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How do satellites stay in orbit? Free fall or centripetal?

  1. Mar 20, 2006 #1
    Hi, I just recently posted this on the astrophysics forum but thought it would probably not quite fit so i decided to move it here.

    Hi, this is my first post. I have been trying to find an answer out of my own interest but is unable to do so. while searching I found this wonderful website. I hope you guys can help.

    I came across a series of questions today and one of the question asks "What keeps satellites in orbit?" The answer choices were inertia, free fall, and centripetal acceleration (I knew it wasnt inertia so im down to free fall and centripetal) I picked free fall. HOWEVER, the answer was actually centripetal acceleration. I think that, with the amount of speed and height, a satellite can travel faster than it can fall and thus continue try to fall but unable to do so because it is traveling too fast about the earth's curvature and thus experience continuous freefall. Can someone explain this? Thanks

    By the way, one of the results i came across is found on:
    http://www.boeing.com/companyoffices...light/iss.html

    Thanks for any explanation , im quite curious.
     
  2. jcsd
  3. Mar 20, 2006 #2

    G01

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    Hmmmm. Free fall is the name of the situation the satellite is in, but its not what keeps it orbiting. The centripetal acceleration causes the object to constantly accelerate toward the center of the earth and thus orbit. Free fall is a result of this acceleration and the objects velocity going around the earth, not the cause of the orbit itself......That help?
     
  4. Mar 20, 2006 #3

    Mk

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    Think of a stone tied to the end of a piece of string held in hand and whirled in a circle. The stone simulates the satellite, and the hand is the Earth. Centrifugal force pulls outward, but the taut string holds the stone in its circular orbit. If the speed of the stone is too low, the stone doesn’t move in a circle, but falls toward the hand holding the string.

    There’s no air resistance in space, so as soon as a satellite has gained the right speed, it retains that speed because of inertia. Satellites fly in stable orbits, for which satellite speed and distance from the Earth are calculated accurately.
     
  5. Mar 21, 2006 #4

    rbj

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    here's another thought (derived from Newton): think of a planet without an atmosphere and with a tall mountain in which an astronaut has a high powered rifle and shoots a bullet horizontally. gravity acts on the bullet and it starts to descend outlining a curved trajectory. if the muzzle velocity was just the right speed, the curvature of that initial trajectory would be the same as the curvature of the planet at that elevation. so the bullet would be "dropping", but the ground underneath it would be "dropping out" underneath the bullet at the same rate (due to the roundness of the planet) therefore the bullet would maintain the same elevation above the mean elevation of the planet's surface.

    for a JAVA app demonstrating this, see: http://www.phy.ntnu.edu.tw/ntnujava/viewtopic.php?t=24 .
     
    Last edited: Mar 21, 2006
  6. Mar 21, 2006 #5

    vanesch

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    ... and then the astronaut better gets off the mountain soon after he fired!

    :smile:


    As to the original question: what keeps satellites in orbit ? Poles, no ? Aren't they all fixed on a very long pole ? :blushing: just kidding.

    What keeps satellites in orbit is gravity. If it weren't for gravity, they'd go straight off in a tangent. Gravity makes them follow a curved orbit. And, the earth being the source of the gravity, it's in the center of the orbit. Now, if the earth is smaller than the orbit, the satellite will not hit the earth's surface, and keep on running on its orbit. On the other hand, if the orbit is smaller, then the satellite will hit the earth's surface. In usual speak, we say it dropped down.
     
  7. Mar 21, 2006 #6
    Then in this case, the only force acting on it is gravity which can cause freefall and/or centripetal force right? Since the earth is the source of gravity AND the center? Then wouldnt free fall and centripetal be the same thing: gravity?
     
  8. Mar 21, 2006 #7

    vanesch

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    What's "free fall" ? Within Newtonian gravity (and there's no need to go beyond that here, so this viewpoint is implicitly assumed), free fall is the state of motion that results from the workings of the force of gravity, and only gravity, on an object. Free fall essentially means: ONLY gravity force, no other forces.
    However, in order to work with forces in the first place, and to use Newton's equation m.a = F, you have to make sure that you apply this law in AN INERTIAL FRAME.

