http://csep10.phys.utk.edu/astr161/lect/history/newton3laws.html

                                   "equilibrium." The American Heritage Science Dictionary. Boston: Houghton Mifflin, 2005. Credo Reference. Web. 08 April 2012.

                                                 http://www.physicsclassroom.com/Class/energy/u5l1c   

                                                 Giancoli, Douglas C. Physics: Principles with Applications. 6th ed. Upper Saddle River, NJ: Pearson/Prentice Hall, 2005. Print

                                                            "kinetic energy." The Crystal Reference Encyclopedia. West Chiltington: Crystal Semantics, 2005. Credo Reference. Web. 11 April 2012.