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Gravitational Force

  1. Jun 11, 2008 #1
    Gravitational Force!!!

    I have an interesting question... im hoping its not to easily answerable cuz it is early in the morning and i may just not be thinking in the right mind.

    Energy cannot be created or destroyed...

    gravity is just a curve in space-time due to a dent made by an amount of matter.

    the analogy is usually that space-time is a trampoline and you put a bowling ball in the middle... small objects put on the trampoline will fall towards the bowling ball. This analogy is pretty good but the reason they fall towards the bowling ball is due to gravity in the first place. if gravity is a curve in space-time then is it really a force? we can describe it as an acceleration on objects but is it really a force technically?

    my second question has to do with the first and creation of energy... if we have some small bits of matter just floating in space not moving (no energy) at all which is probably impossible. The small particles will start "falling" towards whatever particle is largest. Now as they start gathering at the larger one it becomes more and more massive which entails more gravity. now as they all start gathering and the gravity increases they start to become compressed and the temperature rises which is an increase in energy... where exactly is this energy coming from?
  2. jcsd
  3. Jun 11, 2008 #2

    D H

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    Staff Emeritus
    Science Advisor

    Regarding your first, question: Good question. Gravity is not considered a force in general relativity. Assuming Newtonian mechanics, you can use Newton's second law (F=ma) in a non-inertial reference frame but you must add pseudo forces (fictional forces) to the external forces on some particle. An interesting fact about these pseudo forces: Such forces are always proportional to mass. Gravity is the only real force in Newtonian mechanics that is proportional to mass. One of Einstein's leaps was seeing that gravity isn't a real force; it is a pseudo force instead.

    The second question: Energy comes in several forms. While energy is conserved, it can change form. There is potential energy in the "small bits" of matter separated by some distance. This potential energy is converted to kinetic energy as the particles start moving toward each other, and that kinetic energy is converted into thermal energy as the particles collide and compress.
  4. Jun 11, 2008 #3

    Well gentleman i am not too great in physics and i beleive that DH has given the best possible answer, i would like to add some thing to this.Actually answer to second response is itself hidden in your question.
    You are saying when matter particles come closer,they become massive.But how is it possible until the particles interact with each other,this is popularly known as interaction energy,which is emitting in form of thermal energy.
    If anybody find something wrong in my answer,please let me know,as i hav already told that i am not too strong in physics.
    With regards.
  5. Jun 12, 2008 #4
    alright thanks, i guess i just have a hard time contemplating potential energy. thanks for the replies guys!
  6. Jun 15, 2008 #5
    To add, the trampoline analogy is somewhat incomplete. Space is 3D not 2D.
  7. Jun 15, 2008 #6


    Staff: Mentor

    And in GR it is 4D spacetime which is curved.
  8. Jun 15, 2008 #7
    true that
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