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Gravity In Space

  1. May 10, 2008 #1
    I viewed a lot of websites. Most says that there is no gravity in space. Is it true? Coz what i have in my head is that. earth, Sun, Moon and every otehr stars or planets all seems to follow thier path and keep themselves inside that path. Like cars on a traffic road stays in their own lanes. In many space or sci-fict movies, or even real, when there is no gravity - things tend to fly in all dirctions. But how come large objects like earth, sun seems to keep in thier own motion. Instead of flyin insto all dirctions.

    It is said that when a smaller subject is put nxt to a larger object with greater mass, the small object tend to be pulled into the larger object. And that is how us humans tend to be stuck onto earth.... not flying around.

    So, if earth, sun and moon.. or any other planets tend to follow thier own circular motion. Then there must be a gravity force or some sort of similar force acting on those large objects ( as in earth.sun.....etc). So where would the central gravity force be?
  2. jcsd
  3. May 10, 2008 #2
    what i means by path is orbit ^^

    An orbit is a regular, repeating path that an object in space takes around another one. An object in an orbit is called a satellite. A satellite can be natural, like the moon, or human (or extraterrestrial?) -made.
  4. May 10, 2008 #3

    If you are within a space-station orbiting the Earth then both you and the space-station experience a gravitational attraction towards the Earth. The result is that you and the space-station accelerate towards the Earth at the same rate. The combination of inertia (the tendency to keep moving in a straight line at constant speed) and acceleration towards the Earth means that you both follow an orbital trajectory around the Earth.

    If you could suddenly stop both you and the space-station then you would both start to fall towards the centre of the Earth, and as long as there were no other forces acting on the space-station (for example air resistance due to motion through the Earth's atmosphere) then both you and the space-station would accelerate at the same rate. If you tried to stand on a set of scales to weigh yourself then the reading would be zero since you and the scales are falling at the same rate.

    "Weightlessness" does not imply a lack of gravity. For further reading, have a look at the wikipedia article on weightlessness... (Sorry I can't post a direct link due to restrictions on this forum).
  5. May 10, 2008 #4


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    Gravity is everywhere. The effects are most noticeable on universal scales.
  6. May 11, 2008 #5
    Thanks Chronos. But i do not get exactly what you mean...

    what does it mean by ---------In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. - Douglas Adams

    isnt it at the beginin when the universe was created, human-beings havent formed yet??
  7. May 11, 2008 #6
    Gravity is a curvature in space.

    Imagine an elastic fabric and pull all four of its sides.Keep a heavy object in the middle. You will notice circular wrinkles formed. That's exactly how the orbits are formed. But the question is how does two orbits intersect sometimes???
  8. May 11, 2008 #7


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    I take it that you are not familiar with the humor of Douglas Adams.
  9. May 11, 2008 #8


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    It refers to the other beings beside humans, who came down from the trees, and who subsequently invented watches and little pieces of paper called currency.
  10. May 11, 2008 #9


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    I can't imagine that there are many sites that would say such a thing. I suspect you are misinterpreting what you are reading.

    Your instincts are correct: in order for an object to stay in orbit, gravity must pull it into a circular path.
  11. May 12, 2008 #10


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    I meant that gravity permeates the universe, and all mass possessing objects tug upon each other. The quote from Douglass Adams is humor.
  12. May 12, 2008 #11
    ok... i see.... But back to the mainpoint... AS phy6explorer and JimChampion had said. There is a heavy object in the middle acts as the central-force in which the orbits will follow. In another word, all those space-stations or any sort of objects are affected by our earth's gravity. Is that right?

    Also, there is another question - why is all the space-stations, planets rotating in an circular motion?.. as in they are spinning themselves 360 degress all the time when moving.

    The Moon's orbit and the earth's orbit intersect and these two intersecting points are known as North Node ( Rahu ) and the South Node ( Ketu ). These orbits differ by 8 degrees. They are mathematical points which influence human behaviour. That is all I know about orbit intersection. But when orbits intersect with each other, it has to do with gravity,right? (this seems to be a stupid question)
  13. May 13, 2008 #12
    The planets actually rotate in an elliptical path around the sun (and the moons do the same around their planets).

    It is best described by the Laws of Planetary Motion,

    Kepler obtained Tycho Brahe's data after his death despite the attempts by Brahe's family to keep the data from him in the hope of monetary gain. There is some evidence that Kepler obtained the data by less than legal means; it is fortunate for the development of modern astronomy that he was successful. Utilizing the voluminous and precise data of Brahe, Kepler was eventually able to build on the realization that the orbits of the planets were ellipses to formulate his Three Laws of Planetary Motion.

    Kepler's First Law:

    I. The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.

    The Sun is not at the center of the ellipse, but is instead at one focus (generally there is nothing at the other focus of the ellipse). The planet then follows the ellipse in its orbit, which means that the Earth-Sun distance is constantly changing as the planet goes around its orbit. For purpose of illustration we have shown the orbit as rather eccentric; remember that the actual orbits are much less eccentric than this.

    Kepler's Second Law:

    II. The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.

    A line joining the Sun and planet sweeps out equal areas in equal times, so the planet moves faster when it is nearer the Sun. Thus, a planet executes elliptical motion with constantly changing angular speed as it moves about its orbit. The point of nearest approach of the planet to the Sun is termed perihelion; the point of greatest separation is termed aphelion. Hence, by Kepler's second law, the planet moves fastest when it is near perihelion and slowest when it is near aphelion.

    Kepler's Third Law:

    III. The ratio of the squares of the revolutionary periods for two planets is equal to the ratio of the cubes of their semi-major axes.

    Kepler's Third Law implies that the period for a planet to orbit the Sun increases rapidly with the radius of its orbit. Thus, we find that Mercury, the innermost planet, takes only 88 days to orbit the Sun but the outermost planet (Pluto) requires 248 years to do the same.

    I hope this explains everything!
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