Friction. n.d. <http://hyperphysics.phy-astr.gsu.edu/hbase/frict.html>.

Potential Energy. n.d. 2014. <http://www.physicsclassroom.com/class/energy/Lesson-1/Potential-Energy>.

PowerSports, C3. The Effects of Reducing Weight on a Snowmobile. 2014. 25 11 2014. <https://www.c3powersports.com/content/effects-reducing-weight-snowmobile>.

Rotation Kinematics, Moment of Inertia, and Torque. n.d. 2014. <http://bolvan.ph.utexas.edu/~vadim/classes/2008s.homeworks/linang.pdf>.

Roth, Alfred C., Blake J. Fisher and W. Scott Gauthier. "Two and Four cycle engines." Small Gas Engines. The Goodheart-Willcox Co, 2012. Ch 5.