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B Gravity between objects on Earth

  1. Sep 30, 2016 #1
    I always thought the force of gravity between two objects on earth wasn't significant enough for there to be an apparent acceleration between the objects. But I tried calculating the force of gravity for massive objects, like the Great Pyramid of Giza, the force seems to be quite significant. In fact, for a 60 kg person it would be about 24 N when 1 meter away from the pyramid. Why then, is there no observable acceleration?
  2. jcsd
  3. Sep 30, 2016 #2


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    That seems too large.

    There is. Both the magnitude and the direction of the earth's local gravitational field can be affected by mountains or other large objects. This is measurable with careful gravimetric measurements.
  4. Sep 30, 2016 #3


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    That seems waaaay too large. Remember that the pyramid is a very large object and the vast majority of its mass is well beyond 1 meter away from a person standing next to it.
  5. Sep 30, 2016 #4


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  6. Sep 30, 2016 #5
  7. Sep 30, 2016 #6
  8. Sep 30, 2016 #7


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    I think your error is probably that you can't be 1 meter away from the entire mass of the pyramid, since it is roughly 100 meters across. The pyramid will act as though the mass were concentrated at its center, so the closest you can get is roughly 100 meters away, which reduces the force by a factor of 10^4. Try looking up the Shell theorem, which explains why a uniform spherical object acts as though all of the mass is concentrated at the center. The Great pyramid is not a sphere, but the same reasoning will roughly apply.
  9. Sep 30, 2016 #8
    I think the main point to consider is that the great pyramid, or even Mt Everest has a much smaller gravitation than does the entrire Earth.
    Although those do have a gravitation field, it is close to irrelevant when compared to the Earth as a whole for an object standing on the surface of Earth.
    Something like the great pyramid if it were free floating in space would be comparable to a typical asteroid or comet.
    Getting the Rosetta space craft to successfully manage an orbit around something that small was a considerable feat of engineering.
  10. Sep 30, 2016 #9


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