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  1. Standard member vivify
    rain
    02 Mar '15 19:06 / 1 edit
    If I'm correct, the larger an object is, the larger its gravitational pull will be, right? And yes, I know that other factors such as an objects density and magnetic field are to be considered. But most objects have at least some gravitational pull, right? Again, please correct me if I'm wrong.

    Assuming that the above is correct, wouldn't it follow that if a pen is dropped from a skyscraper, that it should fall at an angle (toward the skyscraper and the ground)?

    Basically, if I throw a rock off of a really high mountain, will the rock start to curve back toward the mountain?
  2. 02 Mar '15 19:49
    Originally posted by vivify
    If I'm correct, the larger an object is, the larger it's gravitational pull will be, right? And yes, i know that other factors such as an objects density and magnetic field are to be considered. But most objects have at least some gravitational pull, right? Again, please correct me if I'm wrong.

    Assuming that the above is correct, wouldn't it follow ...[text shortened]... row a rock off of a really high mountain, will the rock start to curve back toward the mountain?
    All objects with mass generate a gravitational field.

    And so a pen dropped from a skyscraper [lets assume a vacuum] will experience a
    pull towards the skyscraper that will deflect it from falling strait towards the centre
    of the earth.

    This is a long known and detectable and important effect.

    In fact it's big enough that it had to be taken into account by Victorian map makers
    who were going around the Himalayas and mapping the mountains.

    In territory where they might be killed if they were detected to be westerners they
    travelled in disguise with important equipment disguised [to, for example, look like a set
    of prayer beads] and made trigonometric readings of the mountains. And when they
    had got around the entire mountain range, a journey of many hundreds of miles, they
    were only a couple of hundred yards out.

    However this was in their eyes an appalling inaccuracy, And unable to determine what
    went wrong, they published their results and asked if anyone could figure out their mistake.

    And the problem it turned out was in accounting for the deflection from vertical in their
    plumb-lines due to the gravitational pull of the mountains.

    Not because they didn't account for gravity, but because the mountains were lighter
    than they thought they were.

    The Earth's crust floats on the molten mantle, and does so because the crust rocks are
    lighter than the mantle materiel. So when you have mountains rising up adding lots of
    extra mass, that pushes down crust materiel into the mantle to create a 'root' to the
    mountain, which creates extra buoyancy to support the mountains above.

    As this root is made of lighter material than the mantle, it means that there is less pull
    towards the mountains than there would be without the root.


    This meant that they were over-compensating for the pull of the mountains on their plumb-lines
    which was why they were 'so far' out.
  3. Standard member DeepThought
    Losing the Thread
    02 Mar '15 20:48
    Originally posted by vivify
    If I'm correct, the larger an object is, the larger its gravitational pull will be, right? And yes, I know that other factors such as an objects density and magnetic field are to be considered. But most objects have at least some gravitational pull, right? Again, please correct me if I'm wrong.

    Assuming that the above is correct, wouldn't it follow th ...[text shortened]... row a rock off of a really high mountain, will the rock start to curve back toward the mountain?
    That's correct, googlefudges answer looks accurate. One subtlety is that in the standard version of General Relativity there is no torsion. So the pen will take a curved trajectory, but assuming it started falling without any rotation there is no reason for it to start rotating. It is possible to write down theories with torsion, but its not included in the standard theory because there is no empirical justification, although there is some interest in this. Ways of detecting it would be to look at the polarization of light - the direction of polarization would rotate as the photon propagated.
  4. Standard member sonhouse
    Fast and Curious
    02 Mar '15 21:29 / 2 edits
    Originally posted by vivify
    If I'm correct, the larger an object is, the larger its gravitational pull will be, right? And yes, I know that other factors such as an objects density and magnetic field are to be considered. But most objects have at least some gravitational pull, right? Again, please correct me if I'm wrong.

    Assuming that the above is correct, wouldn't it follow th ...[text shortened]... row a rock off of a really high mountain, will the rock start to curve back toward the mountain?
    Your statement has one error: you said 'the LARGER an object is, the larger its gravitational pull. The correct usage is 'the larger the mass of an object, the larger the gravitational pull. Other than that, you have a valid question.

    Compare cotton candy 300 mm across VS a lead ball 10 mm across....
  5. 03 Mar '15 08:06
    Gravity measurements are used to map the sea floor:

    https://scripps.ucsd.edu/news/new-map-exposes-previously-unseen-details-seafloor

    http://topex.ucsd.edu/grav_outreach/