https://www.nbclearn.com/science-of-nfl-football/cuecard/50770
http://physics-in-football.weebly.com/forces.html
https://prezi.com/hmghvth2nqjb/football-friction-physics/
http://physicsbuzz.physicscentral.com/2010/11/physics-of-football.html
http://entertainment.howstuffworks.com/physics-of-football.htm
http://physics-in-football.weebly.com/forces.html
https://prezi.com/hmghvth2nqjb/football-friction-physics/
http://physicsbuzz.physicscentral.com/2010/11/physics-of-football.html
http://entertainment.howstuffworks.com/physics-of-football.htm