Gravitational force

  1. I had this doubt right from my school days.
    What is actually gravity? what causes it? is that the core of a planet or any other forces?
    What creates the gravitational forces.?
  2. jcsd
  3. D H

    Staff: Mentor

    Mass, any mass, not just the core of a planet, causes gravity.
  4. If this was the answer then why dont we produce gravitational force? Human body is a mass too right sir..?
  5. Human body does produce gravitational force.
    Last edited: Dec 6, 2013
  6. TumblingDice

    TumblingDice 463
    Gold Member

    We do have our own 'gravity'. You exert a pull on the Earth and the Earth pulls you. The Earth has a lot more mass than you do, so it's pull is much stronger than yours.
    1 person likes this.
  7. PeroK

    PeroK 1,693
    Homework Helper
    Gold Member

    There was a famous experiment where the gravitational force of a mountain in Scotland was measured:

    You'd need something at least as big as a mountain given that the Earth's gravity dominates our enviornment and makes most other gravitational forces difficult/impossible to detect.
  8. I just checked with may bathroom scale.
    I weigh in at 180 pounds.
    Turning the bathroom scale upside down ( not necessary - for illustration purposes ), the earth weighs in also at 180 pounds.
  9. TumblingDice

    TumblingDice 463
    Gold Member

    You're too funny :rofl: !
  10. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Gravity is the allergic reaction of space to the presence of mass/energy.
  11. I seem to have read that gravity isn't really a force - is the shape of space around a mass.
  12. TumblingDice

    TumblingDice 463
    Gold Member

    That's incorrect. Gravity is one of the four fundamental forces of physics. Better reading here:

    Hyperphysics: Fundamental Forces
  13. Well,Pjpic and TumblingDice,each of you is correct in a sense.Gravity is one of the fundamental interactions,in that its one of the fundamental reasons that causes us to see matter change velocity.But Gravity is not a force,in that its an emergent phenomenon emerging from the shape of Space-Time.
  14. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Defining force is an exercise in futility. It is equally valid to define gravity as a distortion of space time geometry. It works the same either way.
  15. TumblingDice

    TumblingDice 463
    Gold Member

    Let's not get involved with a Newtonian vs. GR debate. If you want to teach that gravity is not a force, that's your choice.

    Space (Pjpic) and space-time (Shyan) are not the same. All four dimensions are required for geodesics and gravitation.
    Last edited: Dec 9, 2013
  16. Gravity is best described as curving of space-time for now^^... In GR-Field equation, mass, energy and momentum generates gravitational effect often described as stress-energy tensor, stress-energy-momentum tensor, etc via observation Einstein-Thirring-Lense effect or dragging effect in rotating mass like a gyroscope.

    Any object exerts gravity; The denser the mass the greater the effect but somewhat insignificant in the quantum level than the other forces/phenomenons, in the order of influence/strength.
  17. Like any natural phenomena, gravity is what we observe. We represent such phenomena via models so we can predict some interactions that we haven't yet observed or cannot observe.
    But there are aspects of 'gravity' which are not so clear.

    Here are some model illustrations and some of what we observe:

    Gravity is unique in that it we find it affects everything, even space and time. Other forces, like electromagnetism for example, only affect charged particles [objects]. But gravity affects charged particles.

    In special relativity,SR, no gravity, relative motion causes 'space contraction' and 'time dilation', distortions in space and time.

    In general relativty,GR, we find also that differences in gravitational potential also affect the passage of time.

    It was explained to me some years ago in these forums you can picture world lines [paths, or curves] in special relativty as you would curves on a flat graph paper. When gravitational curvature is involved, as in GR, the graph paper itself on which the curved worldlines are drawn is itself curved.

    On the other hand 'gravity' has some rather indeterminate features in our models: In general we can't define the energy of the gravitational field nor do we have a single overall measure
    for what we call 'gravitational curvature'. Worse is that our model equations seem to break down when space time curvature becomes extreme, that is very high energy situations, like at the moment of the Big Bang and the center of black holes. If we knew everything we'd like to know about gravity, we'd have a theory of quantum gravity...meaning an overall theory and math that includes general relativity and quantum mechanics. That's a work in progress.
  18. Wouldn't it be accurate to say that if you weigh 180 pounds on Earth, the Earth weighs 180 pounds on you?
  19. not really, depends on just what you mean.

    'weights' vary from place to place, from reference to reference, masses don't.

    Better to say they experience the same gravitational attraction, F = GMm/r2

    When you start talking about WEIGHT, W = mg, and so while mass m is constant everywhere, the gravitational strength g your weight differs, say between earth and moon. g is smaller on the moon, so you 'weigh' less.
  20. adjacent

    adjacent 1,540
    Gold Member

    It's Usually Newtonian mechanics which is used.(Like launching Rockets,Calculating Orbits of satellites,Planets etc)
    Newtonian mechanics is still pretty accurate.
    Relativity is used in extreme circumstances.
    So it is OK to consider Gravity as a force which is present between any two masses.

    The OP don't want to get involved in GR and Newtonian mechanics.It will be get more and more confusing,for Newtonian mechanics in much easier to understand.And as I said earlier it gives pretty accurate results if not for extreme situations
  21. That's not a hard-proven fact, it's just what Einstein's theory of gravity says.

    Even though this theory has been quite successful to date, it is not yet as solid a fact as, say, existence of atoms.

    Please do make a distinction between theories and facts when answering newbies' questions. They often do not understand the difference.
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