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Thread: Gravitational Pull

  1. #1
    Join Date
    Feb 2004
    I'll use our sun for example. It exhibits a gravitational pull, which keeps the planets and assorted satellites orbiting it. What I do not understand is why we do not feel/are not affected by, this gravitational pull. Anyone who can point me to an article on the subject, or can explain it, please do so.


  2. #2
    degeneration Guest
    I can answer that one...

    Using the force equation

    F= (G * M1 * M2)/r^2

    Where F is the force, G is the gravitational constant, M1 and M2 are the mass of the two objects in question, and r is the distance to the centre of mass of the object.

    The sun exerts the following force on us:

    Mass sun = 2 x 10^30 kg
    Mass person = 80 kg say.
    r = 1.49 x 10^11 m
    G= 6.67 x 10^-11

    Therefore F=0.48

    For the Earth

    Mass Earth = 5.9742 x 10^24kg
    Radius of Earth = 6378100m

    Therefore F = 783.6

    So the force we feel from the Earth pulling us towards the Earth's surface is over 1600x greater than the force from the sun. So we do feel the force from the sun, but it is negligible by comparison.

  3. #3
    Join Date
    Feb 2004
    ah, so gravitation is scaleable? That clears up quite a few of the questions that I had planned to ask after this one.

  4. #4
    degeneration Guest
    Yeah, gravitation is proportional to the inverse square of the distance.

    Glad my answer helped!

  5. #5
    Join Date
    Mar 2005
    There's actually more to it than that. Even if the sun's gravity at the distance of the Earth were equal to, or stronger than the Earth's gravity we still would not feel it. The reason is that although the Sun is pulling us, it is also pulling the Earth with the same force. Let's just say for example that at the distance of the Earth, the Sun's gravity were 10 times stronger than Earth's gravity. You'd have the sun pulling you at 100 m/s^2 towards the Sun. But you'd also have the Sun pulling the Earth at 100 m/s^2 for a net difference of zero. Then add the Earth pulling you down at 10 m/s^2, and the net result is 10 m/s^2 towards the Earth, exactly what we feel in the real world.

    This is similar to the misconception that a lot of people get when seeing the astronauts in the space shuttle floating around. Many assume that this means that there is no gravity in space. Actually, the Earth's gravity at the altitude of the space shuttle's orbit is almost as strong as gravity here on the ground. But since the Earth is pulling the space shuttle at the same rate that's it's pulling the astronauts inside it, they experience weightlessness, just as you would if you were inside an elevator that was freefalling. When you and your surroundings are falling at the same rate, you feel no force exerted by your surroundings. That is why the Sun could not pull you off the surface of the Earth when it is overhead, even if its gravity was stronger. It's trying, but it's pulling on the ground you're standing on as well.

    Interesting side note, the sun exerts twice as much gravitational force on the Moon than the Earth does. But the Moon does not get stripped away from the Earth because the Sun also pulls the Earth with the same intensity, and in the same direction. (There's small differences since the Moon is sometimes slightly closer or farther from the Sun than the Earth, but these tidal forces are not that significant.)

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