    Now what is "centripetal" force ? It is an ARTIFICIAL FORCE that is introduced when you are working in a rotating reference frame and you'd like to pretend that it is an inertial frame. It is because m.a = F is a priori NOT correct in a non-inertial frame. However, you can PRETEND m.a = F to be correct, on the condition that you add an artificial force to F, which is the centripetal force.

    But I invite you to look upon the satellite and the earth in an INERTIAL frame (NOT rotating with the earth). Then you see the earth rotating, and the satellite swirling around it - simply because of the force of gravity exerted by the earth on that satellite.
     
  9. Mar 21, 2006 #8

    jtbell

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    Are you sure you're not thinking of centrifugal (outward) force? Centripetal (inward) force is very real in this situation. It's just the gravitational force.
     
  10. Mar 21, 2006 #9

    DaveC426913

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    The key here, I believe, is to ask the question: what WOULD the satellite do if otherwise left to its own devices?

    A] fall down
    B] go straight

    If left to its own devices (it has an intrinsic velocity in a specific direction), the satellite would continue on a straight path forever. Going straight is the satellite's DEFAULT behaviour, thus the question is not "what keeps it from falling down?", the question is "what keeps it from flying off into the solar system?". The correct answer is B].

    It is the Earth's gravity that pulls the satellite into a curve. Gravity is the force that provides centripetal acceleration.
     
  11. Mar 21, 2006 #10

    vanesch

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    Eh, yes. Sorry. Mixed up the names - I've never been able to know which is which.
     
  12. Mar 21, 2006 #11
    Not everything in orbit stays in orbit. There are many factors which determines the potential decay and longevity.
     
  13. Mar 21, 2006 #12

    Mk

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    What goes up ends up going down. Satellites end up burning in the atmosphere.
     
  14. Mar 22, 2006 #13

    rbj

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    geez, i dunno. those geosynchronous orbits are pretty far out there, not a lot of atoms to run into that slows the satellite down.

    i suppose there is the density of stuff in space that is normal for our distance from the sun (what is it? a couple atoms per cubic meter?), but i don't expect the earth's orbit around the sun to decay any time in my great-grandchildren's lifetime.
     
  15. Mar 22, 2006 #14

    vanesch

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    I don't think Voyager I and II are going to come down :-)
     
  16. Mar 22, 2006 #15

    rbj

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    didn't know thems were satellites. i think they have exceeded escape velocity for the solar system. i thought i read that in 20,000 years or so, one of them will begin accelerating toward another system. that would mark the point where it last really leaves our solar system (when it "escapes" our system's gravity and is more influenced by another system's gravity.

    maybe they'll "come down", but somewhere else, very far away.
     
  17. Mar 22, 2006 #16

    Mk

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    Ah ha! They aren't satellites! Space probes!!
     
  18. May 2, 2007 #17
    As you know, gravity holds on to the satellite. But if you could turn off gravity, the satellite would keep moving — but in a straight line. It would leave Earth. If you stopped the satellite and turned gravity back on, the satellite would fall straight down to Earth. But if you have both gravity pulling down and the speed of the satellite pushing out, the two balance out so that the satellite can go in a circle around Earth.
     
  19. May 2, 2007 #18
    This ^ is a geostationary orbit - so the satellite looks like its just sitting in the air with nothing whatsoever holding it up (no apparent orbital motion from any transverse velocity). We know there IS velocity, it's just that the earth is rotating at exactly the same speed, so we can't see any motion.

    But what's to say for certain that the Earth IS actually rotating and not the rest of the universe ?
     
  20. May 2, 2007 #19

    jtbell

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    Foucault's pendulum and the Coriolis effect are two phenomena that indicate that the Earth is rotating. Or has someone come up with a calculation that predicts the same effects for a non-rotating Earth and rotating universe?
     
    Last edited: May 2, 2007
  21. May 2, 2007 #20
    Mmm, wiki says Foucult's pendulum was first demonstrated in 1851
    So, before then there was still no logical reason to be absolutely sure that the earth was not the centre of the universe, even if following Newton's and keplers laws ? And 1850, that's only 150 years ago..
     
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