randomly accreting body will end up with a net angular momentum which means it will rotate so the general circular-ish motion of bodies in the solar system is built-in as a consequence of how it was formed. You can easily see that two bodies approaching and then holding together, unless they happen to have equal and opposite momenta, they won't end up stationary, and, unless they hit exactly head on, they will end up turning about their mutual center of mass.

    In terms of Newton's 1st Law, however, the Moon is not in uniform motion ... it is accelerating under the unbalanced force of gravity ... see Newton's 2nd Law. By Newton't 3rd Law, the Earth also orbits the Moon.
    Last edited: Jul 5, 2012
  4. Jul 5, 2012 #3


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Notice how after the impact the cars are spinning...

    Last edited by a moderator: Sep 25, 2014
  5. Jun 16, 2013 #4
    The earth was impacted by a planet-sized object very early in its lifetime. This expelled a lot of matter that formed a ring like Saturn's. Because the matter was far enough away from the earth to form a moon, that's exactly what it did. So the moon started out moving, the necessary kinetic energy provided by the thing that hit the earth all those eons ago.
  6. Jun 16, 2013 #5

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    Moderation note: This thread fits better in our general astronomy forum rather than classical physics.

    That's more or less the giant impact hypothesis of how the Moon formed. Note the word "hypothesis". We don't know. It's a guess. A very good guess, and it is the current consensus view on how the Moon formed, but it remains a guess. It's not good to state it as fact as you did.

    While this hypothesis does explain a large number of problems that other models of the Moon's formation cannot explain, it too has some problems. Per this hypothesis, a Mars-sized body collided with the Earth shortly after the solar system had pretty much finished forming. Material ejected from the collision quickly formed a ring at a distance about six Earth radii. Why a ring? Collisions between bodies would quickly have resulted in objects in circular orbit. The Moon then formed from collisions between these protomoon objects, very similar to how terrestrial planets are thought to form around stars. The Moon formed in orbit because it was already in orbit.

    Again, this is just a hypothesis. There are some problems with it. An alternative model that is garnering some support as of late because it explains some of these open issues involves a collision between two objects of similar size rather than between a Mars-sized object and an Earth-sized object. To me this is just a variation on the giant impact hypothesis.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook