http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation [2]
http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html [3]
http://www.physicsclassroom.com/class/circles/Lesson-3/Newton-s-Law-of-U... [4]
http://theory.uwinnipeg.ca/physics/circ/node7.html [5]
http://physics.weber.edu/amiri/physics1010online/WSUonline12w/OnLineCour... [6]
http://physics.about.com/od/classicalmechanics/a/gravity.htm [7]
Links
[1] https://hootsgo.org/?q=node/6398
[2] http://en.wikipedia.org/wiki/Newton%27s_law_of_universal_gravitation
[3] http://csep10.phys.utk.edu/astr161/lect/history/newtongrav.html
[4] http://www.physicsclassroom.com/class/circles/Lesson-3/Newton-s-Law-of-Universal-Gravitation
[5] http://theory.uwinnipeg.ca/physics/circ/node7.html
[6] http://physics.weber.edu/amiri/physics1010online/WSUonline12w/OnLineCourseMovies/CircularMotion&Gravity/reviewofgravity/ReviewofGravity.html
[7] http://physics.about.com/od/classicalmechanics/a/gravity.htm