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

Satellite orbiting the Earth vs the moon?

  1. Nov 12, 2014 #1
    I know that the the satellite will lose speed(due to perhaps small air resistance), and therefore lose centripetal force? I am not sure how losing centripetal force and speed causes the satellite in orbit to lose radius. If force is the same, and velocity is lowered, than radius would increase.

    I think that the moon does not fall into the earth due to it's pseudo centrifugal force due to the opposing of linear inertia and also due to the fact that it does not lose velocity.
     
  2. jcsd
  3. Nov 12, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    As a satellite gets closer - it gains centripetal force from gravity. The drag is an additional force applied tangentially to the orbit.

    Consider - if removing speed leads to a higher orbit, then that would mean that taking energy away will lift it.
    Usually you expect to give something energy to lift it higher don't you?

    ... please clarify that sentence - it seems pretty mangled.
    You are saying the Moon stays up because ... it does not slow down and it's linear inertia is balanced by centrifugal force? Something like that? It doesn't make a lot of sense. Forces balance forces, not inertia.
    Eg. for a satellite in a circular orbit, the centrifugal force is needed to balance the gravitational force (in a reference frame where the satellite and primary are stationary).
    Also - the Moon does slow down in it's orbit - and gets farther away.
    http://en.wikipedia.org/wiki/Tidal_acceleration

    But you should not be advancing personal theories in PF - try asking a question about why the Moon stays up instead.
     
  4. Nov 13, 2014 #3

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Nope. Same sideways acceleration at less velocity produces a more curved path, with a lower radius of curvature.

    Nope. The centrifugal force, which exists only in rotating reference frames, opposes the gravitational attraction. But a general (frame-independent) explanation shouldn't even involve frame-dependent forces, like the centrifugal force.
     
  5. Nov 13, 2014 #4

    Bandersnatch

    User Avatar
    Science Advisor

    I remember a freshman undergrad physics course that used a sloppily written book which propagated this misconception. Reading it confused me out of the wazoo back then.
    The idea is that since a circular orbit has got a lower orbital speed the larger the radius, then a conclusion is reached that to increase the radius of the orbit one needs to decelerate.
    The above is of course completely incorrect.

    Here's a free browser-based game called Voar:
    http://voar.io/
    It lets you fly a spaceship around a planet. The only control you've got over your craft is the forward thruster. Playing a bit can help gain an intuition of how orbits work.

    There are also other, more involving games out there that can help you learn orbital mechanics first hand. This one here:
    http://orbit.medphys.ucl.ac.uk/
    is a full blown spacecraft simulator.

    For gaining mathematical intuition, probably the best way is, as mentioned by Simon, to use the conservation of energy. Write down the potential+kinetic energy of the ship/satellite at a given radius, add some KE (that is, increase velocity), and try to figure out how the two change, keeping in mind that the total has to remain the same (unless you fire the rockets again).
     
  6. Nov 13, 2014 #5
    Ahh thanks you guys!!!

    Quick question, are the forces on the moon perfectly in balance with the force of gravity, so that it would not come crashing down, and if so, could we possibly have a satellite mimick this phenomena and therefore have it stay up in orbit until the earth gets destroyed?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook