What is gravity?
Sorry to say that I have no really satisfying answer.  I can tell you what I do know...
 

  • It provides a force.  Therefore it must be expressed in Newtons.
  • It is quite weak; things don't just fly at me of their own accord.
  • It is very long range;  Pluto is super far away comets are attracted toward the sun.
  • The force follows an inverse square relationship...you go 2x away, gravity is 4x weaker.

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    I have heard from physics professors that gravity is "delivered" by a teeny tiny particle called a graviton.
    This is similar to the photon "delivering" the electromagnetic phenomena such as light.
    Nobody (as far as I know) has ever found a graviton, but many are looking very diligently for one.

    Gravity, in general has this form:  Fg = m1m2/r2.

    But if you plug numbers into this, you end up with everyday sized numbers.  This cannot be!  If we use 1Kg for the masses and .1m for the distance, we get 100 Kg2/m2. Huh?
    We need the force to be in Newtons, and we know that gravity is not this strong anyway.
    There must be some constant we can stick into the equation that will do 2 things:

  • make the units Newtons
  • make the numbers realistic


    • Enter Henry Cavendish.  He sought to find out what the constant is.  The constant is today called "G" and it is accepted as 6.67E-11 N*m2/Kg2.  Why these units?  If we solved the above version, we got units of Kg2/m2.  By having the constant be in the units N*m2/Kg2, we get a final answer in Newtons.

    The final version, then looks like this:  Fg = Gm1m2/r2.

    What is the attraction due to gravity between two 80Kg people standing two meters apart?

    Fg = Gm1m2/r2 = (6.67E-11*80*80)/22 = 1.1E-7 N

    A very small result! You can see why things don't just fly at each other all over the place.
    Now consider the mass of the earth: 6E24Kg.  It has enough mass to attract stuff!