http://s.hswstatic.com/gif/easy-yo-yo-tricks-13b.jpg Kinetic Energy Once the Yo-Yo begins to travel away from the hand, it begins uncoiling the string and converting it’s potentially energy into kinetic energy, U$\gg$KE. As the potential energy decreases, the kinetic energy increases. Once the Yo-Yo reaches the end of the string, two things can happen. If a tug is given on the string causing an increase in tension and friction on the Yo-Yo axle then the Yo-Yo begins to role its way back up the string converting its kinetic energy back to potential energy, KE$$U.  If no tug is given on the string then is will continue to spin until it slows to due to a small amount of friction from the string, KE$$TH. KE = ½ m v² KE is the translational kinetic energy m is the mass of the Yo-Yo v is the velocity that it moves up or down the string $$