http://www.ultratune.net/
http://www.tazzarisportdivision.com/en/prodotti_scheda.cfm?w_id=66
http://www.bouldernordicsport.com/
http://history.fis-ski.com/
Bauer, Wolfgang, and Gary Westfall. University Physics With Modern Physics. McGraw-Hill Science Engineering, 2010. Print.